[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 distillation 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 composition HClO4,2H2O 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 HClO4,H2O which melts at 50°. On carefully heating this hydrate it breaks up into perchloric acid, which distills over below 100°, and into the liquid hydrate HClO4,2H2O. The acid HClO4 may also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate, carefully distilling and subjecting the crystals of the hydrate HClO4,H2O obtained in the distillate to a fresh distillation. Perchloric acid, HClO4, itself does not distil, and is decomposed on distillation until the more stable hydrate HClO4, H2O is formed; this decomposes into HClO4 and HClO4,2H2O, which latter hydrate distils without decomposition. This forms an excellent example of the influence of water on stability, and of the property of chlorine of giving compounds of the type ClX7, of which all the above hydrates, ClO3(OH), ClO2(OH)3, and ClO(OH)5, are members. Probably further research will lead to the discovery of a hydrate Cl(OH)7.

[46] According to Roscoe the specific gravity of perchloric acid = 1·782 and of the hydrate HClO4,H2O in a liquid state (50°) 1·811; hence a considerable contraction takes place in the combination of HClO4 with H2O.

[47] The decomposition of salts analogous to potassium chlorate has been more fully studied in recent years by Potilitzin and P. Frankland. Professor Potilitzin, by decomposing, for example, lithium chlorate LiClO3, found (from the quantity of lithium chloride and oxygen) that at first the decomposition of the fused salt (368°) takes place according to the equation, 3LiClO3 = 2LiCl + LiClO4 + 5O, and that towards the end the remaining salt is decomposed thus: 5LiClO3 = 4LiCl + LiClO4 + 10O. The phenomena observed by Potilitzin obliged him to admit that lithium perchlorate is capable of decomposing simultaneously with lithium chlorate, with the formation of the latter salt and oxygen; and this was confirmed by direct experiment, which showed that lithium chlorate is always formed in the decomposition of the perchlorate. Potilitzin drew particular attention to the fact that the decomposition of potassium chlorate and of salts analogous to it, although exothermal (Chapter III., Note [12]), not only does not proceed spontaneously, but requires time and a rise of temperature in order to attain completion, which again shows that chemical equilibria are not determined by the heat effects of reactions only.

P. Frankland and J. Dingwall (1887) showed that at 448° (in the vapour of sulphur) a mixture of potassium chlorate and powdered glass is decomposed almost in accordance with the equation 2KClO3 = KClO4 + KCl + O2, whilst the salt by itself evolves about half as much oxygen, in accordance with the equation, 8KClO3 = 5KClO4 + 3KCl + 2O2. The decomposition of potassium perchlorate in admixture with manganese peroxide proceeds to completion, KClO4 = KCl + 2O2. But in decomposing by itself the salt at first gives potassium chlorate, approximately according to the equation 7KClO4 = 2KClO3 + 5KCl + 11O2. Thus there is now no doubt that when potassium chlorate is heated, the perchlorate is formed, and that this salt, in decomposing with evolution of oxygen, again gives the former salt.

In the decomposition of barium hypochlorite, 50 per cent. of the whole amount passes into chlorate, in the decomposition of strontium hypochlorite (Potilitzin, 1890) 12·5 per cent., and of calcium hypochlorite about 2·5 per cent. Besides which Potilitzin showed that the decomposition of the hypochlorites and also of the chlorates is always accompanied by the formation of a certain quantity of the oxides and by the evolution of chlorine, the chlorine being displaced by the oxygen disengaged. Spring and Prost (1889) represent the evolution of oxygen from KClO3 as due to the salt first splitting up into base and anhydride, thus (1) 2MClO3 = M2O + Cl2O5; (2) Cl2O5 = Cl2 + O3; and (3) M2O + Cl = 2MCl + O.

I may further remark that the decomposition of potassium chlorate as a reaction evolving heat easily lends itself for this very reason to the contact action of manganese peroxide and other similar admixtures; for such very feeble influences as those of contact may become evident either in those cases (for instance, detonating gas, hydrogen peroxide, &c.), when the reaction is accompanied by the evolution of heat, or when (for instance, H2 + I2, &c.) little heat is absorbed or evolved. In these cases it is evident that the existing equilibrium is not very stable, and that a small alteration in the conditions at the surfaces of contact may suffice to upset it. In order to conceive the modus operandi of contact phenomena, it is enough to imagine, for instance, that at the surface of contact the movement of the atoms in the molecules changes from a circular to an elliptical path. Momentary and transitory compounds may he formed, but their formation cannot affect the explanation of the phenomena.

[47 bis] See, for example the melting point of NaCl, NaBr, NaI in Chapter II. Note [27]. According to F. Freyer and V. Meyer (1892), the following are the boiling points of some of the corresponding compounds of chlorine and bromine:

BCl317°BBr390°
SiCl359°SiBr4153°
PCl376°PBr3175°
SbCl3223°SbBr3275°
BiCl3447°BiBr3453°
SnCl4606°SnBr4619°
ZnCl2730°ZnBr2650°

Thus for all the more volatile compounds the replacement of chlorine by bromine raises the boiling point, but in the ease of ZnX2 it lowers it (Chapter XV. Note 19).

[48] Even before free fluorine was obtained (1886) it was evident from experience gained in the efforts made to obtain it, and from analogy, that it would decompose water (see first Russian edition of the Principles of Chemistry).