30 RESEARCHES ON THE 



ACID SULPHATE OF PURPUREOCOBALT. 



Our efforts to obtain a neutral sulphate of Purpureocobalt containing two equiva- 

 lents only of sulphuric acid have hitherto been fruitless. When a solution of 

 chloride of Purpureocobalt is treated with sulphate of silver, chloride of silver is 

 formed, and the red supernatant liquid yields, on evaporation, crystals of sulphate of 

 Roseocobalt. Precisely the same result is obtained with the chloride and nitrate 

 of silver; the red solution yielding crystals of nitrate of Roseocobalt. We con- 

 sider it probable that in these cases the sulphate and nitrate of Purpureocobalt, 

 5NH 3 .Co 2 3 ,2S0 3 , and 5NH 3 .Co 2 3 ,2N0 3 , are really formed by double decomposi- 

 tion, but that during evaporation the equivalent of free sulphuric or nitric acid 

 formed at the same time with the sulphate or nitrate, reacts upon this so as to con- 

 vert it into a salt of Roseocobalt with three equivalents of acid. In equations we 

 should have for the sulphate 



5NH 3 .Co 2 Cl 3 +3AgO,S0 3 +HO=5NH 3 .Co 2 3 ,2S0 3 +HO,S0 3 +3AgCl. 

 5NH 3 .Co 2 3 ,2S0 3 + HO,S0 3 = 5NH 3 .Co 2 3 ,3S0 3 + HO. 



When oil of vitriol is poured upon chloride of Purpureocobalt in quantity suffi- 

 cient to make a thick paste, the mass assumes a fine purple color, and swells up very 

 much at first, so that a large vessel is necessary. If the solution, after the evolu- 

 tion of chlorhydric acid has ceased, be diluted with about twice its volume of water, 

 and allowed to stand for a few hours, a large mass of beautiful violet-red needles is 

 deposited. The mother liquor, after standing for a longer time, deposits more crys- 

 tals. These crystals are to be quickly washed with a little cold water, drained 

 and dried by pressure between folds of bibulous paper. They are usually free from 

 chlorine, and are very nearly pure acid sulphate of Purpureocobalt. The mother 

 liquor contains more of the acid sulphate together with small quantities of another 

 sulphate which we shall describe more fully hereafter, and frequently a little unde- 

 composed chloride. By boiling this mixture with chlorhydric acid chloride of 

 Purpureocobalt is formed, which may be employed in preparing a fresh portion of 

 the acid sulphate. 



The acid sulphate of Purpureocobalt may also be prepared by the action of strong 

 sulphuric acid upon the sulphate of Roseocobalt. For this purpose oil of vitriol is 

 to be poured upon the sulphate in quantity sufficient to produce an oily liquid on 

 heating in a water bath. The digestion is to be continued for one or two hours, 

 according to the quantity of salt employed, care being taken that no oxygen is 

 evolved. The dark purple liquid is to be suffered to cool, diluted with an equal 

 bulk of water, and allowed to crystallize. 



The acid sulphate as thus obtained is difficult to purify. By dissolving it in a 

 small quantity of hot water, and evaporating it quickly, fine crystals may some- 

 times be obtained. When, however, the solution is evaporated slowly in the air, 

 crystals of sulphate of Roseocobalt are formed in abundance, while the mother liquor 

 contains free sulphuric acid. When a solution of the acid sulphate is neutralized 



