Detection of Iron

Iron readily lends itself both to detection and estimation. In this chapter the more important methods of detecting and estimating iron in its compounds are discussed.

Iron salts, when moistened with hydrochloric acid and heated on a loop of platinum wire in a Bunsen flame, emit a shower of sparks. When heated on charcoal with sodium carbonate in the blowpipe flame, the compound is converted into a dark-coloured residue. If potassium cyanide is added to the sodium carbonate and iron compound, and the whole heated on charcoal in the inner flame of the blow-pipe, metallic iron is obtained as a grey, magnetic powder.

If a little of the iron compound is heated with borax on platinum wire in the outer blow-pipe flame, on cooling, a yellow transparent bead is obtained, which becomes bottle-green upon heating in the inner or reducing flame of the blow-pipe, in consequence of reduction to the ferrous condition.

The presence of iron in solution may readily be detected by a considerable number of sensitive reactions. Thus ferrous iron gives a green precipitate of ferrous hydroxide upon addition of excess of ammonium hydroxide. With potassium ferricyanide and a trace of acid, a deep blue precipitate - Turnbull's blue - is obtained. With potassium ferrocyanide a white precipitate is obtained in the entire absence of any ferric salt. Ferric iron, on the other hand, is usually characterised by its deep yellow or brown colour. Addition of concentrated hydrochloric acid deepens the colour. With excess of ammonium hydroxide, brown flocculent ferric hydroxide is precipitated. With potassium ferrocyanide solution, a deep blue colour is obtained in acid solution, whilst with potassium ferricyanide there is no action. Potassium thiocyanate gives in acid solution a deep red colour, which is not destroyed by heat. Salicylic acid gives a violet colour, provided no free mineral acid is present.

It is usual, in systematic qualitative analysis, to remove silver, lead, and mercury from the solution to be analysed, by precipitation as chlorides on addition of hydrochloric acid. The metals capable of precipitation as sulphides with hydrogen sulphide in acid solution are next removed, and the filtrate, boiled to expel any dissolved hydrogen sulphide, is treated with ammonium chloride and excess of ammonium hydroxide. In the absence of phosphates, the precipitate may contain the hydroxides of iron, aluminium, and chromium. The presence of iron is evidenced by the brown colour, if the metal is in the ferric condition. In any case, the precipitate is treated with sodium peroxide and water, whereby any aluminium is dissolved as sodium aluminate, any chromium as sodium chromate, the iron remaining as insoluble ferric hydroxide. If desired, a confirmatory test may be applied. For example, the precipitate may be dissolved in hydrochloric acid and potassium ferrocyanide added. The characteristic blue colour indicates iron.

If a phosphate was present, the precipitate obtained on addition of ammonium chloride and hydroxide may contain phosphates of such metals as calcium, etc., which are normally precipitated in later groups in systematic analysis. The precipitate is therefore dissolved in dilute hydrochloric acid, and the solution nearly neutralised with sodium carbonate. Sodium acetate is now added, and the whole boiled. The precipitate contains the phosphates of aluminium, chromium, and iron, and is treated, as already indicated, with sodium peroxide and water.