|Detection of Sulphur in the free state is performed by its general appearance and characteristics, and especially by its combustion to sulphur dioxide. Both in mixtures and compounds the presence of the element can be demonstrated by heating with charcoal and an alkali carbonate, or even better, on a small scale, by heating with an equal bulk of sodium or potassium, or with powdered iron; in each case some of the sulphur is converted into sulphide, which may be detected by the action of an aqueous extract on mercury or silver, or on sodium nitroprusside; the metals are blackened, whilst the nitro-prusside is very sensitive in giving a purple coloration. Alternatively, the solution of the alkali sulphide may be acidified and tests applied for hydrogen sulphide to the vapours evolved on warming. |
Various microchemical tests - are available for the detection of minute quantities of sulphur, both free and combined. The substance under examination may be treated with a little sodium hydroxide solution, extract evaporated just to dryness, a few drops of aqueous sodium cyanide (0.1 per cent.) added and the evaporation repeated. The residue, moistened with dilute sulphuric acid and a drop of ferric chloride, gives the characteristic ferric thiocyanate colour if Detection of Sulphur was Successful (sulphur is present). In the case of minerals, traces of sulphur dioxide produced on heating may be detected by the colour change of an alkaline solution of Bromocresol Green or by the decolorisation of starch-iodine solution.
A micro-test for organically combined sulphur in plants consists in exposing the material to bromine vapour for several hours in order to bring about oxidation. After removing excess of bromine by means of ammonia or by volatilisation, the sulphate is precipitated by calcium chloride, the crystalline precipitate being examined on the following day.