Physical and physico-chemical methods. Physico-chemical methods for the analysis of medicinal products Chemical methods for the analysis of medicinal products

As is known, pharmacopoeial analysis aims to establish the authenticity, determine the purity and quantify the active substance or ingredients of a complex dosage form. Despite the fact that each of these stages of pharmacopoeial analysis solves its specific task, they cannot be considered in isolation. So the performance of the authenticity reaction sometimes gives an answer to the presence or absence of a particular impurity. In the PAS-Na preparation, a qualitative reaction is carried out with a solution of iron (III) chloride (as a derivative of salicylic acid, it forms a violet-red color). But the appearance of a precipitate in this solution after three hours indicates the presence of an admixture of 5-aminosalicylic acid, which is pharmacologically inactive. However, such examples are quite rare.

The determination of some constants - melting point, density, specific absorption rate, allows us to simultaneously draw a conclusion about the authenticity and purity of a given substance. Since the methods for determining certain constants for various preparations are identical, we study them in the general methods of analysis. Knowledge of the theoretical foundations and the ability to carry out the definition will be required in the subsequent analysis of various groups of drugs.

Pharmacopoeial analysis is an integral part of pharmaceutical analysis and is a set of methods for studying medicines and dosage forms set forth in the State Pharmacopoeia and other normative documents (FS, FSP, GOST) and used to determine authenticity, purity and quantitative analysis.

In the quality control of medicines, physical, physico-chemical, chemical and biological methods of analysis are used. ND tests include several main stages:

description;

solubility;

authenticity;

physical constants (melting, boiling or distillation point, refractive index, specific rotation, density, spectral characteristics);

transparency and color of solutions;

acidity or alkalinity, pH of the solution;

determination of impurities;

weight loss on drying;

sulfate ash;

quantitation.

Depending on the nature of the medicinal product, some of these tests may either be absent or others may be included, such as acid value, iodine value, saponification value, etc.

A private monograph for any drug begins with a section "Description", which mainly characterizes the physical properties of matter:

state of aggregation (solid, liquid, gas), if solid, then the degree of its dispersion is determined (fine-crystalline, coarse-crystalline), the shape of the crystals (acicular, cylindrical)

substance color - an important indicator of authenticity and purity. Most drugs are colorless, that is, they are white. Coloring visually when determining the state of aggregation. A small amount of the substance is placed in a thin layer on a Petri dish or watch glass and viewed against a white background. In SP X1 there is an article "Determination of the degree of whiteness of powdered drugs." The determination is carried out by an instrumental method on special photometers "Specol-10". It is based on the spectral characteristic of the light reflected from the drug sample. The so-called reflection coefficient- the ratio of the value of the reflected light flux to the value of the incident. The measured reflectances make it possible to determine the presence or absence of a color or grayish tint in substances by calculating the degree of whiteness (α) and the degree of brightness (β). Since the appearance of shades or a change in color is, as a rule, a consequence of chemical processes - oxidation, reduction, then already this initial stage of the study of substances allows us to draw conclusions. This the method is excluded from the SP X11 edition.

Smell define rarely immediately after opening the package at a distance of 4-6 cm. No smell after opening the package immediately according to the method: 1-2 g of the substance is evenly distributed on a watch glass with a diameter of 6-8 cm and after 2 minutes the smell is determined at a distance of 4-6 cm.

In the Description section, there may be instructions on the possibility of changing substances during storage. For example, in the preparation of calcium chloride it is indicated that it is very hygroscopic and blurs in air, and sodium iodide - in the air it becomes damp and decomposes with the release of iodine, crystalline hydrates, in case of weathering or non-compliance with the conditions of crystallization in production, will no longer have the desired appearance or shape crystals, nor by color.

Thus, the study of the appearance of a substance is the first, but very important step in the analysis of substances, and it is necessary to be able to relate changes in appearance with possible chemical changes and draw the right conclusion.

Solubility(GF XI, issue 1, p. 175, GF XII, issue 1, p. 92)

Solubility is an important indicator of the quality of a drug substance. As a rule, a certain list of solvents is given in the ND, which most fully characterizes this physical property, so that in the future it can be used to assess the quality at one stage or another of the study of this medicinal substance. Thus, solubility in acids and alkalis is characteristic of amphoteric compounds (zinc oxide, sulfonamides), organic acids and bases (glutamic acid, acetylsalicylic acid, codeine). The change in solubility indicates the presence or appearance during storage of less soluble impurities, which characterizes the change in its quality.

In SP XI, solubility means not a physical constant, but a property expressed by approximate data and serving as an approximate characteristic of preparations.

Along with the melting point, the solubility of a substance at constant temperature and pressure is one of the options, according to which authenticity and purity (good quality) of almost all medicines.

It is recommended to use solvents of different polarity (usually three); the use of low-boiling and flammable (diethyl ether) or very toxic (benzene, methylene chloride) solvents is not recommended.

Pharmacopoeia XI ed. accepted two ways of expressing solubility :

In parts (ratio of substance and solvent). For example, for sodium chloride according to FS, the solubility in water is expressed in a ratio of 1:3, which means that no more than 3 ml of water is needed to dissolve 1 g of a medicinal substance.

In conventional terms(GF XI, p.176). For example, for sodium salicylate in PS, solubility is given in conditional terms - “we will very easily dissolve in water”. This means that up to 1 ml of water is needed to dissolve 1 g of a substance.

Pharmacopoeia XII ed. only in conditional (in terms of 1 g)

Conditional terms and their meanings are given in Table. 1. (GF XI, issue 1, p. 176, GF XII, issue 1, p. 92).

Conditional terms of solubility

Conditional terms	Abbreviations	Amount of solvent (ml), required to dissolve 1g substances
Very easily soluble
Easily soluble		More than 1 to 10
Soluble
sparingly soluble
Slightly soluble		» 100 to 1000
Very slightly soluble		» 1000 to 10000
Practically insoluble

The conditional term corresponds to a certain interval of solvent volumes (ml), within which one gram of the medicinal substance should be completely dissolved.

The dissolution process is carried out in solvents at temperature 20°C. In order to save the medicinal substance and the solvent, the mass of the drug is weighed in such a way (with an accuracy of 0.01 g) that no more than 100 ml is spent on establishing the solubility of water, and no more than 10-20 ml of organic solvents.

medicinal substance (substance) considered soluble , if particles of a substance are not detected in a solution when observed in transmitted light.

Methodology . (1 way). The weighed mass of the drug, previously ground into a fine powder, is added to the measured volume of the solvent corresponding to its minimum volume, shaken. Then, in accordance with Table. 1, the solvent is gradually added to its maximum volume and continuously shaken for 10 minutes. After this time, particles of the substance should not be detected in the solution with the naked eye. For example, 1 g of sodium benzoate is weighed, placed in a test tube with 1 ml of water, shaken and 9 ml of water are gradually added, because. sodium benzoate is easily soluble in water (from 1 to 10 ml).

For slowly soluble drugs that require more than 10 minutes for complete dissolution, heating in a water bath up to 30°C is allowed. Observation is carried out after cooling the solution to 20°C and vigorous shaking for 1-2 minutes. For example, caffeine is slowly soluble in water (1:60), codeine is slowly and slightly soluble in water (100-1000), calcium gluconate is slowly soluble in 50 hours of water, calcium lactate is slowly soluble in water, boric acid is slowly soluble in 7 hours glycerin.

2 way. Solubility, expressed in parts, indicates the volume of solvent in ml required to dissolve 1 g of a substance.

Methodology. (Method 2) The mass of the medicinal product weighed on a manual scale is dissolved in the volume of the solvent indicated by the RD. Particles of undissolved substance should not be detected in the solution.

Solubility in parts is indicated in pharmacopoeial monographs for the following preparations: boric acid(soluble in 25 hours of water, 25 hours of alcohol, 4 hours of boiling water); potassium iodide(soluble in 0.75 hours of water, 12 hours of alcohol and 2.5 hours of glycerin); sodium bromide(soluble in 1.5 hours of water, in 10 hours of alcohol); potassium bromide(soluble in 1.7 parts of water and m.p. alcohol); potassium chloride and sodium chloride(r. in 3 hours of water).

In the case of testing, for example, sodium bromide, proceed as follows: weigh 1 g of sodium bromide on a hand scale, add 1.5 ml of water and shake until completely dissolved.

General pharmacopoeial article " Solubility » SP XII ed. Supplemented with a description of methods for determining the solubility of substances with unknown and known solubility.

Melting point (T ° pl)

The melting point is a constant characterizing purity substances and at the same time its authenticity. It is known from physics that the melting point is the temperature at which the solid phase of a substance is in equilibrium with the melt. A pure substance has a clear melting point. Since drugs can have a small amount of impurities, we will no longer see such a clear picture. In this case, the interval at which the substance melts is determined. Usually, this interval lies within 2 ◦ C. A longer interval indicates the presence of impurities within unacceptable limits.

According to the wording of GF X1 under melting point substances understand the temperature interval between the beginning of melting (appearance of the first drop of liquid) and the end of melting (complete transition of the substance to the liquid state).

If the substance has an indistinct beginning or end of melting, determine temperature of only the beginning or end of melting. Sometimes a substance melts with decomposition, in which case it is determined decomposition temperature, that is, the temperature at which sudden change in substance(e.g. foaming).

Methods melting point determination

The choice of method is dictated two points:

the stability of a substance when heated and

ability to be ground into powder.

According to the GF X1 edition, there are 4 ways to determine T ° pl:

Method 1 - for substances that can be triturated into a powder, stable when heated

Method 1a - for substances that can be triturated into powder, not heat resistant

Methods 2 and 3 - for substances that are not triturable

Methods 1, 1a and 2 involve the use of 2 devices:

PTP ( instrument for determining Tm): familiar to you from the course of organic chemistry, allows you to determine the Tm of substances within from 20 ◦ C to 360 ◦ FROM

A device consisting of a round-bottom flask with a test tube sealed into it, into which a thermometer is inserted with a capillary attached to it containing the starting substance. The outer flask is filled with ¾ of the volume of the coolant liquid:

water (allows you to determine Tm up to 80 ◦ C),

vaseline oil or liquid silicones, concentrated sulfuric acid (allows you to determine Tm up to 260 ◦ C),

a mixture of sulfuric acid and potassium sulfate in a ratio of 7:3 (allows you to determine Tm above 260 ◦ C)

The technique is general, regardless of the device.

Finely ground dry matter is placed in a medium-sized capillary (6-8 cm) and introduced into the device at a temperature 10 degrees lower than expected. By adjusting the rate of temperature rise, the temperature range of changes in the substance in the capillary is fixed. At the same time, at least 2 determinations are made and the arithmetic mean is taken.

Tm is determined not only for pure substances, but also for their derivatives– oximes, hydrazones, bases and acids isolated from their salts.

Unlike GF XI in GF XII ed. melting temperature in the capillary method means not the interval between the beginning and end of melting, but end melt temperature , which is consistent with the European Pharmacopoeia.

Temperature limits of distillation (T° kip.)

GF value is defined as interval between the initial and final boiling points at normal pressure. (101.3 kPa - 760 mm Hg). The interval is usually 2°.

Under initial T ° boiling understand the temperature at which the first five drops of liquid were distilled into the receiver.

Under the final- the temperature at which 95% of the liquid passed into the receiver.

A longer interval than indicated in the corresponding API indicates the presence of impurities.

The device for determining the CCI consists of

a heat-resistant flask with a thermometer in which liquid is placed,

refrigerator and

receiving flask (graduated cylinder).

CCI, observed in the experiment, lead to normal pressure according to the formula:

Tisp \u003d Tnabl + K (p - p 1)

Where: p - normal barometric pressure (760 mm Hg)

p 1 - barometric pressure during the experiment

K - increase in Tbp per 1 mm of pressure

Thus, determining the temperature limits of distillation determine authenticity and purity ether, ethanol, chloroethyl, halothane.

OFS GF XII " Determination of temperature limits for distillation » supplemented by the definition boiling points and in private FS recommends defining solidification or boiling point for liquid drugs.

Density(GF XI, issue 1, p. 24)

Density is the mass per unit volume of a substance. Expressed in g/cm 3 .

ρ = m/ V

If the mass is measured in g, and the volume is in cm 3, then the density is the mass of 1 cm 3 of a substance.

Density is determined using a pycnometer (up to 0.001). or hydrometer (measurement accuracy up to 0.01)

See the device of devices in the GF X1 edition.

The purpose of the study of medicinal substances is to establish the suitability of the medicinal product for medical use, i.e. compliance with its regulatory document for this drug.

Pharmaceutical analysis is the science of chemical characterization and measurement of biologically active substances at all stages of production: from the control of raw materials to the assessment of the quality of the resulting medicinal substance, the study of its stability, the establishment of expiration dates and the standardization of the finished dosage form. The peculiarities of pharmaceutical analysis are its versatility and variety of substances or their mixtures, including individual chemicals, complex mixtures of biological substances (proteins, carbohydrates, oligopeptides, etc.). Methods of analysis need to be constantly improved, and if chemical methods, including qualitative reactions, prevailed in the UP Pharmacopoeia, then at the present stage, mainly physicochemical and physical methods of analysis are used.

Pharmaceutical analysis, depending on the tasks, includes various aspects of drug quality control:
1. Pharmacopoeial analysis;
2. Stage-by-stage control of the production of medicines;
3. Analysis of individual drugs.

The main and most significant is the pharmacopoeial analysis, i.e. analysis of medicines for compliance with the standard - a pharmacopoeial monograph or other ND and, thus, confirmation of its suitability. Hence the requirements for high specificity, selectivity, accuracy and reliability of the analysis.

A conclusion about the quality of a medicinal product can only be made on the basis of a sample analysis (a statistically significant sample). The sampling procedure is indicated either in a private article or in a general article of the Global Fund X1 ed. (Issue 2) p.15. To test medicines for compliance with the requirements of regulatory and technical documentation, multi-stage sampling (sampling) is carried out. In multi-stage sampling, a sample (sample) is formed in stages and the products in each stage are randomly selected in proportional quantities from the units selected in the previous stage. The number of steps is determined by the type of packaging.

Stage 1: selection of packaging units (boxes, boxes, etc.);
Stage 2: selection of packaging units in packaging (boxes, bottles, cans, etc.);
Stage 3: selection of products in primary packaging (ampoules, vials, blisters, etc.).

To calculate the selection of the number of products at each stage, use the formula:

where n- the number of packaging units of this stage.

The specific sampling procedure is described in detail in the GF X1 edition, issue 2. In this case, the analysis is considered reliable if at least four samples are reproducible.

Pharmaceutical Analysis Criteria

For various purposes of the analysis, such criteria as the selectivity of the analysis, sensitivity, accuracy, the time of the analysis, the amount of the test substance are important.

The selectivity of the analysis is essential in the analysis of complex preparations consisting of several active components. In this case, the selectivity of the analysis is very important for the quantitative determination of each of the substances.

Requirements for accuracy and sensitivity depend on the object and purpose of the study. When testing for purity or impurities, highly sensitive methods are used. For stepwise production control, the time factor spent on analysis is important.

An important parameter of the analysis method is the sensitivity limit of the method. This limit means the lowest content at which a given substance can be reliably detected. The least sensitive are chemical methods of analysis and qualitative reactions. The most sensitive enzymatic and biological methods to detect single macromolecules of substances. Of those actually used, the most sensitive are radiochemical, catalytic and fluorescent methods, which make it possible to determine up to 10 -9%; sensitivity of spectrophotometric methods 10 -3 -10 -6%; potentiometric 10 -2%.

The term "analysis accuracy" simultaneously includes two concepts: reproducibility and correctness of the results obtained.

Reproducibility - characterizes the dispersion of the results of the analysis compared to the average value.

Correctness - reflects the difference between the actual and found content of the substance. The accuracy of the analysis depends on the quality of the instruments, the experience of the analyst, etc. The accuracy of the analysis cannot be higher than the accuracy of the least accurate measurement. This means that if the titration is accurate to ±0.2 ml plus leakage error is also ±0.2 ml, i.e. in total ±0.4 ml, then when 20 ml of titrant is consumed, the error is 0.2%. With a decrease in the sample and the amount of titrant, the accuracy decreases. Thus, titrimetric analysis allows determination with a relative error of ± (0.2-0.3)%. Each method has its own accuracy. When analyzing, it is important to have an understanding of the following concepts:

Gross mistakes- are a miscalculation of the observer or a violation of the analysis methodology. Such results are discarded as unreliable.

Systematic errors - reflect the correctness of the results of the analysis. They distort the measurement results, as a rule, in one direction by some constant value. Systematic errors can be partially eliminated by introducing corrections, instrument calibration, etc.

Random errors - reflect the reproducibility of the results of the analysis. They are called by uncontrolled variables. The arithmetic mean of random errors tends to zero. Therefore, for calculations, it is necessary to use not the results of single measurements, but the average of several parallel determinations.

Absolute error- represents the difference between the result obtained and the true value. This error is expressed in the same units as the value being determined.

Relative error definition is equal to the ratio of the absolute error to the true value of the determined value. It is usually expressed as a percentage or percentage.

The values of relative errors depend on the method by which the analysis is performed and what the analyzed substance is - an individual substance and a mixture of many components.

The relative error in the study of individual substances by the spectrophotometric method is 2-3%, by IR spectrophotometry - 5-12%; liquid chromatography 3-4%; potentiometry 0.3-1%. Combined methods usually reduce the accuracy of the analysis. Biological methods are the least accurate - their relative error reaches 50%.

Methods for the identification of medicinal substances.

The most important indicator in the testing of medicinal substances is their identification or, as is customary in pharmacopoeial articles, authenticity. Numerous methods are used to determine the authenticity of medicinal substances. All the main and general are described in the GF X1 edition, issue 1. Historically, the main emphasis has been on chemical, incl. qualitative color reactions characterizing the presence of certain ions or functional groups in organic compounds, at the same time, physical methods were also widely used. In modern pharmacopoeias, the emphasis is on physico-chemical methods.

Let's focus on the main physical methods.

A fairly stable constant characterizing a substance, its purity and authenticity is the melting point. This indicator is widely used for the standardization of substances of medicinal substances. Methods for determining the melting point are described in detail in the GF X1, you yourself could try it out in laboratory classes. A pure substance has a constant melting point, however, when impurities are added to it, the melting point, as a rule, decreases very significantly. This effect is called a mixing test, and it is the mixing test that allows you to establish the authenticity of the drug in the presence of a standard sample or a known sample. There are, however, exceptions, as racemic sulphocamphoric acid melts at a higher temperature, and the various crystalline forms of indomethacin differ in melting point. Those. this method is one of the indicators that characterize both the purity of the product and its authenticity.

For some drugs, such an indicator as the solidification temperature is used. Another indicator characterizing a substance is the boiling point or temperature limits of distillation. This indicator characterizes liquid substances, for example, ethyl alcohol. The boiling point is a less characteristic indicator, it strongly depends on the pressure of the atmosphere, the possibility of the formation of mixtures or azeotropes and is used quite rarely.

Among other physical methods, it should be noted the determination density, viscosity. Standard methods of analysis are described in SP X1. The method that characterizes the authenticity of the drug is also the determination of its solubility in various solvents. According to GF X1 ed. This method is characterized as a property that can serve as an indicative characteristic of the test product. Along with the melting point, the solubility of a substance is one of the parameters by which the authenticity and purity of almost all medicinal substances are established. The pharmacopeia establishes an approximate gradation of substances by solubility from very easily soluble to practically insoluble. In this case, a substance is considered to be dissolved, in the solution of which no particles of the substance are observed in transmitted light.

Physical and chemical methods for determining authenticity.

The most informative in terms of determining the authenticity of substances are physicochemical methods based on the properties of the molecules of substances to interact with any physical factors. Physical and chemical methods include:

1.Spectral methods
UV spectroscopy
Spectroscopy in visible light
IR spectroscopy
Fluorescence spectroscopy
Atomic absorption spectroscopy
X-ray methods of analysis
Nuclear magnetic resonance
X-ray diffraction analysis

2. Sorption methods of analysis
Thin layer chromatography
Gas-liquid chromatography
High Performance Liquid Chromatography
Electrophoresis
Iontophoresis
Gel chromatography

3.Mass methods of analysis
Mass spectrometry
Chromatomass spectrometry

4. Electrochemical methods of analysis
Polarography
Electron paramagnetic resonance

5. Use of standard samples

Let us briefly consider the methods of analysis applicable in pharmacy. All these methods of analysis will be read to you in detail at the end of December by Professor V.I. Myagkikh. Some spectral methods are used to determine the authenticity of medicinal substances. The most reliable is the use of the low-frequency region of IR spectroscopy, where the absorption bands most reliably reflect this substance. I also call this area the fingerprint area. As a rule, comparison of IR spectra taken under standard conditions of a standard sample and a test sample is used to confirm authenticity. The coincidence of all absorption bands confirms the authenticity of the drug. The use of UV and visible spectroscopy is less reliable, because the nature of the spectrum is not individual and reflects only a certain chromophore in the structure of an organic compound. Atomic absorption spectroscopy and X-ray spectroscopy are used to analyze inorganic compounds, to identify chemical elements. Nuclear magnetic resonance makes it possible to establish the structure of organic compounds and is a reliable method for confirming authenticity, however, due to the complexity of the instruments and the high cost, it is used very rarely and, as a rule, only for research purposes. Fluorescence spectroscopy is applicable only to a certain class of substances that fluoresce when exposed to UV radiation. In this case, the fluorescence spectrum and the fluorescence excitation spectrum are quite individual, but strongly depend on the medium in which the given substance is dissolved. This method is more commonly used for quantitation, especially of small quantities, as it is one of the most sensitive.

X-ray diffraction analysis is the most reliable method for confirming the structure of a substance, it allows you to establish the exact chemical structure of a substance, however, it is simply not suitable for stream analysis of authenticity and is used exclusively for scientific purposes.

Sorption methods of analysis found a very wide application in pharmaceutical analysis. They are used to determine authenticity, the presence of impurities, and quantification. You will be given a lecture in detail about these methods and the equipment used by Professor V.I. Myagkikh, a regional representative of Shimadzu, one of the main manufacturers of chromatographic equipment. These methods are based on the principle of sorption-desorption of substances on certain carriers in a carrier stream. Depending on the carrier and sorbent, they are divided into thin-layer chromatography, liquid column chromatography (analytical and preparative, including HPLC), gas-liquid chromatography, gel filtration, iontophoresis. The last two methods are used to analyze complex protein objects. A significant drawback of the methods is their relativity, i.e. Chromatography can characterize a substance and its quantity only when compared with a standard substance. However, it should be noted as a significant advantage - the high reliability of the method and accuracy, because. in chromatography, any mixture must be separated into individual substances and the result of the analysis is precisely the individual substance.

Mass spectrometric and electrochemical methods are rarely used to confirm authenticity.

A special place is occupied by methods for determining authenticity in comparison with a standard sample. This method is used quite widely in foreign pharmacopoeias to determine the authenticity of complex macromolecules, complex antibiotics, some vitamins, and other substances containing especially chiral carbon atoms, since it is difficult or even impossible to determine the authenticity of an optically active substance by other methods. A standard sample should be developed and issued on the basis of a developed and approved pharmacopoeial monograph. In Russia, only a few standard samples exist and are used, and the so-called RSOs are most often used for analysis - working standard samples prepared immediately before the experiment from known substances or corresponding substances.

Chemical methods of authentication.

The identification of medicinal substances by chemical methods is used mainly for inorganic medicinal substances, since other methods are most often not available or they require complex and expensive equipment. As already mentioned, inorganic elements are easily identified by atomic absorption or X-ray spectroscopy. Our Pharmacopoeia Monographs usually use chemical authentication methods. These methods are usually divided into the following:

Precipitation reactions of anions and cations. Typical examples are the precipitation reactions of sodium and potassium ions with (zincuranyl acetate and tartaric acid), respectively:

Such reactions are used in great variety and they will be discussed in detail in a special section of pharmaceutical chemistry regarding inorganic substances.

Redox reactions.

Redox reactions are used to reduce metals from oxides. For example, silver from its formalin oxide (silver mirror reaction):

The oxidation reaction of diphenylamine is the basis for testing the authenticity of nitrates and nitrites:

Reactions of neutralization and decomposition of anions.

Carbonates and hydrocarbonates under the action of mineral acids form carbonic acid, which decomposes to carbon dioxide:

Similarly, nitrites, thiosulfates, and ammonium salts decompose.

Changes in the color of a colorless flame. Sodium salts color the flame yellow, copper green, potassium purple, calcium brick red. It is this principle that is used in atomic absorption spectroscopy.

Decomposition of substances during pyrolysis. The method is used for preparations of iodine, arsenic, mercury. Of the currently used, the reaction of basic bismuth nitrate is most characteristic, which decomposes when heated to form nitrogen oxides:

Identification of organoelement medicinal substances.

Qualitative elemental analysis is used to identify compounds containing arsenic, sulfur, bismuth, mercury, phosphorus, and halogens in an organic molecule. Since the atoms of these elements are not ionized, preliminary mineralization is used to identify them, either by pyrolysis, or again by pyrolysis with sulfuric acid. Sulfur is determined by hydrogen sulfide reaction with potassium nitroprusside or lead salts. Iodine is also determined by pyrolysis by the release of elemental iodine. Of all these reactions, the identification of arsenic is of interest, not so much as a drug - they are practically not used, but as a method for monitoring impurities, but more on that later.

Testing the authenticity of organic medicinal substances. The chemical reactions used to test the authenticity of organic medicinal substances can be divided into three main groups:
1. General chemical reactions of organic compounds;
2. Reactions of formation of salts and complex compounds;
3. Reactions used to identify organic bases and their salts.

All these reactions are ultimately based on the principles of functional analysis, i.e. the reactive center of the molecule, which, when reacting, gives the appropriate response. Most often, this is a change in some properties of a substance: color, solubility, state of aggregation, etc.

Let us consider some examples of the use of chemical reactions for the identification of medicinal substances.

1. Reactions of nitration and nitrosation. They are used quite rarely, for example, to identify phenobarbital, phenacetin, dicain, although these drugs are almost never used in medical practice.

2. Diazotization and azo coupling reactions. These reactions are used to open primary amines. Diazotized amine combines with beta-naphthol to give a characteristic red or orange color.

3. Halogenation reactions. Used to open aliphatic double bonds - when bromine water is added, bromine is added to the double bond and the solution becomes colorless. A characteristic reaction of aniline and phenol is that when they are treated with bromine water, a tribromo derivative is formed, which precipitates.

4. Condensation reactions of carbonyl compounds. The reaction consists in the condensation of aldehydes and ketones with primary amines, hydroxylamine, hydrazines and semicarbazide:

The resulting azomethines (or Schiff bases) have a characteristic yellow color. The reaction is used to identify, for example, sulfonamides. The aldehyde used is 4-dimethylaminobenzaldehyde.

5. Oxidative condensation reactions. The process of oxidative cleavage and the formation of azomethine dye underlies ninhydrin reaction. This reaction is widely used for the discovery and photocolorimetric determination of α- and β-amino acids, in the presence of which an intense dark blue color appears. It is due to the formation of a substituted salt of diketohydrindylidene diketohydramine, a condensation product of excess ninhydrin and reduced ninhydrin with ammonia released during the oxidation of the test amino acid:

To open phenols, the reaction of the formation of triarylmethane dyes is used. So phenols interacting with formaldehyde form dyes. Similar reactions include the interaction of resorcinol with phthalic anhydride leading to the formation of a fluorescent dye - fluorescein.

Many other reactions are also used.

Of particular interest are reactions with the formation of salts and complexes. Inorganic salts of iron (III), copper (II), silver, cobalt, mercury (II) and others for testing the authenticity of organic compounds: carboxylic acids, including amino acids, derivatives of barbituric acid, phenols, sulfonamides, some alkaloids. The formation of salts and complex compounds occurs according to the general scheme:

R-COOH + MX = R-COOM + HX

The complex formation of amines proceeds similarly:

R-NH 2 + X = R-NH 2 X

One of the most common reagents in pharmaceutical analysis is a solution of iron (III) chloride. Interaction with phenols, it forms a colored solution of phenoxides, they are colored blue or purple. This reaction is used to discover phenol or resorcinol. However, meta-substituted phenols do not form colored compounds (thymol).

Copper salts form complex compounds with sulfonamides, cobalt salts with barbiturates. Many of these reactions are also used for quantitative determination.

Identification of organic bases and their salts. This group of methods is most often used in ready-made forms, especially in the study of solutions. So, salts of organic amines, when alkalis are added, form a precipitate of a base (for example, a solution of papaverine hydrochloride) and vice versa, salts of organic acids, when a mineral acid is added, give a precipitate of an organic compound (for example, sodium salicylate). To identify organic bases and their salts, the so-called precipitation reagents are widely used. More than 200 precipitating reagents are known, which form water-insoluble simple or complex salts with organic compounds. The most commonly used solutions are given in the second volume of the SP 11th edition. An example is:
Scheibler's reagent - phosphotungstic acid;
Picric acid
Styphnic acid
Picramic acid

All these reagents are used for the precipitation of organic bases (for example, nitroxoline).

It should be noted that all these chemical reactions are used to identify medicinal substances not by themselves, but in combination with other methods, most often physicochemical, such as chromatography, spectroscopy. In general, it is necessary to pay attention to the fact that the problem of the authenticity of medicinal substances is a key one, because this fact determines the harmlessness, safety and effectiveness of the drug, so this indicator needs to be given great attention and it is not enough to confirm the authenticity of the substance by one method.

General requirements for purity tests.

Another equally important indicator of the quality of a medicinal product is purity. All medicinal products, regardless of the method of their preparation, are tested for purity. This determines the content of impurities in the preparation. It is conditionally possible to divide impurities into two groups: the first, impurities that have a pharmacological effect on the body; the second, impurities, indicating the degree of purification of the substance. The latter do not affect the quality of the drug, but in large quantities reduce its dose and, accordingly, reduce the activity of the drug. Therefore, all pharmacopoeias set certain limits for these impurities in drugs. Thus, the main criterion for the good quality of the drug is the absence of impurities, which is impossible by nature. The concept of the absence of impurities is associated with the detection limit of one method or another.

The physical and chemical properties of substances and their solutions give an approximate idea of the presence of impurities in drugs and regulate their suitability for use. Therefore, in order to assess good quality, along with the establishment of authenticity and determination of the quantitative content, a number of physical and chemical tests are carried out to confirm the degree of its purity:

Transparency and degree of turbidity carried out by comparison with a turbidity standard, and transparency is determined by comparison with a solvent.

Chromaticity. A change in the degree of color may be due to:
a) the presence of an extraneous colored impurity;
b) a chemical change in the substance itself (oxidation, interaction with Me +3 and +2, or other chemical processes occurring with the formation of colored products. For example:

Resorcinol turns yellow during storage due to oxidation under the action of atmospheric oxygen to form quinones. In the presence of, for example, iron salts, salicylic acid acquires a purple color due to the formation of iron salicylates.

Color assessment is carried out by comparing the main experience with color standards, and colorlessness is determined by comparison with a solvent.

Very often, a test based on their interaction with concentrated sulfuric acid, which can act as an oxidizing or dehydrating agent, is used to detect organic impurities. As a result of such reactions, colored products are formed. The intensity of the resulting color should not exceed the corresponding color standard.

Determination of the degree of whiteness of powdered drugs– physical method, first included in GF X1. The degree of whiteness (hue) of solid medicinal substances can be assessed by various instrumental methods based on the spectral characteristics of the light reflected from the sample. To do this, reflectances are used when the sample is illuminated with white light obtained from a special source, with a spectral distribution or passed through light filters (with a transmission max of 614 nm (red) or 439 nm (blue)). You can also measure the reflectance of light passed through a green filter.

A more accurate assessment of the whiteness of medicinal substances can be carried out using reflection spectrophotometers. The value of the degree of whiteness and the degree of brightness are characteristics of the quality of whites and whites with shades of medicinal substances. Their permissible limits are regulated in private articles.

Determination of acidity, alkalinity, pH.

The change in these indicators is due to:
a) a change in the chemical structure of the medicinal substance itself:

b) the interaction of the drug with the container, for example, exceeding the permissible limits of alkalinity in a novocaine solution due to glass leaching;
c) absorption of gaseous products (CO 2 , NH 3) from the atmosphere.

Determination of the quality of medicines according to these indicators is carried out in several ways:

a) by changing the color of the indicator, for example, an admixture of mineral acids in boric acid is determined by methyl red, which does not change its color from the action of weak boric acid, but turns pink if it contains impurities of mineral acids.

b) titrimetric method - for example, to establish the permissible limit of the content of hydriodic acid formed during storage of a 10% alcohol solution of I 2, titration is carried out with alkali (no more than 0.3 ml of 0.1 mol / l NaOH by volume of the titrant). (Formaldehyde solution - titrated with alkali in the presence of phenolphthalein).

In some cases, the Global Fund sets the volume of titrant to determine the acidity or alkalinity.

Sometimes two titrated solutions are added in succession: first an acid and then an alkali.

c) by determining the pH value - for a number of drugs (and necessarily for all injection solutions) according to the NTD, it is envisaged to determine the pH value.

Techniques for preparing a substance in the study of acidity, alkalinity, pH

Preparation of a solution of a certain concentration specified in the NTD (for substances soluble in water)
For those insoluble in water, a suspension of a certain concentration is prepared and the acid-base properties of the filtrate are determined.
For liquid preparations immiscible with water, agitation with water is carried out, then the aqueous layer is separated and its acid-base properties are determined.
For insoluble solids and liquids, the determination can be carried out directly in suspension (ZnO)

The pH value approximately (up to 0.3 units) can be determined using indicator paper or a universal indicator.

The colorimetric method is based on the property of indicators to change their color at certain ranges of pH values. To perform the tests, buffer solutions with a constant concentration of hydrogen ions are used, differing from each other by a pH value of 0.2. To a series of such solutions and to the test solution add the same amount (2-3 drops) of the indicator. According to the coincidence of color with one of the buffer solutions, the pH value of the medium of the test solution is judged.

Determination of volatile substances and water.

Volatile substances can enter drugs either due to poor purification from solvents or intermediates, or as a result of the accumulation of degradation products. Water in the medicinal substance can be contained in the form of capillary, absorbed bound, chemically bound (hydrated and crystalline) or free.

Drying, distillation and titration with Fischer's solution are used to determine volatile substances and water.

drying method. The method is used to determine the loss in weight on drying. Losses can be due to the content of hygroscopic moisture and volatile substances in the substance. Dried in a bottle to constant weight at a certain temperature. More often, the substance is kept at a temperature of 100-105 ºС, but the conditions for drying and bringing to a constant mass may be different.

The determination of volatile substances can be carried out for some products by the method of ignition. The substance is heated in a crucible until the volatile substances are completely removed. then gradually increase the temperature until complete calcination at red heat. For example, the GPC regulates the determination of sodium carbonate impurities in the sodium bicarbonate medicinal substance by the calcination method. Sodium bicarbonate decomposes into sodium carbonate, carbon dioxide and water:

Theoretically, the weight loss is 36.9%. According to GPC, the loss in mass should be at least 36.6%. The difference between the theoretical and specified in the GPC mass loss determines the allowable limit of sodium carbonate impurities in the substance.

distillation method in GF 11 is called "Definition of water", it allows you to determine hygroscopic water. This method is based on the physical property of the vapors of two immiscible liquids. A mixture of water and an organic solvent distills at a lower temperature than either of these liquids. GPC1 recommends using toluene or xylene as the organic solvent. The water content in the test substance is determined by its volume in the receiver after the end of the distillation process.

Titration with Fisher's reagent. The method allows to determine the total content of both free and crystalline water in organic, inorganic substances, solvents. The advantage of this method is the speed of execution and selectivity with respect to water. Fisher's solution is a solution of sulfur dioxide, iodine and pyridine in methanol. Among the disadvantages of the method, in addition to the need for strict adherence to tightness, is the impossibility of determining water in the presence of substances that react with the components of the reagent.

Ash definition.

The ash content is due to mineral impurities that appear in organic substances in the process of obtaining auxiliary materials and equipment from the initial products (primarily metal cations), i.e. characterizes the presence of inorganic impurities in organic substances.

a) total ash- is determined by the results of combustion (ashing, mineralization) at high temperature, characterizes the sum of all inorganic substances-impurities.

Ash composition:
Carbonates: CaCO 3, Na 2 CO 3, K 2 CO 3, PbCO 3
Oxides: CaO, PbO
Sulphates: CaSO4
Chlorides: CaCl 2
Nitrates: NaNO 3

When obtaining medicines from plant materials, mineral impurities can be caused by dust pollution of plants, absorption of trace elements and inorganic compounds from soil, water, etc.

b) Ash insoluble in hydrochloric acid, obtained after treatment of total ash with dilute HCl. The chemical composition of the ash is heavy metal chlorides (AgCl, HgCl 2, Hg 2 Cl 2), i.e. highly toxic impurities.

in) sulfate ash- Sulphated ash is determined in assessing the good quality of many organic substances. Characterizes impurities Mn + n in a stable sulfate form. The resulting sulfate ash (Fe 3 (SO 4) 2, PbSO 4, CaSO 4) is used for the subsequent determination of heavy metal impurities.

Impurities of inorganic ions - C1 -, SO 4 -2, NH 4 +, Ca +2, Fe +3 (+2) , Pv +2, As +3 (+5)

Impurities:
a) impurities of a toxic nature (an admixture of CN - in iodine),
b) having an antagonistic effect (Na and K, Mg and Ca)

The absence of impurities that are not allowed in the medicinal substance is determined by a negative reaction with the appropriate reagents. Comparison in this case is carried out with a part of the solution, to which all reagents are added, except for the main one that opens this impurity (control experiment). A positive reaction indicates the presence of an impurity and the poor quality of the drug.

Permissible impurities - impurities that do not affect the pharmacological effect and the content of which is allowed in small quantities established by the NTD.

To establish the permissible limit for the content of ion impurities in medicines, reference solutions are used that contain the corresponding ion in a certain concentration.

Some medicinal substances are tested for the presence of impurities by titration, for example, the determination of the impurity of norsulfazole in the drug fthalazole. The admixture of norsulfazole in phthalazole is determined quantitatively by nitritometrically. Titration of 1 g of phthalazole should consume no more than 0.2 ml of 0.1 mol/l NaNO 2 .

General requirements for reactions that are used in tests for acceptable and unacceptable impurities:
1. sensitivity,
2. specificity,
3. reproducibility of the reaction used.

The results of reactions proceeding with the formation of colored products are observed in reflected light on a dull white background, and white precipitates in the form of turbidity and opalescence are observed in transmitted light on a black background.

Instrumental methods for determining impurities.

With the development of analysis methods, the requirements for the purity of medicinal substances and dosage forms are constantly increasing. In modern pharmacopoeias, along with the considered methods, various instrumental methods are used, based on the physicochemical, chemical and physical properties of substances. The use of UV and visible spectroscopy rarely gives positive results and this is due to the fact that the structure of impurities, especially organic drugs, as a rule. It is close to the structure of the drug itself, so the absorption spectra differ little, and the concentration of the impurity is usually ten times lower than that of the main substance, which makes differential analysis methods unsuitable and allows one to estimate the impurity only approximately, i.e. as it is commonly called semi-quantitatively. The results are somewhat better if one of the substances, especially the impurity, forms a complex compound, while the other does not, then the maxima of the spectra differ significantly and it is already possible to determine the impurities quantitatively.

In recent years, IR-Fourier devices have appeared at enterprises that allow determining the content of both the main substance and impurities, especially water, without destroying the sample, but their use is constrained by the high cost of devices and the lack of standardized analysis methods.

Excellent impurity results are possible when the impurity fluoresces under UV light. The accuracy of such assays is very high, as is their sensitivity.

Wide application for testing for purity and quantitative determination of impurities both in medicinal substances (substances) and in dosage forms, which, perhaps, is no less important, because. many impurities are formed during the storage of drugs, obtained by chromatographic methods: HPLC, TLC, GLC.

These methods make it possible to determine impurities quantitatively, and each of the impurities individually, in contrast to other methods. The methods of HPLC and GLC chromatography will be discussed in detail in a lecture by prof. Myagkikh V.I. We will focus only on thin layer chromatography. The method of thin layer chromatography was discovered by the Russian scientist Tsvet and at the beginning existed as chromatography on paper. Thin layer chromatography (TLC) is based on the difference in the speeds of movement of the components of the analyzed mixture in a flat thin layer of the sorbent when the solvent (eluent) moves through it. Sorbents are silica gel, alumina, cellulose. Polyamide, eluents - organic solvents of different polarity or their mixtures with each other and sometimes with solutions of acids or alkalis and salts. The separation mechanism is due to the distribution coefficients between the sorbent and the liquid phase of the substance under study, which in turn is associated with many, including the chemical and physicochemical properties of the substances.

In TLC, the surface of an aluminum or glass plate is covered with a sorbent suspension, dried in air, and activated to remove traces of solvent (moisture). In practice, commercially manufactured plates with a fixed layer of sorbent are usually used. Drops of the analyzed solution with a volume of 1-10 μl are applied to the sorbent layer. The edge of the plate is immersed in the solvent. The experiment is carried out in a special chamber - a glass vessel, closed with a lid. The solvent moves through the layer under the action of capillary forces. Simultaneous separation of several different mixtures is possible. To increase the separation efficiency, multiple elution is used either in the perpendicular direction with the same or a different eluent.

After the completion of the process, the plate is dried in air and the position of the chromatographic zones of the components is set in various ways, for example, by irradiation with UV radiation, by spraying with coloring reagents, and kept in iodine vapor. On the resulting distribution pattern (chromatogram), the chromatographic zones of the mixture components are arranged in the form of spots in accordance with their sorbability in the given system.

The position of the chromatographic zones on the chromatogram is characterized by the value of R f . which is equal to the ratio of the path l i traversed by the i-th component from the starting point to the path Vп R f = l i / l.

The value of R f depends on the coefficient of distribution (adsorption) K і and the ratio of the volumes of the mobile (V p) and stationary (V n) phases.

Separation in TLC is affected by a number of factors: the composition and properties of the eluent, the nature, fineness and porosity of the sorbent, temperature, humidity, the size and thickness of the sorbent layer, and the dimensions of the chamber. Standardization of experimental conditions allows setting R f with a relative standard deviation of 0.03.

Identification of the components of the mixture is carried out by the values of R f . The quantitative determination of substances in the zones can be carried out directly on the sorbent layer by the area of the chromatographic zone, the fluorescence intensity of the component or its combination with a suitable reagent, by radiochemical methods. Automatic scanning instruments are also used to measure the absorption, transmission, reflection of light, or radioactivity of chromatographic zones. The separated zones can be removed from the plate together with the sorbent layer, the component can be desorbed into the solvent, and the solution can be analyzed spectrophotometrically. Using TLC, substances can be determined in quantities from 10 -9 to 10 -6; the error of determination is not less than 5-10%.

Physico-chemical or instrumental methods of analysis

Physico-chemical or instrumental methods of analysis are based on the measurement of the physical parameters of the analyzed system, which occur or change during the course of the analytical reaction, using instruments (instruments).

The rapid development of physical and chemical methods of analysis was due to the fact that the classical methods of chemical analysis (gravimetry, titrimetry) could no longer satisfy the numerous requests of the chemical, pharmaceutical, metallurgical, semiconductor, nuclear and other industries that required an increase in the sensitivity of methods up to 10-8 - 10-9%, their selectivity and rapidity, which would make it possible to control technological processes according to chemical analysis data, as well as to perform them automatically and remotely.

A number of modern physicochemical methods of analysis make it possible to simultaneously perform both qualitative and quantitative analysis of components in the same sample. The accuracy of the analysis of modern physicochemical methods is comparable to the accuracy of classical methods, and in some, for example, in coulometry, it is significantly higher.

The disadvantages of some physicochemical methods include the high cost of the instruments used, the need to use standards. Therefore, classical methods of analysis still have not lost their value and are used where there are no restrictions on the speed of analysis and where high accuracy is required at a high content of the analyzed component.

Classification of physical and chemical methods of analysis

The classification of physicochemical methods of analysis is based on the nature of the measured physical parameter of the analyzed system, the value of which is a function of the amount of substance. In accordance with this, all physicochemical methods are divided into three large groups:

Electrochemical;

Optical and spectral;

Chromatographic.

Electrochemical methods of analysis are based on the measurement of electrical parameters: current strength, voltage, equilibrium electrode potentials, electrical conductivity, amount of electricity, the values of which are proportional to the content of the substance in the analyzed object.

Optical and spectral methods of analysis are based on measuring parameters that characterize the effects of the interaction of electromagnetic radiation with substances: the intensity of the radiation of excited atoms, the absorption of monochromatic radiation, the refractive index of light, the angle of rotation of the plane of a polarized light beam, etc.

All these parameters are a function of the concentration of the substance in the analyzed object.

Chromatographic methods are methods for separating homogeneous multicomponent mixtures into individual components by sorption methods under dynamic conditions. Under these conditions, the components are distributed between two immiscible phases: mobile and stationary. The distribution of the components is based on the difference in their distribution coefficients between the mobile and stationary phases, which leads to different rates of transfer of these components from the stationary to the mobile phase. After separation, the quantitative content of each of the components can be determined by various methods of analysis: classical or instrumental.

Molecular absorption spectral analysis

Molecular absorption spectral analysis includes spectrophotometric and photocolorimetric types of analysis.

Spectrophotometric analysis is based on the determination of the absorption spectrum or the measurement of light absorption at a strictly defined wavelength, which corresponds to the maximum of the absorption curve of the substance under study.

Photocolorimetric analysis is based on a comparison of the color intensity of the investigated colored and standard colored solutions of a certain concentration.

Molecules of a substance have a certain internal energy E, the components of which are:

Energy of motion of electrons Еel located in the electrostatic field of atomic nuclei;

Vibration energy of atomic nuclei relative to each other E col;

Energy of rotation of the molecule E vr

and mathematically expressed as the sum of all the above energies:

Moreover, if a molecule of a substance absorbs radiation, then its initial energy E 0 increases by the amount of energy of the absorbed photon, that is:

It follows from the above equality that the shorter the wavelength λ, the greater the frequency of oscillations and, therefore, the greater E, that is, the energy imparted to the molecule of the substance when interacting with electromagnetic radiation. Therefore, the nature of the interaction of ray energy with matter depending on the wavelength of light λ will be different.

The totality of all frequencies (wavelengths) of electromagnetic radiation is called the electromagnetic spectrum. The wavelength interval is divided into areas: ultraviolet (UV) approximately 10-380 nm, visible 380-750 nm, infrared (IR) 750-100000 nm.

The energy imparted to a substance molecule by UV and visible radiation is sufficient to cause a change in the electronic state of the molecule.

The energy of infrared rays is less, so it is only sufficient to cause a change in the energy of vibrational and rotational transitions in a molecule of matter. Thus, in different parts of the spectrum it is possible to obtain different information about the state, properties and structure of substances.

Radiation Absorption Laws

Spectrophotometric methods of analysis are based on two main laws. The first of them is the Bouguer-Lambert law, the second law is Beer's law. The combined Bouguer-Lambert-Beer law has the following formulation:

The absorption of monochromatic light by a colored solution is directly proportional to the concentration of the light-absorbing substance and the thickness of the solution layer through which it passes.

The Bouguer-Lambert-Beer law is the basic law of light absorption and underlies most photometric methods of analysis. Mathematically, it is expressed by the equation:

the value lg I / I 0 is called the optical density of the absorbing substance and is denoted by the letters D or A. Then the law can be written as follows:

The ratio of the intensity of the monochromatic radiation flux passing through the test object to the intensity of the initial radiation flux is called the transparency, or transmission, of the solution and is denoted by the letter T: T = I / I 0

This ratio can be expressed as a percentage. The value of T, which characterizes the transmission of a layer 1 cm thick, is called the transmission coefficient. Optical density D and transmission T are related by the relationship

D and T are the main quantities characterizing the absorption of a solution of a given substance with a certain concentration at a certain wavelength and thickness of the absorbing layer.

Dependence D(С) is rectilinear, and Т(С) or Т(l) is exponential. This is strictly observed only for monochromatic radiation fluxes.

The value of the extinction coefficient K depends on the method of expressing the concentration of the substance in the solution and the thickness of the absorbing layer. If the concentration is expressed in moles per liter, and the layer thickness is in centimeters, then it is called the molar extinction coefficient, denoted by the symbol ε and is equal to the optical density of a solution with a concentration of 1 mol / l, placed in a cuvette with a layer thickness of 1 cm.

The value of the molar light absorption coefficient depends on:

From the nature of the solute;

Wavelengths of monochromatic light;

Temperatures;

The nature of the solvent.

Reasons for non-observance of the Bouger-Lambert-Beer law.

1. The law is derived and valid only for monochromatic light, therefore, insufficient monochromatization can cause a deviation of the law, and the more so, the less monochromatization of light.

2. Various processes can occur in solutions that change the concentration of an absorbing substance or its nature: hydrolysis, ionization, hydration, association, polymerization, complex formation, etc.

3. The light absorption of solutions significantly depends on the pH of the solution. When the pH of the solution changes, the following can change:

The degree of ionization of a weak electrolyte;

The form of existence of ions, which leads to a change in light absorption;

The composition of the resulting colored complex compounds.

Therefore, the law is valid for highly dilute solutions, and its scope is limited.

visual colorimetry

The color intensity of solutions can be measured by various methods. Among them, subjective (visual) methods of colorimetry and objective, that is, photocolorimetric, are distinguished.

Visual methods are such methods in which the assessment of the color intensity of the test solution is done with the naked eye. With objective methods of colorimetric determination, photocells are used instead of direct observation to measure the color intensity of the test solution. The determination in this case is carried out in special devices - photocolorimeters, so the method is called photocolorimetric.

Visible light colors:

The study of substances is a rather complex and interesting matter. Indeed, in their pure form, they are almost never found in nature. Most often, these are mixtures of complex composition, in which the separation of components requires certain efforts, skills and equipment.

After separation, it is equally important to correctly determine the belonging of a substance to a particular class, that is, to identify it. Determine the boiling and melting points, calculate the molecular weight, check for radioactivity, and so on, in general, investigate. For this, various methods are used, including physicochemical methods of analysis. They are quite diverse and require the use, as a rule, of special equipment. About them and will be discussed further.

Physical and chemical methods of analysis: a general concept

What are these methods of identifying compounds? These are methods based on the direct dependence of all the physical properties of a substance on its structural chemical composition. Since these indicators are strictly individual for each compound, physicochemical research methods are extremely effective and give a 100% result in determining the composition and other indicators.

So, such properties of a substance can be taken as a basis, such as:

the ability to absorb light;
thermal conductivity;
electrical conductivity;
boiling temperature;
melting and other parameters.

Physicochemical research methods have a significant difference from purely chemical methods for identifying substances. As a result of their work, there is no reaction, that is, the transformation of a substance, both reversible and irreversible. As a rule, the compounds remain intact both in terms of mass and composition.

Features of these research methods

There are several main features characteristic of such methods for determining substances.

The research sample does not need to be cleaned of impurities before the procedure, since the equipment does not require this.
Physicochemical methods of analysis have a high degree of sensitivity, as well as increased selectivity. Therefore, a very small amount of the test sample is needed for analysis, which makes these methods very convenient and efficient. Even if it is required to determine an element that is contained in the total wet weight in negligible amounts, this is not an obstacle for the indicated methods.
The analysis takes only a few minutes, so another feature is the short duration, or rapidity.
The research methods under consideration do not require the use of expensive indicators.

It is obvious that the advantages and features are sufficient to make physicochemical research methods universal and in demand in almost all studies, regardless of the field of activity.

Classification

There are several features on the basis of which the considered methods are classified. However, we will give the most general system, which unites and embraces all the main methods of research related directly to physical and chemical ones.

1. Electrochemical research methods. They are subdivided on the basis of the measured parameter into:

potentiometry;
voltammetry;
polarography;
oscillometry;
conductometry;
electrogravimetry;
coulometry;
amperometry;
dielkometry;
high frequency conductometry.

2. Spectral. Include:

optical;
X-ray photoelectron spectroscopy;
electromagnetic and nuclear magnetic resonance.

3. Thermal. Subdivided into:

thermal;
thermogravimetry;
calorimetry;
enthalpymetry;
delatometry.

4. Chromatographic methods, which are:

gas;
sedimentary;
gel-penetrating;
exchange;
liquid.

It is also possible to divide physicochemical methods of analysis into two large groups. The first are those that result in destruction, that is, the complete or partial destruction of a substance or element. The second is non-destructive, preserving the integrity of the test sample.

Practical application of such methods

The areas of use of the considered methods of work are quite diverse, but all of them, of course, in one way or another, relate to science or technology. In general, several basic examples can be given, from which it will become clear why such methods are needed.

Control over the flow of complex technological processes in production. In these cases, the equipment is necessary for contactless control and tracking of all structural links of the working chain. The same devices will fix malfunctions and malfunctions and give an accurate quantitative and qualitative report on corrective and preventive measures.
Carrying out chemical practical work in order to qualitatively and quantitatively determine the yield of the reaction product.
The study of a sample of a substance in order to establish its exact elemental composition.
Determination of the quantity and quality of impurities in the total mass of the sample.
Accurate analysis of intermediate, main and side participants of the reaction.
A detailed account of the structure of matter and the properties it exhibits.
Discovery of new elements and obtaining data characterizing their properties.
Practical confirmation of theoretical data obtained empirically.
Analytical work with high purity substances used in various branches of technology.
Titration of solutions without the use of indicators, which gives a more accurate result and has a completely simple control, thanks to the operation of the device. That is, the influence of the human factor is reduced to zero.
The main physicochemical methods of analysis make it possible to study the composition of:

minerals;
mineral;
silicates;
meteorites and foreign bodies;
metals and non-metals;
alloys;
organic and inorganic substances;
single crystals;
rare and trace elements.

Areas of use of methods

nuclear power;
physics;
chemistry;
radio electronics;
laser technology;
space research and others.

The classification of physicochemical methods of analysis only confirms how comprehensive, accurate and versatile they are for use in research.

Electrochemical methods

The basis of these methods is reactions in aqueous solutions and on electrodes under the action of an electric current, that is, in other words, electrolysis. Accordingly, the type of energy that is used in these methods of analysis is the flow of electrons.

These methods have their own classification of physico-chemical methods of analysis. This group includes the following species.

Electrical weight analysis. According to the results of electrolysis, a mass of substances is removed from the electrodes, which is then weighed and analyzed. So get data on the mass of compounds. One of the varieties of such works is the method of internal electrolysis.
Polarography. The basis is the measurement of current strength. It is this indicator that will be directly proportional to the concentration of the desired ions in the solution. Amperometric titration of solutions is a variation of the considered polarographic method.
Coulometry is based on Faraday's law. The amount of electricity spent on the process is measured, from which they then proceed to the calculation of ions in solution.
Potentiometry - based on the measurement of the electrode potentials of the participants in the process.

All the processes considered are physicochemical methods for the quantitative analysis of substances. Using electrochemical research methods, mixtures are separated into constituent components, the amount of copper, lead, nickel and other metals is determined.

Spectral

It is based on the processes of electromagnetic radiation. There is also a classification of the methods used.

Flame photometry. To do this, the test substance is sprayed into an open flame. Many metal cations give a color of a certain color, so their identification is possible in this way. Basically, these are substances such as: alkali and alkaline earth metals, copper, gallium, thallium, indium, manganese, lead and even phosphorus.
Absorption spectroscopy. Includes two types: spectrophotometry and colorimetry. The basis is the determination of the spectrum absorbed by the substance. It operates both in the visible and in the hot (infrared) part of the radiation.
Turbidimetry.
Nephelometry.
Luminescent analysis.
Refractometry and polarometry.

Obviously, all the considered methods in this group are methods of qualitative analysis of a substance.

Emission analysis

This causes the emission or absorption of electromagnetic waves. According to this indicator, one can judge the qualitative composition of the substance, that is, what specific elements are included in the composition of the research sample.

Chromatographic

Physicochemical studies are often carried out in different environments. In this case, chromatographic methods become very convenient and effective. They are divided into the following types.

Adsorption liquid. At the heart of the different ability of the components to adsorption.
Gas chromatography. Also based on adsorption capacity, only for gases and substances in the vapor state. It is used in mass production of compounds in similar states of aggregation, when the product comes out in a mixture that should be separated.
Partition chromatography.
Redox.
Ion exchange.
Paper.
Thin layer.
Sedimentary.
Adsorption-complexing.

Thermal

Physical and chemical studies also involve the use of methods based on the heat of formation or decay of substances. Such methods also have their own classification.

Thermal analysis.
Thermogravimetry.
Calorimetry.
Enthalpometry.
Dilatometry.

All these methods allow you to determine the amount of heat, mechanical properties, enthalpies of substances. Based on these indicators, the composition of the compounds is quantified.

Methods of analytical chemistry

This section of chemistry has its own characteristics, because the main task facing analysts is the qualitative determination of the composition of a substance, their identification and quantitative accounting. In this regard, analytical methods of analysis are divided into:

chemical;
biological;
physical and chemical.

Since we are interested in the latter, we will consider which of them are used to determine substances.

The main varieties of physicochemical methods in analytical chemistry

Spectroscopic - all the same as those discussed above.
Mass spectral - based on the action of an electric and magnetic field on free radicals, particles or ions. The physicochemical analysis laboratory assistant provides the combined effect of the indicated force fields, and the particles are separated into separate ionic flows according to the ratio of charge and mass.
radioactive methods.
Electrochemical.
Biochemical.
Thermal.

What do such processing methods allow us to learn about substances and molecules? First, the isotopic composition. And also: reaction products, the content of certain particles in especially pure substances, the masses of the desired compounds and other things useful for scientists.

Thus, the methods of analytical chemistry are important ways of obtaining information about ions, particles, compounds, substances and their analysis.

Non-aqueous solvents have become widely used in modern pharmaceutical analysis. If earlier the main solvent in the analysis was water, now various non-aqueous solvents are also used simultaneously (glacial or anhydrous acetic acid, acetic anhydride, dimethylformamide, dioxane, etc.), which allow changing the strength of basicity and acidity of the analyzed substances. A micromethod has been developed, in particular, the drop method of analysis, which is convenient for use in intra-pharmacy quality control of medicines.

In recent years, such research methods have been widely developed, in which a combination of various methods is used in the analysis of medicinal substances. For example, chromatography-mass spectrometry is a combination of chromatography and mass spectrometry. Physics, quantum chemistry, and mathematics are increasingly penetrating modern pharmaceutical analysis.

The analysis of any medicinal substance or raw material must be started with an external examination, while paying attention to the color, smell, crystal shape, container, packaging, glass color. After an external examination of the object of analysis, an average sample is taken for analysis in accordance with the requirements of the Global Fund X (p. 853).

Methods for the study of medicinal substances are divided into physical, chemical, physico-chemical, biological.

Physical methods of analysis involve the study of the physical properties of a substance without resorting to chemical reactions. These include: determination of solubility, transparency

or the degree of turbidity, color; determination of density (for liquid substances), humidity, melting point, solidification, boiling point. Appropriate techniques are described in SP X .(p. 756-776).

Chemical research methods are based on chemical reactions. These include: determination of ash content, reaction of the environment (pH), characteristic numerical indicators of oils and fats (acid number, iodine number, saponification number, etc.).

For the purposes of identifying medicinal substances, only such reactions are used that are accompanied by a visual external effect, for example, a change in the color of the solution, evolution of gases, precipitation or dissolution of precipitates, etc.

Chemical research methods also include weight and volume methods of quantitative analysis adopted in analytical chemistry (neutralization, precipitation, redox methods, etc.). In recent years, pharmaceutical analysis has included such chemical research methods as titration in non-aqueous media, complexometry.

Qualitative and quantitative analysis of organic medicinal substances, as a rule, is carried out according to the nature of the functional groups in their molecules.

With the help of physico-chemical methods, physical phenomena that occur as a result of chemical reactions are studied. For example, in the colorimetric method, the color intensity is measured depending on the concentration of the substance, in conductometric analysis, the electrical conductivity of solutions is measured, etc.

Physicochemical methods include: optical (refractometry, polarimetry, emission and fluorescent methods of analysis, photometry, including photocolorimetry and spectrophotometry, nephelometry, turbodimetry), electrochemical (potentiometric and polarographic methods), chromatographic methods.

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