Anions - Qualitative Analysis

Anion Identification Tests

§  Carbonate Ions: The most characteristic reaction of carbonate is the formation of carbon dioxide upon treatment with acid:

CO₃^2-(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)

The colorless, odorless carbon dioxide can be identified by bubbling it through a saturated solution of barium hydroxide, with which it forms a white precipitate of barium carbonate.

CO₂(g) + Ba²⁺(aq) + 2OH^-(aq) → BaCO₃(s) + H₂O(l)

Assemble a gas-liberation apparatus from a small test tube and a section of bent tubing. Dissolve or suspend a portion of your compound in a small amount of water and place it in the small test tube. Add about 0.5 mL of 6 M HCl and quickly fit the tube into the small test tube, allowing the gas liberated to bubble into a 6" test tube of saturated Ba(OH)₂ solution. The formation of a white precipitate in the large test tube (if the gas liberated is odorless) is a positive test for carbonate. It is imperative to test the gas-liberation apparatus by adding HCl to Na₂CO₃.

§  Sulfide Ions: When treated with nonoxidizing acids (HCl, CH₃COOH) sulfides react to liberate H₂S gas (rotten-egg odor). If the sulfide is very insoluble liberation of the gas may require concentrated acids (indeed some sulfides, HgS, CuS, are so insoluble that dissolution requires special treatment). The gas is generally identified by its odor and its precipitation of colored sulfides of various metal ions. Sulfides or hydrogen sulfide also are oxidized to elemental sulfur and sulfate by oxidizing agents such as permanganate, nitric acid, sulfuric acid, Fe(III), etc.

3H₂S(aq) + 2H⁺(aq) + 2NO₃^-(aq) → 2NO(g) + 4H₂O + 3S(s)

Acidify a sample with 6 M hydrochloric acid and warm. Cautiously smell the gas evolved and also test the gas with a piece of filter paper moistened with lead acetate solution. A foul smelling gas which turns lead acetate paper black constitutes a positive sulfide test.

§  Sulfate Ions: Sulfate is conveniently identified by precipitation of BaSO₄. Other insoluble barium salts contain anions of weak acids (CO₃^2-, SO₃^2-and PO₄^3-). Precipitation of these anions is prevented by acidifying the solution.
Acidify the test solution with 6 M HCl, and add a few drops of 0.2 M BaCl₂₄^2-. solution. A white precipitate indicates the presence of SO

§  Nitrate Ions: The most notable feature of the chemistry of the nitrate ion is its oxidizing ability as illustrated by the following reactions:

3Fe²⁺(aq) + 4H⁺(aq) + NO₃^-(aq) → NO(g) + 2H₂O + 3Fe³⁺(aq)

In the last reaction the nitrogen oxide reacts with excess Fe²⁺ to give the brown complex ion Fe(NO)²⁺. It is the formation of this brown complex that is used to identify NO₃^- (called the brown ring test).
Acidify about 2 mL of the test solution with 3 M H₂SO₄ and then dissolve one-half spatula full of solid FeSO₄.7H₂O in the acidified solution. Cool the solution and then carefully introduce about 0.5 mL of concentrated H₂SO₄ by allowing it to flow down the side of the tilted test tube. Allow the solution to sit undisturbed so that the sulfuric acid forms a definite layer. The formation of a brown color at the interface of the layer constitutes a positive test for nitrate.

§  Phosphate Ions: The precipitation usually used to identify phosphate is the formation of yellow ammonium molybdophosphate from ammonium molybdate in acidic solution.

12MoO₄^2- + 3NH₄⁺ + PO₄^3- + 24H⁺ → (NH₄)₃[P(Mo₁₂O₄₀)] + 12H₂O

Acidify the sample with concentrated nitric acid and add several drops in excess. Then treat the solution with ammonium molybdate reagent and warm. The formation of a yellow crystalline precipitate confirms the presence of phosphate.

§  Chloride, Bromide, and Iodide Ions: All three of these anions form insoluble silver salts. Although the precipitates are of different colors (AgCl white, AgBr cream, AgI yellow) the colors are difficult to distinguish, and confirmatory tests are necessary.
Silver chloride, the most soluble of the three, dissolves readily in 6 M NH₃₃, a much higher concentration of NH₃ being required to form the complex. solution because of formation of the ammonia complex. Furthermore, when the solution of the ammonia complex is acidified, AgCl reprecipitates. Neither AgBr nor AgI will dissolve readily in 6 M NH

Cl^-(aq) + Ag⁺(aq) → AgCl(s)
AgCl(s) + 2NH₃(aq) → Ag(NH₃)²⁺(aq) + Cl^-(aq)
Ag(NH₃)²⁺(aq) + Cl^-(aq) + 2H⁺(aq) → AgCl(s) + 2NH₄⁺(aq)

Bromide and iodide are usually identified by oxidation to the free elements with chlorine. The elements thus formed are extracted into carbon tetrachloride and identified by their color.

2Br^-(aq) + Cl₂(g) → Br₂(g) + 2Cl^-(aq)
2I^- (aq) + Cl₂(g) → I₂(s) + 2Cl^-(aq)

§  Chloride: Acidify the test solution with 3 M HNO₃. Then add several drops of 0.1 M AgNO₃. If a white precipitate forms, centrifuge and remove the supernatant. To the precipitate add 6 M NH₃ with stirring. If the precipitate dissolves, add 6 M HNO₃ to the solution. A white precipitate will form if the original test solution contained Cl^-.

§  Bromide and Iodide: Acidify the sample with several drops of 6 M HCl and add 4-5 drops of carbon tetrachloride. Then add about 0.5 mL of chlorine water and shake. Appearance of an orange-brown carbon tetrachloride layer indicates the presence of bromide. Formation of a purple layer indicates iodide.
 

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