Note 27 bis — Chapter IV · OZONE AND HYDROGEN PEROXIDE—DALTON'S LAW

It should be mentioned that Schiloff (1893) on taking a 3 per cent. solution of H2O2, adding soda to it, and then extracting the peroxide of hydrogen from the mixture by shaking it with ether, obtained a 50 per cent. solution of H2O2, which, although perfectly free from other acids, gave a distinctly acid reaction with litmus. And here attention should first of all be turned to the fact that the peroxides of the metals correspond to H2O2, like salts to an acid, for instance, Na2O2 and BaO2, &c. Furthermore, it must be remembered that O is an analogue of S (Chapters XV. and XX.), and sulphur gives H2S, H2SO3, and H2SO4. And sulphurous acid, H2SO3, is unstable as a hydrate, and gives water and the anhydride SO2. If the sulphur be replaced by oxygen, then instead of H2SO3 and SO2, we have H2OO3 and OO2. The latter is ozone, while the salt K2O4 (peroxide of potassium) corresponds to the hydrate H2O4 as to an acid. And between H2O and H2O4 there may exist intermediate acid compounds, the first of which would be H2O2, in which, from analogy to the sulphur compounds, one would expect acid properties. Besides which we may mention that for sulphur, besides H2S (which is a feeble acid), H2S2, H2S3, H2S5 are known. Thus in many respects H2O2 offers points of resemblance to acid compounds, and as regards its qualitative (reactive) analogies, it not only resembles Na2O2, BaO2, &c., but also persulphuric acid HSO4 (to which the anhydride S2O7 corresponds) and Cu2O7, &c., which will be subsequently described.