ion

Atom, or group of atoms, that is either positively charged (cation) or negatively charged (anion), as a result of the loss or gain of electrons during chemical reactions or exposure to certain forms of radiation. In solution or in the molten state, ionic compounds such as salts, acids, alkalis, and metal oxides give rise to mobile ions and therefore conduct electricity. These compounds are known as electrolytes.

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

Ion

In Greek mythology, the son of the god Apollo and Creusa, daughter of King Erechtheus of Athens; ancestor of the Ionian or eastern Greeks. In Ion, by the Greek dramatist Euripides, he was carried away as a baby to Delphi in Boeotia, but later reunited with his mother and stepfather Xuthus.

Ion (c. 490–c. 421 BC)

Greek poet of Chios, who also lived at Athens. His first tragedy was produced between 452 and 449 BC and he subsequently gained prizes for both his tragic works and dithyrambs (choral hymns). He is the reputed author of a philsophical treatise on the mystic number three, and is credited with various epigrams, paeans (hymns of thanksgiving), elegies, encomia (eulogies), and comedies. He also wrote five historical or biographical works, including an account of the antiquities of Chios.