Lead chromate, PbCrO 4, occurs native as the mineral crocoisite, and may be obtained as an amorphous pale yellow solid by precipitating a soluble lead salt by an alkaline chromate.
It is a strong reducing agent, giving a precipitate of cuprous oxide from alkaline copper solutions at ordinary temperature, converting mercuric chloride to mercurous chloride, and precipitating metallic silver from solutions of silver salts.
Two series of synthetic hydrates were recognized by Muck and Tommasi: the " red " hydrates, obtained by precipitating ferric salts with alkalis, and the " yellow " hydrates, obtained by oxidizing moist ferrous hydroxide or carbonates.
It is also prepared by precipitating a ferric salt with ammonium sulphide; unless the alkali be in excess a mixture of ferrous sulphide and sulphur is obtained.
Ferri hydroxidum (U.S.P.), the hydrated oxide of iron, made by precipitating ferric sulphate with ammonia, is used solely as an antidote in arsenical poisoning.