THE HALOGENS 485 



perchloric acid) HC10 4 , are produced from 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, potassium chloride and potassium 

 perchlorate have been formed according to the equation 2KC10 3 

 =KC10 4 + KC1 + 2 . 



The formation of this salt is easily observed in the preparation 

 t>f 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 (see Chapter III. 

 Note 12). Under the action of certain acids for instance, sulphuric 

 and nitric potassium chlorate also gives potassium perchlorate. This 

 latter may be easily purified, because it is but sparingly soluble in water, 

 although all the other salts of perchloric acid are very soluble and even 

 deliquesce in the air. The perchlorates, although they contain more 

 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 HC10 4 45 is a colourless arid exceedingly caustic substance 



44 If a solution of chloric acid, HC10 3 , be first concentrated over sulphuric acid under 

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

 and perchloric acid is formed : 4HC1O 3 =2HC10 4 + C1 2 4-8O + H 2 O. Roscoe accordingly 

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

 it from the precipitate of potassium silicofluoride, K 2 SiF 6) concentrated the solution of 

 chloric acid, and then distilled it, perchloric acid being then obtained (see following foot- 

 note). 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 electric 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 solutions 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, being 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 HC10 4 ,H 2 O which 

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

 distils over below 100, and into the liquid hydrate HC1O 4 ,2H 2 0. The acid HC10 4 may 



