Note 21 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

Sulphuretted hydrogen does not saturate the alkaline properties of alkali hydroxides, so that a solution of potassium hydroxide will not under any circumstances give a neutral liquid with sulphuretted hydrogen. In this case the sulphuretted hydrogen forms in solution only an acid salt with the potassium: KHO + H2S = KHS + H2O. It must be supposed that the normal salt is not formed in the solution—that is, that the reaction 2KHO + H2S = K2S + 2H2O does not take place. This is seen from the fact that a development of heat, depending on the formation of potassium hydrosulphide, KHS, is remarked when as much hydrogen sulphide is passed into a solution of potassium hydroxide as it will absorb. But if a further quantity of potassium hydroxide be added to the resultant solution, heat is not developed, whilst if alkali be added to potassium acid sulphate or sodium acid carbonate, heat is developed. It must not be concluded from this that H2S is a monobasic acid, for here there is a question of the decomposing action of water upon K2S; K2S and H2O in reacting on each other should absorb heat if the reaction of KHS upon KHO evolves heat. Furthermore, it must be taken into account that potassium oxide, K2O, and the anhydrous oxides like it, also do not exist in solutions, for whenever they are formed they immediately react with the water, forming caustic potash, KHO, &c. In the same way, directly potassium sulphide, K2S, is formed in water it is decomposed into potassium hydroxide and hydrosulphide: K2S + H2O = KHO + KHS. Potassium sulphide, K2S, in a solid state corresponds with K2O, although neither can exist in solution.