Qualitative reactions for iron (III). Analytical reactions of iron cations Fe(III) Ammonium thiocyanate and iron chloride 3
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| Iron(III) thiocyanate | |
| General | |
|---|---|
| Systematic Name |
Iron(III) thiocyanate |
| Traditional names | iron thiocyanate; thiocyanate iron |
| Chem. formula | Fe(SCN) 3 |
| Physical properties | |
| State | red crystals with a greenish tint |
| Molar mass | 230.09 g/mol |
| Data is based on standard conditions (25 °C, 100 kPa) unless otherwise noted. | |
Iron(III) thiocyanate- an inorganic compound, a metal salt of iron and thiocyanate acid with the formula Fe(SCN) 3 , dissolves in water, forms a crystalline hydrate - red crystals.
Receipt
- exchange reactions:
- Neutralization of a solution of thiocyanic acid with freshly precipitated iron(III) hydroxide:
Physical properties
Iron(III) thiocyanate forms Fe (SCN) 3 3H 2 O crystalline hydrate - paramagnetic red hygroscopic crystals, soluble in water, ethanol, ether, hardly soluble in carbon disulfide, benzene, chloroform, toluene.
Aqueous solutions contain Fe 6H 2 O dimers.
Chemical properties
- With thiocyanates of other metals forms hexathiocyanatoferrate(III) coordination compounds, for example, Li 3 n H 2 O, Na 3 12H 2 O, K 3 4H 2 O, Cs 3 2H 2 O, (NH 4) 3 4H 2 O.
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Literature
- Chemical Encyclopedia / Ed.: Knunyants I.L. and others. - M .: Soviet Encyclopedia, 1990. - T. 2. - 671 p. - ISBN 5-82270-035-5.
- Ripan R., Chetyanu I. Inorganic chemistry. Chemistry of metals. - M .: Mir, 1972. - T. 2. - 871 p.
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An excerpt characterizing iron(III) thiocyanate
- To the next life? - repeated Prince Andrei, but Pierre did not give him time to answer and took this repetition for a denial, especially since he knew the former atheistic convictions of Prince Andrei.– You say that you cannot see the realm of goodness and truth on earth. And I did not see him, and you cannot see him if you look at our life as the end of everything. On earth, precisely on this earth (Pierre pointed to the field), there is no truth - everything is a lie and evil; but in the world, in the whole world, there is a kingdom of truth, and we are now the children of the earth, and forever the children of the whole world. Do I not feel in my soul that I am part of this vast, harmonious whole. Do I not feel that I am in this vast, innumerable number of beings in which the Divine is manifested - the highest power, as you like - that I am one link, one step from lower beings to higher ones. If I see, I clearly see this ladder that leads from the plant to man, then why should I suppose that this ladder is interrupted with me, and does not lead further and further. I feel that not only can I not disappear, just as nothing in the world disappears, but that I will always be and have always been. I feel that besides me, spirits live above me and that there is truth in this world.
“Yes, this is the teaching of Herder,” said Prince Andrei, “but not that, my soul, will convince me, but life and death, that’s what convinces. It is convincing that you see a creature dear to you, who is connected with you, before whom you were guilty and hoped to justify yourself (Prince Andrei trembled in his voice and turned away) and suddenly this creature suffers, suffers and ceases to be ... Why? It cannot be that there is no answer! And I believe he is... That's what convinces, that's what convinced me, - said Prince Andrei.
“Well, yes, yes,” said Pierre, “isn’t that what I say too!”
- No. I only say that it is not arguments that convince you of the need for a future life, but when you walk in life hand in hand with a person, and suddenly this person disappears into nowhere, and you yourself stop in front of this abyss and look into it. And I looked...
- Well, so what! Do you know what is there and what is someone? There is a future life. Someone is God.
Prince Andrew did not answer. The carriage and horses had long since been brought to the other side and had already been laid down, and the sun had already disappeared to half, and the evening frost covered the puddles near the ferry with stars, and Pierre and Andrei, to the surprise of the lackeys, coachmen and carriers, were still standing on the ferry and talking.
- If there is a God and there is a future life, then there is truth, there is virtue; and the highest happiness of man is to strive to achieve them. We must live, we must love, we must believe, - said Pierre, - that we do not live now only on this piece of land, but we have lived and will live forever there in everything (he pointed to the sky). Prince Andrei stood leaning on the railing of the ferry and, listening to Pierre, without taking his eyes off, looked at the red reflection of the sun over the blue flood. Pierre is silent. It was completely quiet. The ferry had landed long ago, and only the waves of the current with a faint sound hit the bottom of the ferry. It seemed to Prince Andrei that this rinsing of the waves was saying to Pierre's words: "True, believe this."
Prince Andrei sighed, and with a radiant, childish, tender look looked into Pierre's flushed, enthusiastic, but still timid in front of his superior friend.
“Yes, if that were the case!” - he said. “However, let’s go sit down,” Prince Andrei added, and leaving the ferry, he looked at the sky, which Pierre pointed out to him, and for the first time, after Austerlitz, he saw that high, eternal sky, which he saw lying on the Austerlitz field, and something long asleep, something the best that was in him, suddenly awoke joyfully and youthfully in his soul. This feeling disappeared as soon as Prince Andrei entered the habitual conditions of life again, but he knew that this feeling, which he did not know how to develop, lived in him. A meeting with Pierre was for Prince Andrei an epoch from which, although in appearance it was the same, but in the inner world, his new life began.
It was already getting dark when Prince Andrei and Pierre drove up to the main entrance of the Lysogorsky house. While they were driving up, Prince Andrei with a smile drew Pierre's attention to the turmoil that had taken place at the back porch. A bent old woman with a knapsack on her back, and a short man in a black robe and with long hair, seeing a carriage driving in, rushed to run back through the gate. Two women ran after them, and all four, looking back at the carriage, ran frightened up the back porch.
“These are God’s Machines,” said Prince Andrei. They took us for their father. And this is the only thing in which she does not obey him: he orders to drive these wanderers, and she accepts them.
- What are God's people? Pierre asked.
Prince Andrei did not have time to answer him. The servants went out to meet him, and he asked where the old prince was and how soon they were waiting for him.
The old prince was still in the city, and they were waiting for him every minute.
Prince Andrei led Pierre to his quarters, which always awaited him in perfect order in his father's house, and he himself went to the nursery.
“Let's go to my sister,” said Prince Andrei, returning to Pierre; - I have not seen her yet, she is now hiding and sitting with her God people. Serve her right, she will be embarrassed, and you will see God's people. C "est curieux, ma parole. [This is curious, honestly.]
- Qu "est ce que c" est que [What is] God's people? Pierre asked.
- But you'll see.
Princess Mary was really embarrassed and blushed in spots when they entered her. In her cozy room with lamps in front of the icon cases, on the sofa, at the samovar, sat next to her a young boy with a long nose and long hair, and in a monastic cassock.
On an armchair, beside him, sat a wrinkled, thin old woman with a meek expression of a child's face.
- Andre, pourquoi ne pas m "avoir prevenu? [Andrey, why didn’t they warn me?] - she said with meek reproach, standing in front of her wanderers, like a hen in front of chickens.
The method is based on determining the wine-red color characteristic of complexes formed by ferric ions and rhodanide ions. These complexes are unstable, therefore, a large excess of rhodanide ions is required to suppress the dissociation of the complex. The process of interaction of ferric ions with rhodanide ions proceeds according to equation (1):
|
Fe 3+ + 6 NH 4 CNS \u003d 6NH 4 + + 3- |
It must be taken into account that, in addition to 3-, other, less intensely colored complexes can be formed, so the concentration of ammonium thiocyanate should be the same in the analyzed and standard solutions. Strong oxidizing agents (potassium permanganate, ammonium persulfate, hydrogen peroxide, etc.), which oxidize the rhodanide anion, as well as substances that reduce iron (III) to iron (II), interfere with the determination. The best medium is nitric acid, while the already low acidity of the solution is sufficient to prevent the hydrolysis of the iron salt (1-2 ml of concentrated nitric acid per 50 ml of solution).
Reagents
Ammonium thiocyanate (NH4CNS), 10% solution;
Nitric acid, concentrated;
Basic standard solution. To prepare the basic standard solution, 0.8634 g of iron ammonium alum is dissolved in a small volume of distilled water. If the solution turns out to be opaque, then add a few drops of concentrated nitric acid and bring the volume to 1 liter. The solution contains 0.1 mg of iron per 1 ml;
Working standard solution. The working standard solution is prepared by diluting the stock standard solution 10 times. The solution contains 0.01 mg of iron per 1 ml.
Working process
In 100 ml volumetric flasks add 1 and 5 ml of the working standard solution, as well as 1; 2.5 and 5 ml of the main standard solution of iron and bring the volume to the mark with distilled water, obtaining solutions with a concentration of 0.1; 0.5; 1.0; 2.5; and 5.0 µg/l, respectively. The prepared solutions and 100 ml of the test sample are poured into 150 ml conical flasks, 5 ml of concentrated HNO 3 and 10 ml of a 10% NH 4 CNS solution are added to each. The solutions are thoroughly mixed and after 3 minutes they are photometered at a wavelength of λ=450 nm, using cuvettes with an optical layer thickness of 5 mm, in relation to distilled water, to which the same reagents are added. The mass concentration of iron is found according to a calibrated graph. A calibration graph is built, plotting the mass concentration of iron in μg / dm 3 along the abscissa axis, and the corresponding optical density value along the ordinate axis.
Determination of chromium content using diphenylcarbazide
Method principle
The method is based on the interaction of chromates and bichromates in an acidic medium with diphenylcarbazide to form a red-violet colored compound, in which chromium is contained in the reduced form of Cr(III), and diphenylcarbazide is oxidized to diphenylcarbazone. The limit of detection is 0.02 mg/l. The range of measured amounts of chromium in the sample is from 1 µg to 50 µg.
When analyzing water in one sample, only Cr(vi) is determined, and in the other, the total content of Cr(iii) and Cr(vi), in which Cr(III) is oxidized to Cr(VI), is determined. Ammonium persulfate is used as an oxidizing agent. The oxidation process proceeds according to equation (2):
|
2Cr 3+ + 3S 2 O 8 2- + 7H 2 O Cr 2 O 7 2- + 6SO 4 2- + 14H + |
The difference in the results determine the content of Cr 3+ .
Reagents
Bi-distilled water (used for the preparation of all reagents);
Sulfuric acid, 1:1;
Phosphoric acid concentrated;
Diphenylcarbazide (C 13 H 14 ON 4), 0.5% solution in acetone (used freshly prepared);
Sodium hydroxide solution, 10% and 25%;
Potassium bichromate basic standard solution K 2 Cr 2 O 7 . The stock standard solution is prepared by dissolving 2.8285 g of the reagent, dried at 150°C, in bidistilled water and making up to 1 L (1 ml solution contains 1 mg Cr(VI);
Working standard solution 1. Prepare by diluting 5 ml of the stock standard solution with bidistilled water to 100 ml (1 ml of the resulting solution contains 50 µg of Cr(VI));
Working standard solution 2. Prepare by diluting 4 ml of working standard solution 1 to 100 ml with bidistilled water (1 ml of the resulting solution contains 2 µg of Cr(VI)).
Construction of a calibration graph
In volumetric flasks with a capacity of 100 ml take 0; 0.5; 1.0; 2.0; 3.0; 5.0; 8.0; 10.0 ml of working standard solution 2, bring the volume of solutions to 50-60 ml, adjust the pH to 8 with an alkali solution, controlling by universal indicator paper. Pour 1 ml of H 2 SO 4 (1:1) and 0.3 ml of H 3 PO 4, bring the volume to 100 ml. The resulting solutions have a concentration of Cr(VI) 0; 10; twenty; 40; 60; 100; 160; 200 µg/l. Add 2 ml of 0.5% diphenylcarbazide solution to each flask and mix well. The resulting solutions after 10-15 minutes. photometry at a wavelength of λ=540 nm, using cuvettes with an optical layer thickness of 30 mm, in relation to distilled water, to which the same reagents are added.
Definition of contentCr(VI)
In a volumetric flask with a volume of 100 ml, such a volume of the sample is placed that it contains from 0.005 to 0.1 mg of chromium, the pH is adjusted to 8 with an acid or alkali solution, controlled by universal indicator paper. Pour 1 ml of H 2 SO 4 (1:1) and 0.3 ml of H 3 RO 4 , bring the volume to 100 ml and mix. Add 2 ml of 0.5% diphenylcarbazide solution to each flask and mix again. The resulting solutions after 10-15 minutes. photometry as above.
Qualitative reactions for iron (III)
Ions of iron (III ) in solution can be determined using qualitative reactions. Let's go through some of them. Take for the experiment a solution of iron chloride ( III).
1. III) - reaction with alkali.
If the solution contains iron ions ( III ), iron hydroxide is formed ( III ) Fe(OH) 3 . The base is insoluble in water and brown in color. (iron hydroxide ( II ) Fe(OH) 2 . - also insoluble, but grey-green in color). A brown precipitate indicates the presence of iron ions in the initial solution ( III).
FeCl 3 + 3 NaOH = Fe(OH) 3 ↓+ 3 NaCl
2. Qualitative reaction to iron ion ( III ) - reaction with yellow blood salt.
The yellow blood salt is hexacyanoferrate potassiumK 4 [ Fe( CN) 6]. (For the determination of iron (II) use red blood saltK 3 [ Fe( CN) 6 ]). To a portion of a solution of iron chloride, add a solution of yellow blood salt. The blue precipitate of Prussian blue* indicates the presence of ferric ions in the initial solution.
3 To 4 +4 FeCl 3 = K Fe ) ↓ + 12 KCl
3. Qualitative reaction to iron ion ( III ) - reaction with potassium thiocyanate.
First, we dilute the test solution - otherwise we will not see the expected color. In the presence of an iron ion (III) when potassium thiocyanate is added, a red substance is formed. It is iron thiocyanateIII). Rhodanide from the Greek "rodeos" - red.
FeCl 3 + 3 KCNS= Fe( CNS) 3 + 3 KCl
Prussian blue was obtained by accident at the beginning of the 18th century in Berlin by the dyemaker Diesbach. Disbach bought an unusual potash (potassium carbonate) from a merchant: a solution of this potash turned blue when iron salts were added. When checking the potash, it turned out that it was calcined with bovine blood. The dye turned out to be suitable for fabrics: bright, stable and inexpensive. Soon the recipe for obtaining paint became known: potash was fused with dried animal blood and iron filings. By leaching such an alloy, yellow blood salt was obtained. Now Prussian blue is used to obtain printing ink and tint polymers. .
Equipment: flasks, pipette.
Safety . Observe the rules for handling alkali solutions and solutions hexacyanoferrates. Avoid contact of solutions of hexacyanoferrates with concentrated acids.
Statement of experience – Elena Makhinenko, text– Ph.D. Pavel Bespalov.
a) Reaction with potassium hexacyanoferrate (II) - potassium ferrocyanide K 4 (pharmacopoeia). Fe 3+ cations in an acidic medium react with potassium ferrocyanide to form a dark blue precipitate of "Prussian blue" - a complex compound of hexacyanoferrate (II) iron (III) Fe 4 3 X H 2 O with a variable number of water molecules. It is shown that, depending on the precipitation conditions, the Prussian Blue precipitate, like the Turnbull blue precipitate (see above), entrains other cations from the solution, so that its composition changes and can correspond to the KFe 3+ formula:
Fe 3+ + K + + 4- →FeK↓
The reaction is specific. The reaction is hindered by oxidizing agents that oxidize the reagent.
Execution of the reaction. 2-3 drops of iron (III) salt solution are added to the test tube, 1-2 drops of HCI solution and 2 drops of K 4 solution are added. The solution turns blue and a dark blue precipitate of "Prussian Blue" precipitates.
b) Reaction with thiocyanate ions (pharmacopoeia). Salts of Fe 3+ form red iron (III) thiocyanate. The reaction is carried out in an acidic medium. The composition of the resulting complex is not constant and can vary from 2+ to 3- depending on the concentration of Fe 3+ and SCN - ions. This reaction is sometimes used to detect iron in combination with reaction 1, with potassium hexacyanoferrate(II). First, by adding NH 4 SCN, a red complex of iron thiocyanate is obtained, which is then transferred by adding potassium hexacyanoferrate (II) to a blue precipitate of iron (III) hexacyanoferrate (II) potassium:
Fe 3+ + 3SCN - →Fe(SCN) 3
The sensitivity of the reaction is 0.25 µg. The reaction is hindered by anions of oxygen acids (phosphoric, arsenic, etc.), fluorides, which form compounds with Fe 3+ and NO 2, giving SCN - a red NOSCN compound.
Execution of the reaction. 3-4 drops of iron (III) salt solution are added to the test tube and 2-3 drops of ammonium thiocyanate NH4NCS or potassium KNCS solution are added. The solution turns blue.
c) Reaction with sodium sulfide (pharmacopoeial). Sodium sulfide precipitates from neutral and slightly alkaline solutions of iron (III) salts a black precipitate Fe 2 S 3:
2Fe 3+ + 3S 2- → Fe 2 S 3 ↓
The Fe 2 S 3 precipitate is soluble in mineral acids.
Execution of the reaction. 3-4 drops of iron (III) salt solution are added to the test tube and 2-3 drops of ammonium sulfide solution or hydrogen sulfide water are added. A black precipitate of iron(III) sulfide is released.
d) Reaction with hydroxides. The precipitate of iron hydroxide (III) Fe (OH) 3, resulting from the interaction of Fe 3+ with hydroxide ions, is insoluble in alkali solutions and therefore, according to the acid-base classification, Fe 3+ belongs to the group of cations, the hydroxides of which are insoluble in alkalis. The Fe(OH) 3 precipitate is soluble in dilute acids; we will not dissolve in a saturated solution of ammonium chloride (in contrast to the white precipitate of Fe (OH) 2).
Execution of the reaction. 3-4 drops of iron (III) salt solution are added to the test tube and 3-4 drops of NaOH are added. A red-brown precipitate of iron hydroxide (III) Fe (OH) 3 falls out.
e) Reaction with sulfosalicylic acid (pharmacopoeial). The Fe 3+ cation reacts in aqueous solutions with sulfosalicylic acid at pH ≈ 9-11.5 with the formation of yellow complexes: Fe 3+ + L 2- → 3- ,
where L 2- is the designation of the sulfosalicylate anion formed from sulfosalicylic acid upon elimination of two protons, presumably from groups
–COOH and –SO 3 H.
The most stable complex is yellow, containing iron (III) and anions of sulfosalicylic acid in a molar ratio of iron (III): sulfosalicylate anions, equal to 1:3, i.e. There are three sulfosalicylate ligands per iron atom. This complex dominates in the ammonia solution. The exact structure of the complexes in solution is unknown. The sensitivity of the reaction is 5-10mkg.
Execution of the reaction. ~5 drops of iron (III) salt solution are added to the test tube, ~10 drops of sulfosalicylic acid solution and ~0.5 ml of concentrated ammonia solution are added. The solution takes on a yellow color.
Analytical reactions of magnesium (II) cations.
a) Reaction with alkalis. Alkali solutions separate from solutions of magnesium salts a white gelatinous precipitate of magnesium hydroxide Mg (OH) 2, easily soluble in acids and solutions of ammonium salts:
Mg(OH) 2 ↓+ 2HCI→MgCI 2 + 2H 2 O
Mg(OH) 2 ↓+ 2NH 4 CI→ MgCI 2 + 2NH 4 OH
Execution of the reaction. To 1-2 drops of a solution containing magnesium ions, add 2-3 drops of 1M NaOH. A white gelatinous precipitate forms. The resulting precipitate is divided into 2 test tubes. Add 3-4 drops of HCl to the 1st tube, the precipitate dissolves. We add 3-4 drops of NH 4 Cl to the 2nd tube, the precipitate also dissolves.
b) Reaction with potassium hypoiodite. When iodine interacts with alkali, potassium hypoiodite KIO is formed; in this case, the equilibrium in the solution shifts to the right and it becomes colorless:
I 2 + 2OH - ↔I - + IO - + H 2 O
When a magnesium salt is added, Mg 2+ ions form a precipitate of Mg (OH) 2 with OH ions - a precipitate of Mg (OH) 2, which causes a shift in equilibrium to the left. The iodine released during this is adsorbed by the Mg (OH) 2 precipitate and stains it in a red-brown color.
Execution of the reaction. Lugol's solution is decolorized by adding dropwise KOH solution. A magnesium salt solution is added to the colorless solution obtained. An amorphous precipitate, colored reddish-brown, immediately stands out.
c) Reaction with sodium hydrogen phosphate (pharmacopoeia). Sodium hydrogen phosphate forms a white crystalline precipitate with magnesium ions in the presence of NH 3 at pH ~ 9:
At pH> 10, Mg(OH) 2 and Mg 3 (PO 4) 2 can be formed. It is recommended to add NH 3 to the acidic analyzed solution to pH ~9. Due to the formation of NH 4 C1, the pH of the solution is kept constant. The precipitate is soluble in strong acids and in acetic acid:
MgNH 4 PO 4 ↓+ 3HCI → H 3 PO 4 + MgCI 2 + NH 4 CI
MgNH 4 PO 4 ↓+ 2CH 3 COOH → Mg(CH 3 COO) 2 + NH 4 H 2 PO 4
The detection limit for magnesium is 10 μg. Interfere with ions that form sparingly soluble phosphates; NH 4 + , K(I) and Na(I) do not interfere.
Execution of the reaction. To 1-2 drops of a solution containing magnesium ions, add 2 - 3 drops of 2 M HCl, 1 drop of Na 2 HPO 4 solution and, with stirring, add 2 M NH 3 dropwise until the smell of ammonia appears (pH ~ 9). A white crystalline precipitate falls out.
d) Reaction with 8-hydroxyquinoline (luminescent reaction). 8-Oxyquinoline forms with magnesium ions at pH 9 - 12 a fluorescent green light oxyquinoline:
The detection limit for magnesium is 0.025 µg. The intensity of the glow increases when a wet spot is treated with magnesium oxyquinolinate with a solution of NH 3 . Al(III), Zn(II) interfere.
Executing a reaction. A drop of a solution containing magnesium ions and a drop of an ethanol solution of the reagent are applied to the filter paper. The resulting magnesium hydroxyquinolinate is treated with a drop of 10% ammonia solution. When viewing a wet spot in ultraviolet light, a green glow is observed.
e) Reaction with quinalizarin (1,2,5,8-tetraoxyanthraquinone)(I). Quinalizarin (1,2,5,8-tetrahydroxyanthraquinone)(I) with magnesium ions forms a poorly soluble blue compound in an alkaline solution, to which the structure (II) is attributed:
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It is assumed that quinalizarin varnish is an adsorption compound of magnesium hydroxide with a reagent. The formation of chelates of variable composition is very likely.
The limit of detection of magnesium is 5 μg. Detection is not interfered with by alkaline earth metal ions; in the presence of a sufficiently large amount of alkali, aluminum ions do not interfere.
The ammonium ion interferes with the detection of the magnesium ion because it prevents the formation of magnesium hydroxide. The reagent solution in an alkaline medium is colored purple, so a control experiment is necessary.
Execution of the reaction. To 1 - 2 drops of a solution containing magnesium ions, add 1 drop of quinalizarin solution and 2 drops of a 30% NaOH solution. A blue precipitate is formed. To conduct a control experiment, add one drop of a quinalizarin solution, 2 drops of a 30% NaOH solution to 1 - 2 drops of water. The solution turns purple.
4. Test control 1
