98 



Persulphuric Acids. 



[Mar. 13, 



which indicates that the ratio Ci/C 2 of persulphuric oxygen to peroxide 

 oxygen depends not only on the ratio C3/C4 of sulphuric acid to water 

 and on the concentration C^ of the water in the solution, but also on 

 the actual concentration Co of the hydrogen peroxide. The fact that 

 the equilibrium is independent of total oxidising power can only be 

 explained if the chief products of interaction are members of the series 

 HoCV^SOs, or hydrates thereof. 



As there was no sufficient evidence to justify the assumption that a 

 third persulphuric acid was present in the solutions they examined, 

 Lowry and West had no alternative but to regard Caro's acid as 

 pertetrasulphuric acid, and their simpler member of the series as the 

 acid corresponding to Marshall's salts. The determination of the 

 ratio of sulphur to active oxygen by Baeyer and Villiger has rendered 

 such a limitation impracticable, and we now feel not only that it is 

 justifiable but that we are compelled to postulate the existence of at 

 least three persulphuric acids, viz. : — 



Pertetrasulphuric acid S0 3 : = 4 : 1 



Perdisulphuric acid S0 3 : = 2 : 1 



Peranhydrosulphuric acid (Caro's acid) S0 3 : = 1 : 1 



In carrying out his experiments, Mr. Cook nearly neutralised solu- 

 tions of Caro's acid by means of a carbonate and then neutralised the 

 liquid by means of either sulphuric acid or caustic soda, portions being 

 taken out and tested with methyl orange. Measured portions of the 

 neutral solution were run as quickly as possible into flasks contain- 

 ing a little dilute sulphuric acid to arrest decomposition ; the per- 

 sulphuric acids were then estimated by Baeyer and Villiger 's method. 

 Portions of the same solution were heated at 100° until all oxidising 

 power was lost, and the acidity developed was estimated by caustic 

 soda, using methyl orange as indicator. 



Solution I. — This was prepared by digesting potassium persulphate 

 at 60 — 70° with a solution containing only 10 per cent, of sulphuric 

 acid. At the end of about 2 hours the cooled solution was diluted 

 and neutralised with chalk. The amount of iodine liberated at once 

 by the Caro's acid present, expressed in terms of decinormal thio- 

 sulphate solution, was equivalent to 41-54 c.c. ; the amount of iodine 

 liberated slowly by the perdisulphuric acid present was equivalent to 

 1*05 c.c. The acid liberated on warming the solution was equivalent 

 to 22-06 c.c. The calculated amount, assuming the ratio 20 : S0 3 , 

 would be 1-05 + 141*54 = 21*82 c.c. 



In the subsequent experiments 15 grammes of potassium persulphate 

 was digested with from 35 to 25 c.c. of concentrated sulphuric acid at 

 the ordinary temperature during 1 — 2 hours. The solution was then 

 diluted with ice and neutralised : in Experiments 2, 3, 4 and 6 with 

 chalk, in 5 with sodium bicarbonate, and in 7 with sodium carbonate, 



