Experiment: salt analysis, anions

How are anions identified in salt analysis?

Where this experiment comes from. The JEE Main 2027 and NEET (UG) 2026 syllabuses list the chemical principles of qualitative salt analysis in Unit 20, for the anions CO₃²⁻, S²⁻, SO₄²⁻, NO₃⁻, NO₂⁻, Cl⁻, Br⁻ and I⁻, with insoluble salts excluded. The NCERT textbooks do not describe the method, so this page follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 7. That unit also covers sulphite, acetate, oxalate and phosphate, which these syllabuses leave out.

In plain words: an acid pushes a weaker or more volatile acid out of its salt, and the gas or vapour that comes off identifies the anion. The wet tests then confirm it with a precipitate or a colour.

• Water extract or sodium carbonate extract. Confirmatory tests use a solution of the salt in water if it dissolves. If not, they use a sodium carbonate extract: boil 1 g of the salt with about 3 g of sodium carbonate and 15 mL of distilled water for about 10 minutes, cool and filter (Lab Manual, page 52). Carbonate itself is tested on the solid or its water solution, because the extract contains carbonate (page 52).

Dilute sulphuric acid group: CO_₃^²⁻, S^²⁻ and NO_₂^⁻

Preliminary test. Take 0.1 g of the salt and add 1–2 mL of dilute sulphuric acid. Watch in the cold, then warm, and identify any gas (Lab Manual, page 51):

• Carbonate: the CO₂ turns lime water milky, and more CO₂ clears it again, because soluble calcium hydrogencarbonate forms (Lab Manual, pages 52–53).
• Sulphide: H₂S blackens lead acetate paper (lead sulphide). To confirm, make the water extract alkaline with ammonium hydroxide, or use the sodium carbonate extract, and add sodium nitroprusside: a purple or violet colour (Lab Manual, pages 52–54).
• Nitrite:
  • acidify the water extract with acetic acid and add potassium iodide and starch: iodine is set free, and the starch turns blue;
  • or add sulphanilic acid and then 1-naphthylamine to the acidified extract: a red azo dye forms (Lab Manual, pages 52, 54–55).

These are the Lab Manual’s equations (pages 53–55).

• NCERT slip (Laboratory Manual, page 55): the sulphanilic acid and 1-naphthylamine test for nitrite is printed as the “Griss-Ilosvay test”. The test is named after Peter Griess, who first described the reaction in 1858 (Wikipedia, “Griess test”), so the name is spelt Griess. In an exam, write Griess test.

Concentrated sulphuric acid group: Cl^⁻, Br^⁻, I^⁻ and NO_₃^⁻

Preliminary test. If dilute acid gives nothing, add 3–4 drops of concentrated sulphuric acid to 0.1 g of the salt, watch in the cold, then warm (Lab Manual, page 56):

Confirmatory tests (Lab Manual, pages 57–61):

• Silver nitrate (sodium carbonate extract acidified with dilute HNO₃, or the water extract):
  • chloride gives a curdy white AgCl, soluble in ammonium hydroxide;
  • bromide gives a pale yellow AgBr, soluble in ammonium hydroxide only with difficulty;
  • iodide gives a yellow AgI, insoluble in ammonium hydroxide.
• Chlorine water and an organic layer: add 1 mL of carbon tetrachloride or chloroform and excess chlorine water, and shake. An orange-brown layer shows bromide, and a violet layer shows iodide (carbon disulphide or dichloromethane may be used instead).
• Chloride, with MnO₂: a pinch of MnO₂ and concentrated H₂SO₄ give greenish-yellow chlorine, which bleaches.
• Chloride, chromyl chloride test: heat the salt with solid potassium dichromate and concentrated H₂SO₄, and pass the vapour into NaOH solution, which turns yellow (chromate). Acetic acid and lead acetate then give a yellow precipitate of lead chromate. Use the smallest possible amount, to avoid pollution by chromium.
• Nitrate, brown ring test: mix 1 mL of the salt solution with 2 mL of concentrated H₂SO₄ and cool. Pour freshly prepared iron(II) sulphate solution down the side of the tube: a dark brown ring forms where the layers meet, from [Fe(NO)]SO₄.

These are the Lab Manual’s equations (pages 58–61).

Independent group: SO_₄^²⁻

• If neither acid gives a result, test for sulphate (Lab Manual, page 62). Acidify the water extract or sodium carbonate extract and add barium chloride: a white precipitate of BaSO₄, insoluble in concentrated HCl or concentrated HNO₃.
• With acetic acid and lead acetate sulphate gives a white precipitate of PbSO₄ (Lab Manual, pages 62–63).
• Which acid? The NCERT Lab Manual’s Table 7.5 (page 62) acidifies with dilute hydrochloric acid, but the chemistry section on page 63 says acetic acid. Dilute HCl is the safer choice: barium sulphite also forms a white precipitate, but it dissolves in dilute HCl (Lab Manual, page 54), so HCl leaves only the sulphate (reasoned). The Lab Manual’s own specimen record also acidifies with dilute HCl (page 85).

Confirmatory tests

Safety

• The Lab Manual’s margins (pages 53–61) give these warnings:
  • hydrogen sulphide, given off in the sulphide test, is marked flammable and toxic (page 53);
  • bromine is very toxic by inhalation and corrosive;
  • chlorine is toxic by inhalation;
  • iodine is harmful by inhalation and contact with the skin;
  • chloroform is harmful and toxic by inhalation;
  • chromyl chloride is marked oxidising and corrosive, and lead chromate harmful (page 59);
  • nitric acid is marked oxidising and corrosive (page 61);
  • barium compounds, potassium permanganate, manganese dioxide, silver nitrate and copper sulphate also carry hazard symbols.
• Concentrated sulphuric acid, heated with the salt in the second step, is corrosive: the same Laboratory Manual marks it so in Unit 11 (page 116). Always pour acid into water, never water into acid (Lab Manual, page 81).
• Carbon tetrachloride is also hazardous: NCERT notes some evidence that exposure to it causes liver cancer (NCERT Class 12 Chemistry, Unit 6, page 187).
• Do the gas tests in a fume cupboard (reasoned), and use every reagent in the smallest amount that works (Lab Manual, page 82).

Solved examples

Common mistakes to avoid

Quick revision summary

• Dilute H₂SO₄: CO₂ (carbonate; lime water milky), H₂S (sulphide; lead acetate black), NO₂ (nitrite; KI–starch blue).
• Concentrated H₂SO₄: HCl (chloride; white fumes with NH₃), Br₂ (bromide), I₂ (iodide), NO₂ with Cu (nitrate).
• Confirm: AgNO₃ (white, pale yellow, yellow); Cl₂ water + CCl₄ (orange-brown Br₂, violet I₂); chromyl chloride (Cl⁻); brown ring (NO₃⁻); nitroprusside (S²⁻).
• Sulphate: BaCl₂ in acid gives white BaSO₄, insoluble in acids.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why do bromides and iodides not give a test like the chromyl chloride test?

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   Hot concentrated sulphuric acid oxidises bromide and iodide to the free halogens, 2NaBr + 2H₂SO₄ → Br₂ + SO₂ + Na₂SO₄ + 2H₂O, and iodide likewise to I₂ (Lab Manual, pages 59–60). No chromyl compound distils over, so no chromium reaches the NaOH and lead acetate gives no yellow lead chromate: the test is specific to chloride (reasoned; compare Lab Manual Discussion Question (xiv), page 82). Bromide and iodide are oxidised so easily because the halide ions grow stronger reducing agents down group 17 (reasoned).

2. Q2. Why must the iron(II) sulphate solution for the brown ring test be freshly prepared, and why is the mixture cooled first?

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   The brown ring is [Fe(NO)]SO₄, formed from iron(II) sulphate and NO, so the iron must still be in the +2 state; an old solution may have been oxidised. The mixture is cooled because concentrated H₂SO₄ heats it as it mixes, and the brown compound needs to form in the cold, as the Lab Manual’s procedure does (Lab Manual, pages 58 and 61; reasoned).