Tests for negative ions
The presence of negative ions can be determined by performing a number of different tests.
Bromide (Br−): addition of silver nitrate solution to bromide solution immediately yields a whitish precipitate of silver bromide, which is partially soluble in concentrated ammonia solution, for example:
KBr + AgNO3 AgBr + KNO3
Carbonate (CO32−): a solid carbonate treated with dilute hydrochloric acid gives off carbon dioxide gas, which turns limewater milky:
CaCO3 + 2HCl CaCl2 + H2O + CO2
Chloride (Cl−): treatment of a chloride with concentrated sulphuric acid produces colourless hydrogen chloride gas, which forms thick white fumes of ammonium chloride on mixing with gaseous ammonia:
NH3 + HCl
⇌
NH4Cl(s)
Hydrogen carbonate (HCO3−): heating a solution of a hydrogen carbonate produces carbon dioxide, which turns limewater milky:
Ca(HCO3)2 CaCO3 + H2O + CO2
Hydrogen carbonates react with dilute hydrochloric acid giving off carbon dioxide, in a similar way to carbonates.
Iodide (I−): on addition of silver nitrate solution to an acidified solution of an iodide, a yellow precipitate of silver iodide is formed immediately, which is insoluble in ammonia solution:
KI + AgNO3 AgI + KNO3
Nitrate (NO3−): there are two tests for the nitrate ion in solution.
Sodium hydroxide solution and aluminium powder (or Devarda's alloy, which contains aluminium) are added to a solution of the nitrate. The mixture is warmed and the ammonia gas produced turns red litmus paper blue:
3NO3− + 5OH− + 2H2O + 8Al 3NH3 + 8AlO2−
The brown ring test: an equal volume of iron(II) sulphate solution (acidified with dilute sulphuric acid) is added to the nitrate solution in a test tube. Concentrated sulphuric acid is carefully poured down the side of the test tube, so that it forms a separate layer at the bottom of the tube. A brown ring is formed at the junction of the two layers. This is FeSO4.NO, which is produced by the reduction of nitrate ions to nitrogen monoxide by the iron(II) ions:
NO3− + 4H+ + 3Fe2+ NO(g) + 3Fe3+ + 2H2O
Care should be taken with this test, as nitrites and bromides can give similar results.
Nitrite (NO2−): addition of dilute sulphuric acid to a nitrite produces brown nitrogen dioxide gas, which turns blue litmus paper red without bleaching it. The solution turns pale blue. No heating is required.
Sulphate (SO42−): addition of dilute hydrochloric acid and barium chloride solution to a solution of a sulphate results in the immediate precipitation of barium sulphate:
Na2SO4 + BaCl2 BaSO4 + 2NaCl
Sulphide (S2−): addition of dilute hydrochloric acid to a sulphide results in the production of colourless hydrogen sulphide gas, which smells of rotten eggs and turns lead nitrate (soaked into filter paper) black.
Na2S + 2HCl 2NaCl + H2S
Sulphite (SO32−): addition of dilute hydrochloric acid to a sulphite, with heating, produces colourless sulphur dioxide gas. This turns potassium dichromate from orange to green, but does not change the colour of lead nitrate solution.
K2SO3 + 2HCl 2KCl + SO2 + H2O
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