Potassium rhodanide and iron chloride 3. Rhodanide method for the determination of iron

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Iron (III) thiocyanate
Are common
Systematic name	Iron (III) thiocyanate
Traditional names	thiocyanic iron; thiocyanic iron
Chem. formula	Fe (SCN) 3
Physical properties
condition	red crystals with a greenish tint
Molar mass	230.09 g / mol
Data are based on standard conditions (25 ° C, 100 kPa) unless otherwise noted.

Iron (III) thiocyanate - an inorganic compound, a salt of the metal of iron and rhodanic acid with the formula Fe (SCN) 3, dissolves in water, forms a crystalline hydrate - red crystals.

Receiving

• Exchange reactions:

$\backslash \backslash \; mathsf\; (Fe\_2\; (SO\_4)\; \_3\; +\; 3Ba\; (SCN)\; \_2\; \backslash \backslash \; \backslash \backslash \; xrightarrow\; ()\; \backslash \backslash \; 2Fe\; (SCN)\; \_3\; +\; 3BaSO\_4\; \backslash \backslash \; downarrow)$

• Neutralization of thiocyanate solution with freshly precipitated iron (III) hydroxide:

$\backslash \backslash \; mathsf\; (Fe\; (OH)\; \_3\; +\; 3HSCN\; \backslash \backslash \; \backslash \backslash \; xrightarrow\; ()\; \backslash \backslash \; Fe\; (SCN)\; \_3\; +\; 3H\_2O)$

Physical properties

Iron (III) thiocyanate forms crystalline hydrate Fe (SCN) 3 3H 2 O - 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

• Forms coordination compounds with thiocyanates of other metals hexathiocyanatoferrates (III), 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 / Editorial board: I.L. Knunyants 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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Excerpt Characterizing Iron (III) Thiocyanate

- In the future life? - repeated Prince Andrew, but Pierre did not give him time to answer and took this repetition for denial, especially since he knew the former atheistic convictions of Prince Andrew.
- You say that you cannot see the kingdom of good and truth on earth. And I have not seen him and he cannot be seen if we look at our life as the end of everything. On the ground, on this very ground (Pierre pointed out in the field), there is no truth - all lies and evil; but in the world, in the whole world, there is a kingdom of righteousness, and we are now children of the earth, and forever children of the whole world. Don't I feel in my soul that I am part of this huge, harmonious whole. Do I not feel that I am in this huge, countless number of beings in which the Divine is manifested - a higher power, if you like - that I am one link, one step from lower beings to higher ones. If I see, clearly see this staircase that leads from plant to person, then why should I suppose that this staircase is interrupted with me, and does not lead further and further. I feel that not only can I not disappear, just as nothing disappears in the world, but that I always will and always have been. I feel that besides me, spirits live above me and that there is truth in this world.
“Yes, this is Herder’s teaching,” said Prince Andrew, “but not that, my soul, will convince me, but life and death, that’s what convinces. What convinces you is that you see a creature dear to you, which is connected with you, before which you were guilty and hoped to justify yourself (Prince Andrey trembled and turned away) and suddenly this creature suffers, suffers and ceases to be ... Why? It cannot be that there was no answer! And I believe that he is…. That's what convinces, that's what convinced me, 'said Prince Andrey.
- Well, yes, well, yes, - said Pierre, - isn't it the same thing I say!
- No. I only say that it is not arguments that convince of the need for a future life, but when you walk hand in hand with a person in life, and suddenly this person disappears there in nowhere, and you yourself stop in front of this abyss and look there. And, I looked ...
- Well then! 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 the horses had long since been taken to the other side and had already been laid, and the sun had already disappeared by half, and the evening frost covered the puddles near the ferry with stars, while Pierre and Andrei, to the surprise of the footmen, coachmen and carriers, were still standing on the ferry and talking.
- If there is God and there is a future life, that is, truth, there is virtue; and man's highest happiness is to strive to attain them. We must live, we must love, we must believe, - said Pierre, - that we are not living only on this piece of land today, but we have lived and will live forever there in everything (he pointed to the sky). Prince Andrew stood with his elbows on the rail 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 fell silent. It was completely quiet. The ferry had stopped long ago, and only the waves of the current hit the bottom of the ferry with a faint sound. It seemed to Prince Andrew that this rinsing of waves was saying to Pierre's words: "really, believe this."
Prince Andrew sighed, and with a radiant, childish, gentle gaze looked into the flushed, enthusiastic, but still timid face of Pierre's preeminent friend.
- Yes, if only it were! - he said. `` But let's go sit down, '' added Prince Andrew, and as he got off the ferry he looked at the sky, which Pierre had 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 that had long since fallen asleep, something that was best in him, suddenly woke up joyfully and young in his soul. This feeling disappeared, as soon as Prince Andrey entered his usual conditions of life, but he knew that this feeling, which he could not develop, lived in him. The meeting with Pierre was for Prince Andrei an era from which, although in appearance, the same one began, but in the inner world his new life.

It was already getting dark when Prince Andrey and Pierre drove up to the main entrance of the Lysogorsk house. As they drove up, Prince Andrey with a smile drew Pierre's attention to the commotion 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 the carriage driving in, rushed back into the gate. Two women ran out after them, and all four, looking back at the carriage, ran in fright onto the back porch.
- These are Cars god's people- said Prince Andrew. “They took us for a father. And this is the only thing in which she does not obey him: he orders to drive these pilgrims, and she accepts them.
- What are God's people? - asked Pierre.
Prince Andrew did not have time to answer him. The servants came out to meet him, and he asked about where the old prince was and whether they were expecting him soon.
The old prince was still in the city, and they were waiting for him every minute.
Prince Andrew led Pierre to his quarters, which were always waiting for him in perfect order at his father's house, and he himself went to the nursery.
“Let's go to our sister,” said Prince Andrew, returning to Pierre; - I have not seen her yet, she is now hiding and sitting with her God's people. Serves 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 are] God's people? - asked Pierre
- But you'll see.
Princess Marya was really embarrassed and blushed with 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 monk's cassock.
On an armchair, beside, sat a wrinkled, thin old woman with a gentle expression of a child's face.
“Andre, pourquoi ne pas m” avoir prevenu? [Andrei, why didn't you warn me?], She said with meek reproach, standing in front of her pilgrims like a mother hen before chickens.

Qualitative reactions to iron (III)

Iron ions (III ) in solution can be determined using qualitative reactions. Let's carry out some of them. Take for the experiment a solution of ferric chloride (III).

1. III) - reaction with alkali.

If the solution contains iron ions (III ), iron hydroxide (III ) Fe (OH) 3. The base is insoluble in water and brown in color. (Iron hydroxide (II ) Fe (OH) 2. - also insoluble, but gray-green). A brown precipitate indicates the presence of iron ions in the initial solution (III).

FeCl 3 + 3 NaOH \u003d Fe (OH) 3 ↓ + 3 NaCl

2. Qualitative reaction per iron ion ( III ) - reaction with yellow blood salt.

Yellow blood salt is potassium hexacyanoferrateK 4 [ Fe( CN) 6]. (To determine iron (II) use red blood saltK 3 [ Fe( CN) 6]). To a portion of the ferric chloride solution, a solution of yellow blood salt. The blue Prussian blue precipitate * indicates the presence of ferric ions in the initial solution.

3 TO 4 +4 FeCl 3 \u003d K Fe ) ↓ + 12 KCl

3. Qualitative reaction for 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. This is iron thiocyanate (III). Rodanid from the Greek "rodeos" is red.

FeCl 3 + 3 KCNS= Fe( CNS) 3 + 3 KCl

Prussian blue was obtained by accident in the early 18th century in Berlin dyeing by master Diesbach. Disbach bought an unusual potash (potassium carbonate) from a merchant: the solution of this potash, when added with iron salts, turned out to be blue. When checking the potash, it turned out that it was calcined with bovine blood. The dye was found to be suitable for fabrics: bright, long-lasting and inexpensive. Soon the recipe for obtaining paint became known: potash was fused with dried animal blood and iron filings. Leaching of this alloy produced yellow blood salt. Prussian blue is now used to produce printing ink and tint polymers .

Equipment:flasks, pipette.

Safety engineering ... Observe the rules for handling alkaline solutions and solutions hexacyanoferrates. Avoid contact of hexacyanoferrate solutions with concentrated acids.

Experience setting - Elena Makhinenko, text- Ph.D. Pavel Bespalov.

The method is based on the determination of the wine-red color characteristic of the complexes formed by ferric ions and thiocyanate ions. These complexes are unstable; therefore, a large excess of thiocyanate ions is required, which suppress the dissociation of the complex. The process of interaction of ferric ions with thiocyanate ions proceeds according to the equation (1):

Fe 3+ + 6 NH 4 CNS \u003d 6NH 4 + + 3-

It should be borne in mind that, in addition to 3-, other, less intensely colored complexes can be formed, therefore the concentration of ammonium thiocyanate should be the same in the analyzed and standard solutions. The determination is interfered with by strong oxidants (potassium permanganate, ammonium persulfate, hydrogen peroxide, etc.) that oxidize the thiocyanate anion, as well as substances that reduce iron (III) to iron (II). The best medium is nitric acid, while the already low acidity of the solution is sufficient, which prevents the hydrolysis of the iron salt (1-2 ml of concentrated nitric acid per 50 ml of solution).

Reagents

Ammonium thiocyanate (NH4CNS), 10% solution;

Concentrated nitric acid;

Basic standard solution. To prepare the basic standard solution, 0.8634 g of ammonium iron is dissolved in a small volume of distilled water. If the solution turns out to be cloudy, then add a few drops of concentrated nitric acid and adjust the volume to 1 liter. The solution contains 0.1 mg of iron in 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 in 1 ml.

Working process

In volumetric flasks of 100 ml add 1 and 5 ml of the working standard solution, as well as 1; 2.5 and 5 ml of the basic standard iron solution 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 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 \u003d 450 nm using cuvettes with an optical layer thickness of 5 mm, in relation to distilled water to which the same reagents have been added. The mass concentration of iron is found from a calibrated graph. A calibration graph is plotted by plotting the mass concentration of iron in μg / dm 3 along the abscissa and the corresponding optical density value along the ordinate.

1. Determination of chromium content using diphenylcarbazide

Method principle

The method is based on the interaction of chromates and dichromates in an acidic medium with diphenylcarbazide to form a red-violet-colored compound in which chromium is contained in the reduced form Cr (III), and diphenylcarbazide is oxidized to diphenylcarbazone. The detection limit 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, only Cr (vi) is determined in one sample, and in the other, the total content of Cr (iii) and Cr (vi), in which Cr (III) is oxidized to Cr (VI). Ammonium persulfate is used as an oxidizing agent. The oxidation process proceeds according to equation (2):

2Сr 3+ + 3S 2 O 8 2- + 7H 2 O  Сr 2 O 7 2- + 6SO 4 2- + 14Н +

The Cr 3+ content is determined from the difference in the results.

Reagents

Double-distilled water (used for the preparation of all reagents);

Sulfuric acid, 1: 1;

Concentrated phosphoric acid;

Diphenylcarbazide (C 13 H 14 OH 4), 0.5% solution in acetone (used freshly prepared);

Sodium hydroxide solution, 10% and 25%;

Basic standard solution of potassium dichromate K 2 Cr 2 O 7. The basic standard solution is prepared by dissolving 2.8285 g of the reagent, dried at 150C, in bidistilled water and adjusting the volume to 1 liter (1 ml of 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 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)).

Building 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, bring the pH to 8 with an alkali solution, monitoring with a universal indicator paper. 1 ml of H 2 SO 4 (1: 1) and 0.3 ml of H 3 PO 4 are added, the volume is adjusted to 100 ml. The resulting solutions have a concentration of Cr (VI) 0; ten; 20; 40; 60; 100; 160; 200 μg / l. Add 2 ml of 0.5% diphenylcarbazide solution to each flask and mix well. The resulting solutions in 10-15 minutes. photometry at a wavelength λ \u003d 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)

A volume of sample is placed in a 100 ml volumetric flask so that it contains from 0.005 to 0.1 mg of chromium, the pH is adjusted to 8 with an acid or alkali solution, monitoring with a universal indicator paper. 1 ml of H 2 SO 4 (1: 1) and 0.3 ml of H 3 PO 4 are added, the volume is adjusted to 100 ml and the mixture is stirred. To each flask add 2 ml of 0.5% diphenylcarbazide solution and mix again. The resulting solutions in 10-15 minutes. photometric as indicated above.

a) Reaction with potassium hexacyanoferrate (II) - potassium ferrocyanide K 4 (pharmacopoeial). Fe 3+ cations in an acidic medium react with potassium ferrocyanide to form a dark blue precipitate "Prussian blue" - a complex compound of iron (III) hexacyanoferrate (II) Fe 4 3 x H 2 O with a variable number of water molecules. It has been shown that, depending on the deposition conditions, the Prussian blue precipitate, like the Turnboolean blue precipitate (see above), carries away 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 interfered with by oxidizing agents that oxidize the reagent.

Performing a reaction. Add 2-3 drops of iron (III) salt solution to the test tube, add 1-2 drops of HCI solution and 2 drops of K 4 solution. The solution turns blue and a dark blue "Prussian blue" precipitate forms.

b) Reaction with thiocyanate ions (pharmacopoeial).Fe 3+ salts form red iron (III) thiocyanate. The reaction is carried out in an acidic environment. The composition of the resulting complex is unstable 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 converted into a blue precipitate of iron (III) potassium hexacyanoferrate (II) by adding potassium hexacyanoferrate (II):

Fe 3+ + 3SCN - → Fe (SCN) 3

The sensitivity of the reaction is 0.25 μg. The reaction is interfered with by anions of oxygen acids (phosphoric, arsenic, etc.), fluorides, which form compounds with Fe 3+ and NO 2, giving SCN, a red NOSCN compound.

Performing a reaction. 3-4 drops of a solution of an iron (III) salt are added to a test tube and 2-3 drops of a solution of ammonium thiocyanate NH4NCS or potassium KNCS are added. The solution turns blue.

c) Reaction with sodium sulfide (pharmacopoeial).Sodium sulfide precipitates black Fe 2 S 3 from neutral and weakly alkaline solutions of iron (III) salts:

2Fe 3+ + 3S 2- → Fe 2 S 3 ↓

The Fe 2 S 3 precipitate is soluble in mineral acids.

Performing a 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 precipitated.

d) Reaction with hydroxides.The precipitate of iron (III) hydroxide 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+ is referred to the group of cations whose hydroxides are insoluble in alkalis. The Fe (OH) 3 precipitate is soluble in dilute acids; insoluble in a saturated solution of ammonium chloride (in contrast to the white precipitate Fe (OH) 2).

Performing a 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 (III) hydroxide 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 by the elimination of two protons, presumably from the groups
–COOH and –SO 3 N.

The most stable complex is yellow, containing iron (III) and sulfosalicylic acid anions 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 ammonia solution... The exact structure of the complexes in solution is unknown. The sensitivity of the reaction is 5-10mkg.

Performing a reaction. Add ~ 5 drops of iron (III) salt solution to the test tube, add ~ 10 drops of sulfosalicylic acid solution and ~ 0.5 ml of concentrated ammonia solution. The solution turns yellow.

Analytical reactions of magnesium (II) cations.

a) Reaction with alkalis.Alkaline solutions are isolated from solutions of magnesium salts 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

Performing a reaction. To 1-2 drops of a solution containing magnesium ions, add 2 to 3 drops of 1M NaOH. A white gelatinous precipitate is formed. Divide the resulting sediment into 2 test tubes. Add 3-4 drops of HCl to the 1st test tube, the precipitate dissolves. Add 3-4 drops of NH 4 Cl to the 2nd test tube, the precipitate also dissolves.

b) Reaction with potassium hypoioditis. When iodine interacts with alkali, potassium hypoioditis KIO is formed; in this case, the equilibrium in the solution shifts to the right and it becomes discolored:

I 2 + 2OH - ↔I - + IO - + H 2 O

When magnesium salt is added, Mg 2+ ions form with OH ions - a precipitate of Mg (OH) 2, which causes the balance to shift to the left. The iodine released during this is adsorbed by the Mg (OH) 2 precipitate and stains it red-brown.

Performing a reaction. Lugol's solution is decolorized by adding KOH solution dropwise. A magnesium salt solution is added to the resulting colorless solution. An amorphous red-brown precipitate immediately emerges.

c) Reaction with sodium hydrogen phosphate (pharmacopoeial).Sodium hydrogen phosphate forms with magnesium ions in the presence of NH 3 at pH ~ 9 a white crystalline precipitate:

At pH\u003e 10, Mg (OH) 2 and Mg 3 (PO 4) 2 can form. It is recommended to add NH 3 to the acidic test solution to pH ~ 9. Due to the formation of NH 4 C1, the pH of the solution is kept constant. The precipitate dissolves 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. Ions that form poorly soluble phosphates interfere; NH 4 +, K (I) and Na (I) do not interfere.

Performing a 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 add 2 M NH 3 dropwise with stirring until the smell of ammonia appears (pH ~ 9). A white crystalline precipitate is formed.

d) Reaction with 8-hydroxyquinoline (luminescent reaction).8-hydroxyquinoline forms fluorescent with magnesium ions at pH 9 - 12 green light hydroxyquinolinate:

The detection limit for magnesium is 0.025 μg. The luminescence intensity increases when a wet spot is treated with magnesium hydroxyquinolinate with an NH 3 solution. A1 (III), Zn (II) interfere.

Performing 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 oxyquinolinate is treated with a drop of 10% ammonia solution. When viewed from a wet spot in ultraviolet light, a green glow is observed.

e) Reaction with quinalizarin (1,2,5,8-tetraoxyanthrachinone) (I).Quinalizarin (1,2,5,8-tetraoxyanthraquinone) (I) with magnesium ions forms a poorly soluble blue compound in an alkaline solution, which is attributed to the structure (II):

It is believed that the quinalizarin lacquer is an adsorption compound of magnesium hydroxide with a reagent. Formation of chelates of variable composition is highly probable.

The detection limit for magnesium is 5 μg. Detection is not interfered with by alkaline earth metal ions; in the presence of enough a large number alkalis do not interfere with aluminum ions.

The ammonium ion interferes with the detection of the magnesium ion as it prevents the formation of magnesium hydroxide. The solution of the reagent in an alkaline medium is colored violet, therefore a control experiment is necessary.

Performing a reaction. To 1 - 2 drops of a solution containing magnesium ions, add 1 drop of quinalizarin solution and 2 drops of 30% NaOH solution. A blue precipitate is formed. To carry out a control experiment, add one drop of quinalizarin solution, 2 drops of 30% NaOH solution to 1 - 2 drops of water. The solution turns purple.

4. Test control 1