32 Dr. H. J. S. Sand on the Measurement of the 



Results obtained with Aqueous Solutions of Copper Sulphate, 

 Silver Nitrate, and Zinc Sulphate. 



The first series of experiments we have to refer to is one 



carried out with aqueous solutions of metal salts, the substances 



employed being solutions of copper sulphate, silver nitrate, 



and zinc sulphate. As long as metal-salt is left in the layer 



of liquid touching the cathode, the metal is deposited from 



such solutions at a cathode-potential, probably either identical 



with or differing only slightly from the potential at which it 



is in equilibrium with the solution of its salt. But when the 



salt has been removed from the electrode by the current, a 



sudden drop of the potential of the electrode takes place. A 



marked difference is, however, observed in this connexion 



between solutions of copper sulphate and silver nitrate. In 



the case of the former, the cathode-potential drops about one 



volt, and then becomes fairly constant again. At the same 



time the resistance of the layer of liquid around the cathode 



increases by about 175 ohms per sq. cm., the combined effect 



being that the difference of potential between the standard 



electrode and the cathode, as measured in the experiments, 



increases by 1*2 to 1*3 volts above its original value. The 



increase of resistance is found to be due to the formation of a 



deposit of copper hydroxide arising from hydrolysis of the 



copper sulphate at great dilutions. In the case of the silver 



nitrate, however, when depletion of the silver occurs a drop 



of the potential of the cathode amounting to only about 0*2 



volt takes place, after which the potential approaches its 



former value again more or less rapidly. The cathode is then 



found to have become covered with a mass of silver crystals 



which grow into the solution. The curves in fig. 5 show 



results obtained with a silver nitrate, and with a copper 



sulphate solution. As has been shown in the previous 



communication referred to, the diffusion coefficient of the 



salts experimented on may be calculated when the time is 



known which elapses until the concentration of the salt in 



the layer of liquid at the electrode becomes zero, and this 



time may be taken as that at which the break in the curve 



takes place. The actual points chosen on this break were 



5 centivolts in the case of the silver nitrate, and 50 centi- 



volts in the case of the copper sulphate solutions. The 



formula employed is — 



,_ 1-1284 2 (V 



96540 2 c 2 ' 



cm. 



k being the diffusion coefficient of the salt in when c is 



° sec. 



