1902.] 



Per sulphuric Acids. 



95 



Only the anhydride, S2O7, was isolated by Berthelot, but he con- 

 cluded that the corresponding acid was formed (1) on dissolving 

 the anhydride in water, (2) on electrolysing strong solutions of sul- 

 phuric acid, and (3) by the interaction of hydrogen peroxide and 

 ordinary sulphuric acid. The correctness of these conclusions appears 

 to have been regarded as beyond question after Marshall had dis- 

 covered in 1891 that well-defined salts of "Berthelot's acid " could be 

 prepared by electrolysing solutions of potassium or ammonium hydrogen 

 sulphate. Doubt arose, however, when Caro, in 1898, discovered that 

 if Marshall's salts were acted on by sulphuric acid, a new acid was 

 obtained having properties markedly different from those associated 

 with Berthelot's acid. " Caro's acid " soon acquired importance as an 

 oxidising agent, owing to the good use that was made of it by Bam- 

 berger and by von Baeyer and Villiger. 



Having found that Caro's acid liberated iodine very rapidly whereas 

 Berthelot's acid acted but slowly on iodides, these latter chemists were 

 able to devise a process by which the one acid could be estimated in 

 presence of the other. By removing sulphuric acicl by means of 

 barium phosphate, they obtained a solution containing Caro's acid and 

 Berthelot's acid, in which they determined the amount of each of these 

 substances as well as the amount of sulphate to which the persulphuric 

 acids gave rise when decomposed. Their results led them to conclude 

 that the ratio of sulphur to active oxygen in Caro's acid was 



S0 3 : = 1 : 1. 



Placing the simplest possible interpretation upon this result, they 

 assigned to the acid the formula — 



H 2 S0 5 = H2CVSO3.* 



Meanwhile, the problem had been studied from a somewhat different 

 point of view by Lowry and West, f who had determined the equilibrium 

 subsisting between hydrogen peroxide and "persulphuric acid" in 

 presence of sulphuric acid and water, and had found that the ratio 

 which the hydrogen peroxide bore to total " persulphuric acid " was 

 entirely dependent on the ratio which the water bore to the sulphuric 

 acid — the ratio of hydrogen peroxide to " persulphuric acid " being 

 ultimately the same in a mixture prepared from hydrogen peroxide and 

 sulphuric acid as in a solution of equal strength prepared by electro- 

 lysis. The experimental curve approximated very closely to a curve 

 deduced from an equation of the fourth order, and assuming that the 

 chief product of interaction was a persulphuric acid of the series 

 ELCV^SOg, it was to be supposed that it was the fourth term of the 



VOL. LXX. 



* 'Ber. Dent. Chem. Ges.,' 1901, 853. 

 f 'Chem. Soc. Trans.,' 1900, 950. 



H 



