• Domestic science and medicine in the 19th - early 20th centuries Chemistry of the 19th century in medicine
• What are peptides? Types and types of peptides. All about peptides and anti-aging cosmetics with peptides Additional peptides are not formed
• Toxic effect on humans of hazardous chemicals
• Radioautography Method of autoradiography in cytology
• Identification of bacteria by antigenic structure
• Organic matter in wastewater What are organic compounds in water
• Hydrogen index (pH factor)
• What is the pH of water and why is it important to know it?
• Redox potential Redox systems
• Reversible redox system Reversible redox systems

9.1. Objective

Determination of the dependence of thermoelectromotive force of a thermocouple on the temperature difference of the junctions.

In a closed circuit (Fig. 9.1), consisting of dissimilar conductors (or semiconductors) A and B, an electromotive force (emf) E T arises and a current flows if contacts 1 and 2 of these conductors are maintained at different temperatures T 1 and T 2 . This emf is called thermoelectromotive force (thermo-emf), and an electric circuit of two dissimilar conductors is called a thermocouple. When the sign of the temperature difference between the junctions changes, the direction of the thermocouple current changes. it
the phenomenon is called the Seebeck phenomenon.

There are three known reasons for the occurrence of thermo-EMF: the formation of a directed flow of charge carriers in a conductor in the presence of a temperature gradient, the dragging of electrons by phonons, and a change in the position of the Fermi level depending on temperature. Let's consider these reasons in more detail.

In the presence of a temperature gradient dT / dl along the conductor, the electrons at its hot end have a greater kinetic energy, and hence a greater speed of chaotic motion compared to the electrons of the cold end. As a result, there is a predominant flow of electrons from the hot end of the conductor to the cold one, a negative charge accumulates at the cold end, and an uncompensated positive charge remains at the hot end.

The accumulation continues until the resulting potential difference causes an equal flow of electrons. The algebraic sum of such potential differences in the circuit creates the volumetric component of the thermo-emf.

In addition, the existing temperature gradient in the conductor leads to the predominant movement (drift) of phonons (quanta of the vibrational energy of the crystal lattice of the conductor) from the hot end to the cold one. The existence of such a drift leads to the fact that the electrons scattered by phonons themselves begin to make a directed movement from the hot end to the cold one. The accumulation of electrons at the cold end of the conductor and the depletion of electrons from the hot end leads to the appearance of a phonon component of the thermo-emf. Moreover, at low temperatures, the contribution of this component is the main one in the occurrence of thermal emf.

As a result of both processes, an electric field arises inside the conductor, directed towards the temperature gradient. The intensity of this field can be represented as

E = -dφ / dl = (-dφ / dT) (-dt / dl)=-β (-dT / dl)

where β = dφ / dT.

Relation (9.1) relates the electric field strength E to the temperature gradient dT / dl. The resulting field and the temperature gradient have opposite directions, so they have different signs.

The field determined by expression (9.1) is the field of external forces. Integrating the strength of this field over the section of the circuit AB (Fig. 9.1) from junction 2 to junction 1 and assuming that T 2 > T 1, we obtain an expression for the thermo-emf acting on this section:

(The sign changed when the integration limits changed.) Similarly, we determine the thermo-emf acting in section B from junction 1 to junction 2.

The third reason for the occurrence of thermo-emf. is temperature dependent position of the Fermi level, which corresponds to the highest energy level occupied by electrons. The Fermi level corresponds to the Fermi energy E F that electrons can have at this level.

The Fermi energy is the maximum energy that conduction electrons can have in a metal at 0 K. The Fermi level will be the higher, the greater the density of the electron gas. For example (Figure 9.2), E FA is the Fermi energy for metal A, and E FB is for metal B. The values ​​of E PA and E PB are the highest potential energy of electrons in metals A and B, respectively. When two dissimilar metals A and B come into contact, the presence of a Fermi level difference (E FA > E FB) leads to the transition of electrons from metal A (with a higher level) to metal B (with a low Fermi level).

In this case, metal A is charged positively, and metal B is negatively charged. The appearance of these charges causes a shift in the energy levels of metals, including the Fermi levels. As soon as the Fermi levels are equalized, the cause that causes the preferential transition of electrons from metal A to metal B disappears, and a dynamic equilibrium is established between the metals. From fig. 9.2 it can be seen that the potential energy of an electron in metal A is less than in B by the value E FA - E FB . Accordingly, the potential inside metal A is higher than inside B by the value)

U AB = (E FA - E FB) / l

This expression gives the internal contact potential difference. The potential decreases by this amount during the transition from metal A to metal B. If both thermocouple junctions (see Fig. 9.1) are at the same temperature, then the contact potential differences are equal and directed in opposite directions.

In this case, they cancel each other out. It is known that the Fermi level, although weakly, depends on temperature. Therefore, if the temperature of junctions 1 and 2 is different, then the difference U AB (T 1) - U AB (T 2) on the contacts gives its contact contribution to the thermal emf. It can be compared with volumetric thermo-emf. and is equal to:

E cont \u003d U AB (T 1) - U AB (T 2) \u003d (1 / l) ( + )

The last expression can be represented as follows:

The resulting thermo-emf. (ε T) is composed of emf acting in contacts 1 and 2 and emf acting in sections A and B.

E T = E 2A1 + E 1B2 + E cont.

Substituting expressions (9.3) and (9.6) into (9.7) and performing transformations, we obtain

where α = β - ((1/l) (dE F / dT))

The value of α is called the thermo-emf coefficient. Since both β and dE F / d T depend on temperature, the coefficient α is also a function of T.

Taking into account (9.9), the expression for thermo-EMF can be represented as:

The quantity α AB is called differential or at separate thermo-EMF this pair of metals. It is measured in W/K and essentially depends on the nature of the contacting materials, as well as the temperature range, reaching about 10 -5 ÷10 -4 V/K. In a small temperature range (0-100°C) specific thermo-emf. weakly dependent on temperature. Then formula (9.11) can be represented with a sufficient degree of accuracy in the form:

E T \u003d α (T 2 - T 1)

In semiconductors, unlike metals, there is a strong dependence of the concentration of charge carriers and their mobility on temperature. Therefore, the effects discussed above, leading to the formation of a thermal emf, are more pronounced in semiconductors; much more and reaches values ​​of the order of 10 -3 V/K.

9.3. Description of the laboratory setup

To study the dependence of thermo-emf. on the temperature difference of the junctions (contacts), in this work we use a thermocouple made of two pieces of wire, one of which is a chromium-based alloy (chromel), and the other is an aluminum-based alloy (alumel). One junction, together with a thermometer, is placed in a vessel with water, the temperature T 2 of which can be changed by heating on an electric stove. The temperature of the other junction T 1 is maintained constant (Fig. 9.3). The resulting thermal emf. measured with a digital voltmeter.

9.4. Experimental technique and processing of results
9.4.1. Experimental technique

We use direct measurements of the emf arising in the thermocouple. The temperature of the junctions is determined by the temperature of the water in the vessels using a thermometer (see Fig. 9.3)

9.4.2. Work order

1. Connect the power cord of the voltmeter to the mains.
2. Press the mains button on the front panel of the digital voltmeter. Let the device warm up for 20 minutes.
3. Loosen the clamp screw on the thermocouple stand, lift it up and secure. Pour cold water into both glasses. Release the thermocouple junctions into the beakers to approximately half the depth of the water.
4. Write in the table. 9.1 the value of the initial temperature T 1 of the junctions (water) according to the thermometer (for the other junction it remains constant throughout the experiment).
5. Turn on the electric stove.
6. Record emf values. and temperature T 2 in the table. 9.1 every ten degrees.
7. When water boils, turn off the electric stove and voltmeter.

9.4.3. Processing of measurement results

1. Based on the measurement data, plot the dependence of the emf. thermocouples 8T (ordinate axis) on the temperature difference of the junctions ΔT \u003d T 2 - T 1 (abscissa axis).
2. Using the resulting graph of the linear dependence of E T on ∆T, determine the specific thermal emf. according to the formula: α = ∆E T / ∆(∆T)

9.5. Checklist

1. What is the essence and what is the nature of the Seebeck phenomenon?
2. What causes the occurrence of the volumetric component of the thermo-emf?
3. What causes the appearance of the phonon component of the thermo-emf?
4. What causes a contact potential difference?
5. What devices are called thermocouples and where are they used?
6. What is the essence and what is the nature of the Peltier and Thomson phenomena?

1. Savelyev I.V. Course of general physics. T.3. - M.: Nauka, 1982. -304 p.
2. Epifanov G.I. Solid state physics. M.: Higher school, 1977. - 288 p.
3. Sivukhin DV General course of physics. Electricity. T.3. - M.: Nauka, 1983. -688 p.
4. Trofimova T. I. Physics course. M. : Higher school, 1985. - 432 p.
5. Detlaf A. A., Yavorsky V. M. Physics course. M. : Higher school, 1989. - 608 p.

ziruemogo solution per unit. In production measurements, hydrogen electrodes are not used, since they are inconvenient to use.

8.1.1. measuring cell pH meter

AT Due to the fact that the electrode potential cannot be directly measured, a galvanic cell is used in the potentiometric method, in which one electrode is a measuring one, and the other is a reference electrode (or auxiliary), the potential of which does not depend on the concentration of the investigated ions of the solution. The measuring electrode is placed in the analyzed

liquid medium, a potential jump EX is created on it, determined by the concentration of ions in this medium. The potential of the reference electrode must always remain constant regardless of changes in the composition of the medium.

AT glass electrodes are used as measuring electrodes, the indicator part of which is made of special types of glass with a hydrogen function. As a reference or auxiliary electrode, calomel or silver chloride electrodes are usually used. They belong to the electrodes of the so-called second kind, which consist of a metal, its sparingly soluble salt, and a sparingly soluble salt with the same anion as the sparingly soluble salt.

The general view of the cell with a glass measuring electrode is shown in fig. 1, where 1 is a glass indicator electrode, 2 is a calomel reference electrode.

The EMF of the electrode sensor of the pH meter consists of a number of potentials:

E cell \u003d E k + E vn + E x + E cf + E d,

where E k is the potential difference between the contact auxiliary electrode and the solution filling the glass electrode; E ext - potential difference between the solution and the inner surface of the measuring membrane; E x is the potential difference between the outer surface of the glass membrane and the controlled medium (pH function); E cf is the potential difference at the interface mercury (Hg) - calomel (Hg 2 Cl 2); E d - diffusion potential at the contact boundary of two media - KCl and controlled environment. Chloe

Potassium ride KCl plays the role of an electrolytic key that connects the analyzed solution to the electrode.

Rice. 1. The electrical circuit of the measuring cell of the pH meter

In this case, the values ​​of E k , E vn , E v are constant and do not depend on the composition of the analyzed medium. The diffusion potential E d is very small and can be neglected. Thus, the total EMF is determined by the activity of hydrogen ions: E cell \u003d E x + E.

Thus, E cell \u003d f (pH), that is, E cell is a linear function of pH, which is used in the electrical measurement of pH.

The dependence of the EMF of the electrode cell E cell on pH is determined by the electrode properties of the glass and is characterized by the slope coefficient S of the characteristics of the electrode system S= E/ pH. A change in the temperature of the analyzed solution affects the EMF of the electrode system, changing the slope of the nominal static characteristic (NSH) of the measuring electrode. If we express this dependence graphically (Fig. 2), then we get a bunch of intersecting lines. The coordinates of the point of intersection of the straight lines are called the coordinates of the isopotential point (Е Н , рН Н ) and are the most important characteristics of the electrode system, which are guided by the calculation of the temperature compensation circuit of the pH meter. Temperature compensation of changes in the EMF of the electrode system, as a rule, is carried out automatically (with the help of a TS included in the circuit of an industrial converter of a pH meter).

>> R ST.

Rice. 2. NSH of the measuring electrode

A measuring cell with a glass electrode can be represented as an equivalent circuit (Fig. 3). The resistance R cell is very high due to the high resistance of the glass electrode membrane R st (R cell 500 MΩ), Therefore, the flow of small currents through the internal resistance of the cell will cause a large measurement error:

UВХ \u003d EJCH - ICH RCH ; UВХ \u003d EYACH.

It can be seen from the last equality that the main measurement requirement U IN = E YCH can be met if R IN >> RCH , i.e.

R IN

Rice. 3. Equivalent circuit of the measuring cell

8.1.2. Industrial converters for pH meters GSP

The set of automatic industrial pH-meter consists of a submersible sensor (DPg-4M type) or a main sensor (DM-5M type), a measuring high-resistance transducer and a GSP secondary device for general industrial purposes. The task of the measuring device included in the pH meter kit is to measure the EMF of the electrode system, which, under constant temperature conditions, is a function of pH.

Accurate measurement of the EMF of the measuring cell of a pH meter, which is a low-power source, is associated with significant difficulties. Firstly, a current whose density exceeds 10–7 A/cm2 cannot be passed through the measuring cell, since the phenomenon of electrode polarization may occur, as a result of which the electrodes fail. The second significant difficulty lies in the fact that when directly measuring the EMF of a pH-meter cell with current consumption, for example, a millivoltmeter, an electrical circuit is created through which a current flows, which is determined by the sum of the internal resistance of the measuring electrode (about 500 ... 1000 MΩ) and the resistance of the measuring device. In this case, a number of conditions must be met: the measuring current must be less than the polarization current of the electrodes; the internal resistance of the device must be at least 100 times higher than the resistance of the glass electrode, which, however, conflicts with the requirement for high sensitivity of the device. In this regard, converters with direct measurement of EMF are practically not used.

The only method that satisfies all the requirements for measuring the EMF of a pH meter cell is the compensation (potentiometric) or zero method of measurement, the main advantage of which is the absence of current at the time of reading. However, it should not be assumed that with the compensation method the electrode is not loaded at all, and therefore the phenomenon of electrode polarization is excluded. Here, the current flow (within 10-12 A) is explained by the fact that during the measurement process there is always an unbalance, and at the time of measurement, compensation is achieved only with the accuracy with which the sensitivity of the null indicator allows.

Currently, only electronic null indicators (measuring transducers) with static compensation are used to measure the EMF of an electrode system with a glass electrode. A simplified block diagram explaining the principle of operation of such a converter is shown in fig. 4. The converter is a DC amplifier covered by a deep negative feedback of the feedback on the output current, which ensures a large input resistance. The amplifier is built according to the circuit for converting direct voltage into alternating voltage with subsequent demodulation.

Rice. Fig. 4. Structural diagram of the method for measuring the EMF of the pHmeter cell

The measured EMF E IA is compared with the voltage U OUT formed from the flow of the output current of the amplifier I OUT through the resistor R OS. The difference between these voltages is fed to the input of the amplifier U IN = E IJ -U OUT . If the gain k \u003d U OUT / U IN, then E IA \u003d U OUT / (1 + 1 / k). With a sufficiently large value of k (k 500) E IA U OUT I OUT R OS , i.e. the output current strength is practically proportional to the input signal from the pH-meter measuring cell.

The use of static compensation allows many times to reduce the current consumed from the measuring cell during the measurement process.

This principle is implemented in almost all industrial pH converters - meters: pH-201, P201, P202, P205 (semiconductor element base) and in P215 (using standard microcircuits).

8.1.3. Description of the converter P - 201

Industrial transducers type P201 are designed to measure the activity of hydrogen ions (pH value) of solutions and pulps in systems of automatic control and regulation of technological processes.

The transducers are designed to work with any commercially available pH sensitive elements, such as DPg-4M; DM-5M and others.

The converter has voltage and current outputs for connecting secondary devices with the corresponding input

	signals.
		Main technical characteristics:
		measurement limits	-1 to 14 pH
		limit of allowed basic reduced
	errors:
		a) DC output signals and
		DC voltage
		b) according to the indicating device
		measuring glass resistance
	electrode
		auxiliary electrode resistance
		settling time	no more than 10 s
		output current
		output voltage
			0 to 10 100mV

The converter is designed for installation in close proximity to industrial units. The transducer may consist of a narrow-profile indicating device and the transducer itself, installed on one common panel or separately, or only one transducer. The appearance of the device is shown in fig. 5.

The casing 1 is made of sheet steel, the cover 2 is cast, made of aluminum alloy. On the front side of the cover there is an inscription with the index of the device, a cap 3 and a screw plug 4.

Rice. 5. Appearance of the converter P201

A frame is installed inside the casing, which serves as the basis for installing all the blocks and elements of the device. On the front panel of the converter, located under the cover, the axes of variable resistors are displayed, designed to change the measurement limits of the converters. The block with clamps for external electrical connections is located in a closed compartment, access to it is provided from the rear wall of the housing. The wires are introduced into the compartment through four glands in the lower wall of the device (Fig. 6).

Rice. Fig. 6. Scheme of external electrical connections of the converter P-201: TRM - universal meter-regulator; TKR - block of temperature compensation resistors

8.1.4. Verification and calibration of an automatic pH meter

The current verification of an automatic pH meter consists in comparing its readings with those of a control device. With a significant discrepancy, the readings of the device under test are corrected using a compensator or by changing the calibration of the transducer using the tuning knobs. Except

In addition, a more detailed check of the sensor and transducer should be carried out periodically.

Checking the sensor includes the following operations:

1) careful external examination, especially of those parts that come into contact with the measured medium;

2) checking electrical circuits, in particular the insulation resistance of the glass and reference electrode circuits from

relative to the case, which must be at least 1012 ohms and 2108 ohms, respectively;

3) checking the characteristics of the electrode system by buffer solutions with a known pH value using a control laboratory pH meter.

Converter verification includes:

1) determination of the main measurement error of the transducer and correction of its calibration;

2) determination of additional measurement errors of the transducer from a change in the resistance of the glass electrode R ST , changes in the resistance of the reference electrode RSR

and change in the potential of the controlled solution E X .

To calibrate the scale of pH meters, it is necessary to have an I-01 or I-02 electrode system simulator.

The electrode system simulator allows you to check the performance of the pH-meter sensor; the influence of changes in the resistance of the electrodes and the voltage between the solution and the body of the unit on the readings of the device; noise immunity of pH meters.

Using the simulator, you can reproduce the following parameters of the electrode system:

a) voltage equivalent to the EMF of the electrode system, in the range from 0 to 1000 mV;

b) resistance equivalent to the glass electrode resistance: 0; 500 and 1000 MΩ;

c) resistance equivalent to the resistance of the auxiliary electrode: 10 and 20 kOhm;

d) voltage equivalent to the EMF "ground - solution": 0 and

The simulator is the electrical equivalent of the electrode system (Fig. 7) and is designed as a portable device housed in a steel case with a removable cover.

E W Rv

Rice. Fig. 7. Equivalent circuit of the electrode system simulator: R I – resistance of the measuring glass electrode; R B is the resistance of the auxiliary electrode; E - total EMF of the electrode system: E G - EMF "ground - solution".

On the front panel of the simulator there are terminals for connecting it to the verified pH meter using the cable that is included in the kit. The knobs for setting the required output voltage, electrode resistance, controlled solution potential, etc. are also located there.

8.2. EQUIPMENT AND INSTRUMENTS

1. industrial converter P-201.

2. Electrode system simulator I-02.

3. Meter-regulator universal multi-channel TPM 138.

8.3. WORK SEQUENCE

1. Assemble the installation for verification of the converter P-201 using the I-02 simulator in accordance with the scheme of fig. 8 by connecting the output of the simulator to the input “Meas” and “Aux” of the transducer via a coaxial cable.

2. Prepare the simulator for work. To do this, press on the switches of the simulator: “R And ” – button 500; “EЗР”, “RВ” - buttons

“00” for EZP and “010” for RB ; “POWER” – button “INTERNAL” and “ON”.

3. Apply power to the stand.

Rice. 8. Verification scheme: 1 – I-02 electrode system simulator; 2 – electrode system; 3 - high-resistance converter P-201; 4 - multichannel meter-regulator TPM 138

4. Use the arrows ^ v on TPM 138 to select channel No. 5, through which the EMF is counted.

5. Check the converter.

For this:

5.1. Dial on the buttons of the switch “E, mV” of the simulator the EMF value corresponding to the pH value of the digitized scale mark. The switch “EX , mV” is set to the position “+” or “-“ depending on the sign of the EMF in the calibration table.

5.2. To make a reading of the indications on the I-02 simulator. Determine the basic measurement error at RВ = 10

kOhm; EZ =0. The main error is checked on all digitized scale marks during forward and reverse stroke and is calculated by the formula = [(E -E 0) / (E K -E H)] 100%, where E 0 is tabular (the actual value of the EMF of the electrode system corresponding to this digitized scale mark, mV, E – actual EMF value, mV, E K , E N – EMF values ​​corresponding to the final and initial scale marks.

6. Present the verification results in a report.

Ministry of Education and Science of the Russian Federation

Federal Agency for Education

Saratov State

Technical University

Electrode measurement

potentials and emf

Guidelines

on the course "Theoretical electrochemistry"

for students of the specialty

direction 550800

Local distribution electronic edition

Approved

editorial and publishing

Council of Saratov

state

technical university

Saratov - 2006

All rights to reproduction and distribution in any form remain with the developer.

Illegal copying and use of this product is prohibited.

Compiled by:

Edited by

Reviewer

Scientific and technical library of SSTU

Registration number 060375-E

© Saratov State

technical university, 2006

Introduction

One of the fundamental concepts of electrochemistry is the concepts of electrochemical potential and EMF of an electrochemical system. The values ​​of electrode potentials and EMF are associated with such important characteristics of electrolyte solutions as activity (a), activity coefficient (f), transfer numbers (n+, n-). By measuring the potential and EMF of the electrochemical system, one can calculate a, f, n+, n - electrolytes.

The purpose of the guidelines is to familiarize students with theoretical ideas about the causes of potential jumps between an electrode and a solution, with the classification of electrodes, mastering the theoretical foundations of the compensation method for measuring electrode potentials and EMF, using this method to calculate activity coefficients and ion transport numbers in electrolyte solutions.

Basic concepts

When a metal electrode is immersed in a solution, a double electric layer appears at the interface and, consequently, a potential jump appears.

The emergence of a potential jump is caused by various reasons. One of them is the exchange of charged particles between the metal and the solution. When a metal is immersed in an electrolyte solution, metal ions, leaving the crystal lattice and passing into the solution, bring their positive charges into it, while the metal surface, on which an excess of electrons remains, becomes negatively charged.

Another reason for the appearance of potentials is the selective adsorption of anions from an aqueous salt solution on the surface of some inert metal. Adsorption leads to the appearance of an excess negative charge on the metal surface and, further, to the appearance of an excess positive charge in the nearest solution layer.

The third possible reason is the ability of polar uncharged particles to be oriented adsorbed near the phase boundary. In oriented adsorption, one of the ends of the dipole of a polar molecule faces the interface, and the other, towards the phase to which the given molecule belongs.

It is impossible to measure the absolute value of the potential jump at the electrode-solution interface. But it is possible to measure the EMF of an element composed of the electrode under study and the electrode, the potential of which is conditionally taken as zero. The value obtained in this way is called the "intrinsic" potential of the metal - E.

A standard hydrogen electrode serves as an electrode, the equilibrium potential of which is conventionally taken as zero.

An equilibrium potential is a potential characterized by an established equilibrium between a metal and a salt solution. The establishment of an equilibrium state does not mean that no processes occur at all in the electrochemical system. The exchange of ions between the solid and liquid phases continues, but the rates of such transitions become equal. The equilibrium at the metal-solution boundary corresponds to the condition

iTo= iBUT=iO , (1)

where iTo is the cathode current;

iO – exchange current.

To measure the potential of the electrode under study, other electrodes can be used, the potential of which is known relative to the hydrogen standard electrode - reference electrodes.

The main requirements for reference electrodes are the constancy of the potential jump and good reproducibility of the results. Examples of reference electrodes are electrodes of the second kind: calomel:

Cl- / hg2 Cl2 , hg

Silver chloride electrode:

Cl- / AgCl, Ag

mercury sulfate electrode and others. The table shows the potentials of the reference electrodes (according to the hydrogen scale).

The potential of any electrode - E, is determined at a given temperature and pressure by the value of the standard potential and the activities of the substances involved in the electrode reaction.

If the reaction proceeds reversibly in an electrochemical system

υAA+υBB+…+.-zF→υLL+υMM

then https://pandia.ru/text/77/491/images/image003_83.gif" width="29" height="41 src=">ln and Cu2+ (5)

Electrodes of the second kind are metal electrodes coated with a sparingly soluble salt of this metal and immersed in a solution of a highly soluble salt that has a common anion with a sparingly soluble salt: an example is silver chloride, calomel electrodes, etc.

The potential of an electrode of the second kind, for example, a silver chloride electrode, is described by the equation

EAg, AgCl/Cl-=E0Ag, AgCl/Cl-ln aCl - (6)

A redox electrode is an electrode made of an inert material and immersed in a solution containing some substance in oxidized and reduced forms.

There are simple and complex redox electrodes.

In simple redox electrodes, a change in the valency of the particle charge is observed, but the chemical composition remains constant.

Fe3++e→Fe2+

MnO-4+e→MnO42-

If we denote the oxidized ions as Ox, and the reduced ions as Red, then all the above reactions can be expressed by one general equation

Ox+ e→Red

A simple redox electrode is written as a diagram Red, Ox/ Pt, and its potential is given by the equation

E Red, Ox=E0 Red, Ox+https://pandia.ru/text/77/491/images/image005_58.gif" width="29" height="41 src=">ln (8)

The potential difference between the two electrodes when the external circuit is turned off is called the electromotive force (EMF) (E) of the electrochemical system.

E= E+ - E- (9)

An electrochemical system consisting of two identical electrodes immersed in a solution of the same electrolyte of different concentrations is called a concentration cell.

EMF in such an element arises due to the difference in the concentrations of electrolyte solutions.

Experimental technique

Compensation method for measuring EMF and potential

Instruments and accessories: R-37/1 potentiometer, galvanometer, battery, Weston cells, carbon, copper, zinc electrodes, electrolyte solutions, silver chloride reference electrode, electrolytic key, electrochemical cell.

Assemble the installation diagram (Fig. 2)

e. I. – electrochemical cell;

e. and. – investigated electrode;

e. With. – reference electrode;

e. k. - electrolytic key.

DIV_ADBLOCK84">

concentrations of CrO42- and H+ ions are constant and equal to 0.2 g-ion/l and 3-ion/l concentration of H+ varies and is: 3; 2; one; 0.5; 0.1 g-ion/l;

the concentration of CrO42-, Cr3+ ions are constant and equal to 2 g-ion/l and 0.1 g-ion/l, respectively, the concentration of H+ ions varies and is: 2; one; 0.5; 0.1; 0.05; 0.01 g-ion/l.

Task 4

Measurement of the potential of a simple redox system Mn+7, Mn2+ graphite.

the concentration of the Mn2+ ion is constant and equal to 0.5 g-ion/l

the concentration of MnO2-4 ions changes and is 1; 0.5; 0.25; 0.1; 0.01 g-ion/l;

the concentration of MnO-4 ions is constant and equal to 1 g-ion/l

the concentration of Mn2+ ions v changes and is: 0.5; 0.25; 0.1; 0.05; 0.001 g-ion/l.

Processing of experimental data

1. All obtained experimental data must be converted to the hydrogen scale.

3. Construct a graphical dependence of the potential on the concentration in the coordinates E, lgC, draw a conclusion about the nature of the influence of the concentration of potential-determining ions on the value of the electrode potential.

4. For concentration elements (task 2), calculate the diffusion potential jump φα using the equation

φα = (10)

when measuring EMF by the compensation method

1. The potentiometer must be grounded before operation.

2. When working with batteries, you must:

Use to check the voltage at the terminals with a portable voltmeter;

When assembling batteries into a battery, avoid shorting the case and terminals in order to avoid severe burns.

3. After work, turn off all devices.

Literature

1. Antropov electrochemistry:

textbook / .- 2nd ed. revised add.-M.: Higher school, 1984.-519s.

2.-Rotinyan electrochemistry: textbook / ,

L.: Chemistry, p.

3. Damask /, .- M .: Higher School, 1987.-296s.

What EMF(electromotive force) in physics? Electric current is not understood by everyone. Like space distance, only under the very nose. In general, it is not fully understood by scientists either. Enough to remember Nikola Tesla with his famous experiments, centuries ahead of their time and even today remaining in a halo of mystery. Today we are not solving big mysteries, but we are trying to figure out what is emf in physics.

Definition of EMF in physics

EMF is the electromotive force. Denoted by letter E or the small Greek letter epsilon.

Electromotive force- scalar physical quantity characterizing the work of external forces ( forces of non-electric origin) operating in electrical circuits of alternating and direct current.

EMF, like tension e, measured in volts. However, EMF and voltage are different phenomena.

Voltage(between points A and B) - a physical quantity equal to the work of the effective electric field performed when transferring a unit test charge from one point to another.

We explain the essence of EMF "on the fingers"

To understand what is what, we can give an analogy example. Imagine that we have a water tower completely filled with water. Compare this tower with a battery.

Water exerts maximum pressure on the bottom of the tower when the tower is full. Accordingly, the less water in the tower, the weaker the pressure and pressure of the water flowing from the tap. If you open the tap, the water will gradually flow out at first under strong pressure, and then more and more slowly until the pressure weakens completely. Here stress is the pressure that the water exerts on the bottom. For the level of zero voltage, we will take the very bottom of the tower.

It's the same with the battery. First, we include our current source (battery) in the circuit, closing it. Let it be a clock or a flashlight. While the voltage level is sufficient and the battery is not discharged, the flashlight shines brightly, then gradually goes out until it goes out completely.

But how to make sure that the pressure does not run out? In other words, how to maintain a constant water level in the tower, and a constant potential difference at the poles of the current source. Following the example of the tower, the EMF is presented as a pump, which ensures the influx of new water into the tower.

The nature of the emf

The reason for the occurrence of EMF in different current sources is different. According to the nature of occurrence, the following types are distinguished:

• Chemical emf. Occurs in batteries and accumulators due to chemical reactions.
• Thermo EMF. Occurs when contacts of dissimilar conductors at different temperatures are connected.
• EMF of induction. Occurs in a generator when a rotating conductor is placed in a magnetic field. EMF will be induced in a conductor when the conductor crosses the lines of force of a constant magnetic field or when the magnetic field changes in magnitude.
• Photoelectric EMF. The occurrence of this EMF is facilitated by the phenomenon of an external or internal photoelectric effect.
• Piezoelectric emf. EMF occurs when a substance is stretched or compressed.

Dear friends, today we have considered the topic "EMF for Dummies". As you can see, the EMF force of non-electric origin, which maintains the flow of electric current in the circuit. If you want to know how problems with EMF are solved, we advise you to contact our authors– scrupulously selected and proven specialists who will quickly and clearly explain the course of solving any thematic problem. And by tradition, at the end we invite you to watch the training video. Happy viewing and good luck with your studies!

Instruments for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM Sensor Lab are designed to measure thermo-EMF coming from primary thermoelectric converters that measure the temperature of liquid metals (cast iron, steel, copper and others) and EMF generated by oxygen activity sensors.

Description

Operating principle

The thermo-EMF signals from the primary thermoelectric converter (thermocouple) and the EMF from the oxygen activity sensors (mV) supplied to the "measuring" input of the device for measuring the temperature of liquid metals and the EMF of the oxygen activity sensors iM2 Sensor Lab are converted into digital form and, according to the corresponding program, are converted into temperature and oxygen activity values. These signals are received in cycles up to 250 s-1. The device has 4 inputs: Ch0 and Ch2 - for measuring signals from thermocouples, and Ch1, Ch3 - for measuring EMF signals from oxygen activity sensors.

In the process of temperature measurements, an analysis is made of the change in the incoming input signal in order to determine its output to stable readings (characterized by the parameters of the so-called "temperature area", determined by the length (time) and height (temperature change). If during the time specified by the length of the area, the actual the change in temperature does not exceed its specified height (i.e., the allowable temperature change), then the area is considered selected.Next, the iM Sensor Lab device for measuring the temperature of liquid metals and the EMF of oxygen activity sensors averages the temperature clock values ​​measured over the length of the selected area, and displays average value as a result of measurements on the screen.

In a similar way, areas are allocated corresponding to the EMF output to stable readings, the dimensions of which are also set by the length (time) and height (permissible change in the EMF value).

In addition to measuring the temperature of the bath, the device allows you to determine the liquidus temperature of liquid steel, which can be recalculated according to the empirical equation in the carbon content. According to the results of measurements of the EMF generated by oxygen activity sensors, the oxygen activity in liquid steel, cast iron and copper, the carbon content in steel, the sulfur and silicon content in cast iron, the activity of FeO (FeO + MnO) in liquid metallurgical slags and some other parameters are determined by calculation associated with the thermal state and chemical composition of liquid metals. The device also has the ability to determine the level of the bath (the position of the slag-metal boundary) by analyzing the rate of temperature changes when the thermocouple is immersed in the bath and determining the thickness of the slag layer with special probes.

Instruments for measuring the temperature of liquid metals and the EMF of the iM2 Sensor Lab oxygen activity sensors have two modifications, which differ in the presence or absence of a touch LCD screen (Figure 1). In the absence of a screen, the device is controlled from an external computer or from an industrial tablet. In this case, special software is supplied for communication between them.

The touch screen is located on the front panel of the instrument housing and displays the progress of measurements, its results and other information related to measurements in digital and graphical forms. The screen also displays a menu in the form of text tabs, which is used to control the device, its diagnostics and view data on the execution.

Sheet No. 2 Total sheets 4

previous measurements. In the "without screen" modification, all of the above information is displayed on the screen of a computer or industrial tablet.

The electronic boards of the device for measuring the temperature of liquid metals and the EMF of the iM2 Sensor Lab oxygen activity sensors are installed in a dust-proof steel case made according to the 19” standard for mounting on a mounting rack or mounting in a shield.

Signals from primary transducers can be transmitted to the device in two ways - by cable and by radio. In the latter case, the device is connected to the receiving unit (Receiver Box) via a serial interface, and a transmitter (QUBE) is installed on the handle of the submersible rods, which converts the signals from the sensors into radio signals transmitted to the receiving unit. The latter receives them and transfers them to the device for processing.

The device is not sealed.

Software

Installation of software (SW) is carried out at the manufacturer. Access to the metrologically significant part of the software is not possible.

The design of the MI excludes the possibility of unauthorized influence on the software of the measuring instrument and measurement information.

Firmware protection level against unintentional and intentional modifications

High according to R 50.2.077-2014.

Specifications

The metrological and technical characteristics of devices for measuring the temperature of liquid metals and the EMF of the iM2 Sensor Lab oxygen activity sensors are given in Table 1. Table 1

* - without taking into account the error of the primary converter, extension cable and EMF sensor.

Type approval mark

The type approval mark is typographically applied to the title page of the operational documentation by typographical method and on the front panel of the instrument by offset printing method.

Completeness

The completeness of the measuring instrument is given in Table 2. Table 2

Verification

is carried out according to MP RT 2173-2014 “Instruments for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM2 Sensor Lab. Verification Methodology”, approved by the GCI SI FBU “Rostest-Moscow” on October 26, 2014.

The main means of verification are given in Table 3. Table 3

Information about measurement methods

Information about measurement methods is contained in the instruction manual.

Regulatory and technical documents that establish requirements for instruments for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM2 Sensor Lab

1 Manufacturer's technical documentation Heraeus Electro-Nite GmbH & Co. kg.

2 GOST R 52931-2008 “Instruments for monitoring and regulating technological processes. General technical conditions".

3 GOST R 8.585-2001 “GSP. Thermocouples. Nominal static conversion characteristics.

4 GOST 8.558-2009 “GSP. State Verification Scheme for Temperature Measuring Instruments.

when performing work on assessing the conformity of products and other objects with mandatory requirements in accordance with the legislation of the Russian Federation on technical regulation.

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