Identification of cations

part 2 of 12.5 Identification of ions and gases

How to identify cations with aqueous sodium hydroxide

• Add a few drops of sodium hydroxide solution

• Record any colour changes or precipitates formed

• Add excess sodium hydroxide solution

• Record any colour changes or changes to precipitates

How to identify cations with ammonia solution

• Add a few drops of ammonia solution

• Record any colour changes or precipitates formed

• Add excess ammonia solution

• Record any colour changes or changes to precipitates

aluminium

Effect of aqueous NaOH: white precipitate
Effect of excess aqueous NaOH: soluble in excess, giving a colourless solution
Effect of ammonia: white precipitate
Effect of excess ammonia: insoluble in excess

ammonium

ammonia gas produced on warming with aqueous sodium hydroxide, turns damp red litmus paper blue

calcium

Effect of aqueous NaOH: no precipitate or very slight white precipitate
Effect of excess aqueous NaOH: insoluble in excess
Effect of ammonia: no precipitate or very slight white precipitate
Effect of excess ammonia: insoluble in excess

chromium (III)

Effect of aqueous NaOH: green precipitate
Effect of excess aqueous NaOH: soluble in excess
Effect of ammonia: grey-green precipitate
Effect of excess ammonia: insoluble in excess

copper (II)

Effect of aqueous NaOH: pale blue precipitate
Effect of excess aqueous NaOH: insoluble in excess
Effect of ammonia: pale blue precipitate
Effect of excess ammonia: soluble in excess, giving a dark blue solution

iron (II)

Effect of aqueous NaOH/ ammonia: green precipitate
Effect of excess aqueous NaOH/ ammonia: insoluble in excess

*turns brown near surface on standing

iron (III)

Effect of aqueous NaOH/ ammonia: red-brown precipitate
Effect of excess aqueous NaOH/ ammonia: insoluble in excess

zinc

Effect of aqueous NaOH/ ammonia: white precipitate
Effect of excess aqueous NaOH/ ammonia: soluble in excess giving a colourless solution