1890-91.] Dr H. Marshall on Potassium Persulphate. 
63 
Note on Potassium Persulphate. 
By Hugh Marshall, D.Sc. 
(Read February 16, 1891.) 
Persulphuric anhydride and the corresponding acid have been 
known for some time. Berth elot obtained the former by subjecting 
a mixture of sulphurous anhydride and oxygen to the effluve electrique 
(as in the preparation of ozone), and a mixture of the latter with 
sulphuric acid, by adding the anhydride to water. He also pre- 
pared a similar mixture by the electrolysis of sulphuric acid solu- 
tion in a cell where the electrodes were separated by a porous pot. 
Both substances he found to he very easily decomposed, spon- 
taneously evolving oxygen. Up till now, however, the correspond- 
ing salts have not been prepared. In fact, Mendeleef, while 
commenting on Berthelot’s results, expresses the opinion that per- 
sulphuric anhydride is not a true acid-forming oxide, hut a peroxide 
similar to those of the metals barium, lead, &c., and that Berthelot’s 
persulphuric acid is analogous to peroxide of hydrogen. Becently, 
however, I have obtained the potassium salt, and have since 
succeeded in preparing it in quantity. 
While oxidising a solution of cobaltous sulphate in presence of 
potassium sulphate and sulphuric acid, by electrolysing it in a 
divided cell, as in Berthelot’s experiment, I obtained a quantity of 
white feathery crystals. These were filtered off, washed with cold 
water, and dried on porous plate over sulphuric acid. The sub- 
stance was found to possess powerful oxidising properties. When 
heated it fused and soon decomposed, evolving acid fumes and 
leaving a white residue which proved to be potassium sulphate. A 
solution of the substance gave only a faint precipitate with barium 
chloride solution, but on boiling a dense precipitate of barium 
sulphate separated gradually while chlorine was simultaneously 
evolved. These properties .seemed to point to the salt being a per- 
sulphate, and analysis confirmed this opinion. 
A known quantity was ignited and the resulting sulphate of 
potassium weighed. The residue amounted to 64*2 per cent, of the 
original. For potassium persulphate theory requires 64'4. The 
