Ions are produced during electrolysis, for example the salt zinc chloride (ZnCl₂) dissociates into the positively-charged Zn²⁺ and negatively-charged Cl⁻ when electrolysed.

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

tables

flame test colours

essays

Molecular and Ionic Compounds

Periodic table: reactivity in metals and non-metal groups

pH scale

Titration method

animations

electrolysis of aluminium oxide