Estimation of Platinum

Platinum may be estimated by acidulating the solution of its salts with hydrochloric acid and passing hydrogen sulphide through. The platinum is precipitated as disulphide, ignited and weighed as metal. The precipitated sulphide is difficult to filter. Gaze recommends addition of mercuric chloride to the solution, whereby the platinum sulphide comes down with mercuric sulphide in a manageable form. The latter disappears by volatilisation during ignition.

An obvious disadvantage lies in the fact that the bulk of the precipitate is increased, and of course the mercuric vapours evolved upon ignition are poisonous. For these reasons magnesium chloride has been recommended, since the presence of this salt in solution assists precipitation of the platinum sulphide, which is obtained in a pure state upon washing with acidulated water.

Platinum is conveniently estimated by almost neutralising an acid solution of its salts with ammonia, and concentrating until crystallisation sets in. Sufficient water is now added to just dissolve the crystals, and excess of saturated ammonium chloride solution. After thorough mixing an excess of alcohol is added, and the whole allowed to stand for twenty-four hours, after which the precipitate of ammonium chlor-platinate, (NH₄)₂PtCl₆, is collected on asbestos in a crucible, washed with 80 per cent, alcohol, and either weighed as such after drying, or ignited in a current of hydrogen and weighed as metallic platinum.

A useful volumetric method consists in adding platinic chloride or an alkali chlor-platinate to a cold concentrated solution of potassium iodide. The iodine liberated, according to the equation

PtCl₄ + 4KI = PtI₂ + 4KCl + I₂,

is estimated by titration with sodium thiosulphate in the usual way, with starch as indicator.

Colorometric Methods have been recommended.

The purity of a sample of platinum may be checked by determining the thermo-electric force of a couple consisting of the sample and a known standard platinum alloy.

A Microchemical Method of estimating traces of platinum in gold and silver is described by van Breukeleveen.