Introduction Besides the quantitative analysis of chemical compounds, the qualitative analysis of un — Physical Chemistry — Thermodynamics Chemistry Question

Introduction

Besides the quantitative analysis of chemical compounds, the qualitative analysis of unknown substances or mixtures of substances in order to identify the cations and/or anions is also an important procedure in analytical chemistry. Cations have to be separated prior to identification, however, this is not the case for anions.

In this exercise, the anions in an analytical sample are to be identified. Some of these anions can be identified by direct analysis of the solid sample, however, for others it is necessary to identify them in the filtrate of a soda extract. Several reagents are provided that can either be used in the initial identification of the anions present, or to perform the necessary confirmation tests for a particular anion.

The reactions of the anions with the reagents that are available, as far as is necessary for your analysis, are described below.

List of potential anions:
acetate, H₃CCOO–
nitrate, NO₃–
carbonate, CO₃₂–
oxalate, C₂O₄₂–
chloride, Cl–
perchlorate, ClO₄–
chromate, CrO₄₂–
sulphate, SO₄₂–

Preparation of the soda extract

One spatulaful of the sample (about 1 g) is mixed with 2 – 3 times the amount of sodium carbonate. The mixture is suspended in water and heated for 10 minutes. After cooling, the residue is filtered off and washed with water. The filtrate is used in the anion identification. It is always a good idea to use blind samples for comparison and to check the purity of soda.

Selected reactions of the anions that may be present:

Acetate
Theory: Acetate anions react with potassium hydrogensulfate to form acetic acid:
H₃CCOO– + HSO₄– → H₃CCOOH + SO₄₂–
Dilute sulfuric acid also forms acetic acid upon reaction with acetate anions.
Procedure: The solid sample is ground with four times the amount of potassium hydrogensulfate in a mortar. In the presence of acetate anions, there is the characteristic smell of acetic acid.

Carbonate
Theory: Carbonate anions react with dilute hydrochloric acid to form unstable carbonic acid that decomposes into water and carbon dioxide:
CO₃₂– + 2 H⁺ → {H₂CO3} → CO₂ + H₂O
Carbon dioxide reacts with barium hydroxide to form barium carbonate:
CO₂ + Ba(OH)₂ → BaCO₃ + H₂O
Procedure: In a test tube, dilute hydrochloric acid is added to a small amount of the sample. The test tube is closed immediately connected to a fermentation tube filled with freshly prepared barium hydroxide solution. The test tube is gently heated. In the presence of carbonate anions, white flakes of barium carbonate are observed in the solution in the fermentation tube within 3 - 5 minutes.
[VISUAL]
Schematic representation of a fermentation tube

Chloride
Theory: Chloride anions in a nitric acid solution react with silver nitrate to form silver chloride:
Ag⁺ + Cl– → AgCl
Silver chloride is soluble in concentrated ammonia solution. It is insoluble in concentrated nitric acid.
Procedure: An aqueous solution of silver nitrate is added to 5 cm₃ of the soda extract acidified with dilute nitric acid. In the presence of chloride anions, white silver chloride precipitates from solution. The latter decomposes into elementary silver within a few hours if it is exposed to sunlight.

Chromate
Theory: Chromate anions react with silver nitrate in a neutral or dilute nitric acid solution to form silver chromate:
2 Ag⁺ + CrO₄₂– → Ag₂CrO₄
Silver chromate is soluble in acids and ammonia solution.
Procedure: An aqueous solution of silver nitrate is added to 5 cm₃ of the soda extract that is acidified with dilute nitric acid. In the presence of chromate anions, reddish brown silver chromate precipitates from the solution.
Theory: Chromate anions react with barium chloride in an acetic acid solution buffered by ammonium acetate to form barium chromate:
Ba²⁺ + CrO₄₂– → BaCrO₄
Barium chromate is soluble in strong mineral acids.
Procedure: A spatulaful of ammonium acetate is added to 5 cm₃ of the soda extract that has been acidified with acetic acid. An aqueous solution of barium chloride is added and the mixture boiled for 2 minutes. In the presence of chromate anions, yellow barium chromate precipitates from the solution.
Concentrated, yellow coloured, chromate containing solutions form orange coloured dichromates upon acidification with dilute sulfuric acid. The addition of more highly concentrated sulfuric acid leads to the formation of dark coloured oligo^- and polychromates.

Nitrate
Theory: Nitrate anions are reduced to nitrogen monoxide (NO) by iron(II) sulfate in solutions acidified with sulfuric acid. Nitrogen monoxide reacts with iron(II) cations to form the brownish nitrosyl complex [Fe(NO)(H₂O)₅]2+.
Procedure: 2.5 cm₃ of an iron(II) sulfate solution acidified with sulfuric acid is added to 2.5 cm₃ of the soda extract. After mixing, the test tube is brought into a skew position and concentrated sulfuric acid is poured carefully along the inner surface. In the presence of nitrate anions, a brownish ring forms at the phase boundary between the solution and the sulfuric acid.

Oxalate
Theory: In a neutral solution, oxalate anions react with silver nitrate solution to form silver oxalate:
2 Ag⁺ + C₂O₄₂– → Ag₂C₂O₄
Silver oxalate is sparingly soluble in acetic acid. It is soluble in nitric acid and ammonia solution.
Procedure: An aqueous solution of silver nitrate is added to 5 cm₃ of the soda extract neutralized with acetic acid. In the presence of oxalate anions, a white precipitate of silver oxalate is formed.
Theory: Oxalate anions react in an ammoniacal or acetic acid solution that is buffered by sodium acetate, with calcium chloride to form calcium oxalate:
Ca²⁺ + C₂O₄₂– → CaC₂O₄
Calcium oxalate is insoluble in dilute acetic acid. It is soluble in strong mineral acids. Calcium oxalate is oxidized to carbon dioxide by potassium permanganate in an acidic solution. In this reaction, the manganese(VII) cations are reduced to manganese(II) cations.
Oxalates and oxalic acid decompose by reaction with concentrated sulfuric acid into carbon monoxide and carbon dioxide:
H₂C₂O₄ →[H₂SO₄] H₂O + CO + CO₂
Procedure: 5 cm₃ of the soda extract are acidified with acetic acid. Ammonia solution is added until the mixture is slightly ammoniacal followed by the addition of an aqueous solution of calcium chloride. In the presence of oxalate anions, white calcium oxalate precipitates from solution. The precipitate is filtered off and dissolved in sulfuric acid. A solution of potassium permanganate is added dropwise to the solution. The potassium permanganate solution rapidly decolourizes and a gas is formed.
Theory: In a neutral solution, oxalate anions react with barium chloride to form barium oxalate:
Ba²⁺ + C₂O₄₂– → BaC₂O₄
Barium oxalate dissolves in dilute acetic acid.
Procedure: An aqueous solution of barium chloride is added to 5 cm₃ of the soda extract neutralized with dilute hydrochloric acid. In the presence of oxalate anions, white barium oxalate precipitates from the solution.

Perchlorate
Theory: In a solution slightly acidified with nitric acid, perchlorate anions react with potassium nitrate to form potassium perchlorate:
ClO₄– + K⁺ → KClO₄
Potassium perchlorate is insoluble in cold water and cold dilute acid.
Procedure: An aqueous solution of potassium nitrate is added to 5 cm₃ of the soda extract slightly acidified with nitric acid. In the presence of perchlorate anions, a white precipitate of potassium perchlorate forms.
Theory: In a neutral and slightly alkaline solution perchlorate anions are reduced by iron(II) hydroxide (formed by the reaction of iron(II) sulfate with sodium hydroxide) to chloride anions.
Procedure: 4 cm₃ of an aqueous iron(II) sulfate solution are added to 5 cm₃ of the soda extract acidified with dilute nitric acid. Dilute sodium hydroxide solution is added until some iron(II) hydroxide begins to precipitate from solution or the solution is slightly alkaline. The mixture is boiled for a few minutes and the resulting precipitate is filtered off. In the presence of perchlorate anions, the filtrate of the reaction contains chloride anions, which can be confirmed by reaction with silver nitrate in a solution acidified with nitric acid.

Sulfate
Theory: In an acidic solution acidified with hydrochloric acid sulfate anions react with barium chloride to form barium sulfate:
Ba²⁺ + SO₄₂– → BaSO₄
Barium sulfate is insoluble in concentrated hydrochloric acid and in concentrated nitric acid. It is sparingly soluble in hot concentrated sulfuric acid, 12 percent of barium sulfate dissolves.
Procedure: An aqueous solution of barium chloride is added to 5 cm₃ of the soda extract acidified with dilute hydrochloric acid. In the presence of sulfate anions white barium sulfate precipitates from the solution.
Theory: In an acidic solution acidified with hydrochloric acid, sulfate anions react with calcium chloride to form calcium sulfate:
Ca²⁺ + SO₄₂– → CaSO₄
Calcium sulfate dissolves in concentrated sulfuric acid and concentrated hydrochloric acid.
Procedure: An aqueous solution of calcium chloride is added to 5 cm₃ of the soda extract acidified with dilute hydrochloric acid. In the presence of sulfate anions, white calcium sulfate precipitates from the solution. The precipitation is not quantitative!

List of chemicals:
- acetic acid, aqueous solution, w(H₃CCOOH) = 0.99
- acetic acid, aqueous solution, w(H₃CCOOH) = 0.05
- ammonia, aqueous solution, w(NH₃) = 0.25
- ammonium acetate, solid
- barium chloride, aqueous solution, c(BaCl₂) ~ 1.5 mol dm–3
- barium hydroxide, aqueous solution, w(Ba(OH)₂ · 8 H₂O) ~ 0.02
- calcium chloride, aqueous solution, c(CaCl₂ · 2 H₂O ) = 1 mol dm–3
- hydrochloric acid, w(HCl) = 0.36
- hydrochloric acid, c(HCl) = 2 mol dm–3
- iron(II) sulfate, aqueous solution, c(FeSO₄) = 1 mol dm–3
- nitric acid, aqueous solution, w (HNO₃) = 0.65
- nitric acid, aqueous solution, c(HNO₃) = 2 mol dm–3
- potassium hydrogensulfate, solid
- potassium nitrate, aqueous solution, saturated
- potassium permanganate. aqueous solution, c(KMnO₄) = 0.02 mol dm–3
- silver nitrate, aqueous solution, c(AgNO₃) = 0.2 mol dm–3
- sodium acetate. solid
- sodium carbonate, solid
- sodium hydroxide, aqueous solution, w(NaOH) ~ 0.05
- sulfuric acid, aqueous solution, (95-97 %)
- sulfuric acid, aqueous solution, c(H₂SO₄) = 2 mol dm–3

Preparation of the sample:
To avoid interferences in the qualitative determinations only certain selected counter ions should be present in the analytical sample. The following salts guarantee the determination of anions without any interference: LiCl, LiClO₄, Na(OOCCH₃), Na₂CO₃, NaCl, NaNO₃, Na₂C₂O₄, NaClO₄, Na₂SO₄, K₂CO₃, K₂Cr₂O₇, KNO₃, K₂SO₄, AlCl₃, Al₂(SO₄)₃, FeCl₂, FeSO₄, CoCl₂, Co(NO₃)₂, CoSO₄, NiCl₂, Ni(NO₃)₂, NiSO₄. Certain other salts can be used. The salts must not form sparingly soluble residues. If salts are to be used that are not mentioned in the following table, then the hazard and safety data sheets for the compounds must first be consulted.
aluminium(III) chloride, AlCl₃ · 6 H₂O
aluminium(III) sulfate, Al₂(SO₄)₃ · x H₂O
cobalt(II) chloride, CoCl₂ · 6 H₂O
cobalt(II) nitrate, Co(NO₃)₂ · 6 H₂O
cobalt(II) sulfate, CoSO₄ · 7 H₂O
iron(II) chloride, FeCl₂ · 4 H₂O
iron(II) sulfate, FeSO₄ · 7 H₂O
lithium chloride, LiCl
lithium perchlorate, LiClO₄
nickel(II) chloride, NiCl₂ · 6 H₂O
nickel(II) nitrate, Ni(NO₃)₂ · 6 H₂O
nickel(II) sulfate, NiSO₄ · 6 H₂O
potassium carbonate, K₂CO₃
potassium dichromate, K₂Cr₂O₇
potassium nitrate, KNO₃
potassium sulfate, K₂SO₄
sodium acetate, NaH₃CCOO
sodium carbonate, Na₂CO₃
sodium chloride, NaCl
sodium nitrate, NaNO₃
sodium oxalate, Na₂C₂O₄
sodium perchlorate, NaClO₄ · H₂O
sodium sulfate, Na₂SO₄