K2O + H2S = K2S + H2O.
The latter is the more general opinion, and it receives support from the fact that when sulphuretted hydrogen is passed into the solution of a metallic salt, an insoluble precipitate of a sulphide of the metal is thrown down. Thus, when the gas is passed into a solution of cupric sulphate, the precipitate consists of hydrated cupric sulphide, the liberated sulphuric acid renders the liquid which was before neutral, acid. The larger number of sulphides so formed, combining with water at the instance of their precipitation, occur as hydrates.
There is also a class of sulphides known as hydrosulphides, sulphydrates, or double sulphides, in which an equivalent of the metal is replaced by an equivalent of hydrogen. Examples of these are the potassic hydrosulphide (KHS), sodic hydrosulphide (NaHS), and ammonic hydrosulphide (H4NHS). No such combinations occur with hydrogen and the metals of the earth proper, and of the iron group.
Tests. Many of the hydrosulphates or sulphides may be detected, by dropping on them some hydrochloric acid, when the characteristic smell of sulphuretted hydrogen will be immediately evolved from them. Very small quantities of a sulphide may be detected as follows:—Place the suspected sulphide in a small test tube, on the upper part of which is inserted a piece of blotting paper moistened with a solution of plumbic acetate, then carefully pour some hydrochloric acid on to the substance, when, if it be a sulphide, the
paper will become. immediately browned or blackened.
Many small quantities of the soluble sulphides are revealed in neutral or alkaline solutions by the rich purple colour which they form on the addition of a solution of sodic nitro-prusside. Most of them, when heated before the blow-pipe, give off the smell of sulphurous acid.
The quantitative determination of free sulphuretted hydrogen, or of a soluble sulphide in any solution, is conducted as follows:—The liquid to be tested is mixed with a small quantity of a cold solution of starch, made slightly acid with acetic acid. A solution of iodine of known strength, dissolved in potassic iodide is then added, until the liquid just begins to turn blue from the action of the excess of iodine on the starch. In this process the sulphuretted hydrogen converts the iodine into hydriodic acid, whilst sulphur is liberated.
Of course the quantity of sulphuretted hydrogen is calculated from the quantity of iodine employed. The reaction is—
2H2S + 2I2 = 4HI + S2.
The value of sulphuretted hydrogen as a reagent has already been alluded to. It throws down most of the metals from solutions of their salts in the form of insoluble sulphides; and each of the sulphides so produced in many cases being distinguished from the others by a special and characteristic colour. The sulphuretted hydrogen thus presents the metal in a form in which it can, in many instances, be easily and with certainty recognised. Thus sulphide of lead is black, of arsenic yellow, of antimony orange, of manganese salmon colour, and of zinc white. By means of sulphuretted hydrogen, also, the chemist is enabled to separate the metals into groups.