218 On the Electrolysis of Dilute Sulphuric Acid, fyc. [Jan. 17, 



Copper Sulphate. 



A careful examination of the chemical changes which accompany 

 the electrolysis of a solution of copper sulphate appeared, however, 

 capable of throwing additional light on the value of this electrolytic 

 method for ascertaining the composition of hydrates in aqueous 

 solution. It is well known that water forms with CuS0 4 a definite 

 hydrate, CuS0 4 5H 2 0. The anhydrous salt is white, the hydrate blue. 

 It is reasonable to suppose that the blue solution of this salt contains 

 molecules of this or of some higher hydrate. Now, if in the electro- 

 lytic process the water of hydration suffers decomposition along 

 with the CuS0 4 , the primary chemical changes might be expected 

 to be — 



Positive pole. Negative pole. 



(A.) CuS0 4 5H 2 = S0 3 + 6 Cu + 5H 2 . 



iter of h 

 3r of sol 



manifestly be- 



But, if the water of hydration takes no more part in the electrolysis 

 than the water of solution does, then the chemical changes would 



Positive pole. Negative pole. 



(B.) CuS0 4 = SO^+O Cu. 

 Of course the collateral action — 



Positive pole. Negative pole. 



H 3 =0 H 2 



might also take place, but this would occur only with currents of 

 considerable density. The method is obviously capable of discrimin- 

 ating between these two actions, even supposing a considerable 

 quantity of the electrolyte travelled unchanged from one compart- 

 ment of the apparatus to the other. Eor, in the first case, either free 

 hydrogen would be liberated at the negative pole, or free acid formed 

 in the negative compartment, equal to five-sixths of the total copper 

 deposited; the free acid, and the five-sixths of the total copper, to 

 which it is equivalent, being produced by the chemical action — 

 5H 2 -r-5CuS0 4 =Cu 5 -r-5H 2 S0 4 ; equation A becoming — 



Positive pole. Negative pole. 



CuS0 4 ,5H 2 + 5CuS0 4 = S0 3 + 6 Cu 6 + 5H 2 S0 4 . 



On the other hand, if the action was in accordance with B there 

 would be only a deposition of copper on the negative electrode, and 

 no formation of free acid in the negative compartment. In the 

 annexed table the results and particulars of several experiments are 

 set out. In each experiment 25 cub. centims. of a 10 per cent. 



