Detection and Estimation of Hydrogen Sulphide

Even when only small quantities are present in a gaseous mixture or in aqueous solution, free hydrogen sulphide can be detected readily by its odour (a concentration of 1 in 700,000 is detectable), by its effect on paper moistened with lead acetate solution and by the "methylene blue" reaction. The last test, by which it is possible to detect less than one milligramme of the substance in 40 litres of water, is applied by adding to the solution (obtained, if necessary, by bubbling the suspected gas through water) one-fiftieth of its volume of concentrated hydrochloric acid, a very little p-aminodimethylaniline sulphate, and then two or three drops of dilute ferric chloride solution; in the presence of hydrogen sulphide the liquid becomes coloured, from the formation of Methylene Blue, an organic dye containing sulphur. In the presence of alkali, hydrogen sulphide reacts with a dilute solution of sodium nitroprusside to give an intense purple coloration. In the presence of a sulphide (soluble or insoluble), sodium azide and iodine react in solution with vigorous evolution of nitrogen,

2NaN₃ + I₂ = 2NaI + 3N₂,

and the reaction is claimed to be more sensitive as a test for sulphide than any of the tests already mentioned; thiosulphates, tri-, tetra- and penta-thionates also catalyse the reaction, however, but not dithionates, sulphites, selenides, tellurides, arsenides or antimonides, nor does free sulphur. The test is useful in detecting sulphides in minerals and hydrogen sulphide in natural waters. Other tests which are less satisfactory are based on the blackening of metallic silver and on the reduction of weak iodine solution (plus starch) or of potassium ferricyanide in the presence of ferric chloride; in the former case the blue colour of the solution is bleached, whilst in the latter the solution becomes coloured with Prussian Blue.

When present in considerable quantity in a gaseous mixture, hydrogen sulphide can be estimated by the increase in weight of suitable absorption tubes, containing, for example, moist granulated salts of lead or copper. More convenient, however, is the oxidation of the gas to sulphuric acid by passage through bromine water or other suitable oxidising agent, when the quantity of sulphuric acid may subsequently be determined either volumetrically or gravimetrically; the same process obviously may be applied to aqueous solutions of hydrogen sulphide. Another type of method is based on the conversion of the hydrogen sulphide into some insoluble sulphide such as that of lead, copper or mercury, when the amount of metallic sulphide may be determined by oxidation of its sulphur to sulphuric acid; if only traces of hydrogen sulphide are under examination, this type of method may be applied colorimetrically.

Hydrogen sulphide is sometimes estimated volumetrically by treating a solution containing less than 0.04 per cent, with excess of standard iodine and then after a short period titrating the residual iodine in the usual manner with sodium thiosulphate or arsenious oxide; accurate direct titration is not possible:

H₂S + I₂ = 2HI + S.

A modification of this method is to effect the oxidation by a known quantity of bromine (obtained by the addition of standard potassium bromate solution together with potassium bromide and hydrochloric acid) and subsequently add excess of potassium iodide; the superfluous bromine liberates a corresponding amount of iodine, which can be estimated titrimetrically; hence the bromine consumed and the hydrogen sulphide oxidised can be calculated. The chemical change in this case is:

H₂S + 4H₂O + 4Br₂ = 8HBr + H₂SO₄.

The sulphur in sulphides from which hydrogen sulphide can be liberated by acids can naturally be estimated by suitable application of the foregoing methods.

An accurate electrometric method applicable to soluble sulphides consists in precipitating as silver sulphide in alkaline solution by titration with standard ammoniacal silver solution. The change of E.M.F. at the end-point is considerable. The method is satisfactory in the presence of sulphite, sulphate, thiosulphate, polysulphide or chloride.

Insoluble sulphides must be examined by other processes of which examples are given under Sulphur.