THK ii A LOO F.NS 



As the salts of chloric acid, HC1O 3 , are produced by the oxidation of 

 the salts of hypochlorous acid, so, in a similar manner, the salts of per- 

 chloric acid, HC1O 4 , are produced by the oxidation of the salts of chloric 

 acid, HC1O 3 . But this is the highest form of the oxidation of HC1. 

 Perchloric acid, HC1O 4 , is the most stable of all the acids of chlorine. 

 When fused potassium chlorate begins to swell up and solidify, after 

 having parted with one-third of its oxygen ; then potassium chloride 

 and potassium perchlorate are formed, 2KClO 3 = KClO 4 + KCl-f O 2 . 



The formation of this salt is clearly remarked in the preparation- 

 of oxygen from potassium chlorate, owing to the fact that the potas- 

 sium perchlorate fuses with greater difficulty than the chlorate, and 

 therefore appears in the molten salt as solid grains. (Under the action, 

 of certain acids for instance, sulphuric and nitric potassium chlorate 

 also gives potassium perchlorate.) It may be easily purified, because it 

 is but sparingly soluble in water, although all the other salts of per- 

 chloric acid are very soluble and even effloresce in the air. It is a 

 remarkable fact that the perchlorates, although they contain mor& 

 oxygen than the chlorates, are decomposed with greater difficulty, and, 

 even when thrown on ignited charcoal, give a much feebler deflagration 

 than the chlorates. Sulphuric acid (at a temperature not below 100) 

 evolves volatile and, to a certain extent, stable perchloric acid from 

 potassium perchlorate. Neither sulphuric nor any other acid will 

 further decompose perchloric acid as it decomposes chloric acid. Of 

 all the acids of chlorine, perchloric acid alone can be distilled. 44 The 

 pure hydrate HC1O 4 45 is a colourless and exceedingly caustic substance 



44 If a solution of chloric acid, HC1O 5 , be first concentrated over sulphuric acid under 

 the receiver of an air-pump and afterwards distilled, then chlorine and oxygen are evolved 

 and perchloric acid formed: 4HC1O 5 = 2HC1O 4 + C1 3 + 8O + H 2 O. Eoscoe accordingly 

 directly decomposed a solution of potassium chlorate by hydrofluosilicic acid, decanted it 

 from the precipitate of potassium silicofluoride, K.>SiF 6 , concentrated the solution of chloric 

 acid, and then distilled it, perchloric acid being then obtained (see following footnote). 

 That chloric acid is capable of passing into perchloric acid is also seen from the fact that 

 potassium permanganate is decolorised, although slowly, by the action of a solution of 

 chloric acid. On decomposing a solution of potassium chlorate by the action of an elec- 

 tric current, potassium perchlorate is obtained at the positive electrode (where the oxygen 

 is evolved). Perchloric acid is also formed by the action of an electric current on solu- 

 tions of chlorine and chlorine monoxide. Perchloric acid was obtained by Count Stadion 

 and afterwards by Serullas, and was studied by Roscoe and others. 



45 Perchloric acid, which is obtained in a free state by the action of sulphuric acid on 

 its salts, may be separated from a solution very easily by distillation, because it is volatile, 

 although it is partially decomposed by distillation. The solution obtained after distilla- 

 tion may be concentrated by evaporation in open vessels. In the distillation the solution 

 reaches a temperature of 200, and then a very constant liquid hydrate of the composi- 

 tion HC1O 4 ,2H 2 O is obtained in the distillate. If this hydrate be mixed with sulphuric 

 acid, it begins to decompose at 100, but, nevertheless, a portion of the acid passes over 

 into the receiver without decomposing, forming a crystalline hydrate HC1O 4 ,H.,O which 

 melts at 50. On carefully heating this hydrate it breaks up into perchloric acid, which 



