at the Electrodes in a Solution. 63 



In the calculation of i 2 t Wiedeburg's value for the diffusion- 

 coefficient of copper sulphate at 1 8°, 



cm - 



K l8 = 44-79 x 10- 7 (1 -3-467 c) ~^- 

 18 v sec. 



was employed. Here the concentration c is supposed given 



. g. CuS0 4 , ., . • mg.-equiv. 



in kl — i. when it is given m h the number 



cm. 2 ' to cm. d 



0*2761 must be taken instead of 3*467. The value of K 

 for c = has been taken in calculating the numbers contained 

 in the columns '* i 2 t max/' and " i\ ace. to (16) " ; and the 

 value of K for the initial concentration in calculating the 

 number i\m\n. From the quantities i 2 t given in the table, 

 which are based upon the diffusion-coefficient of copper 

 sulphate at 18°, we should find i*t at 6 degrees by the 

 formula 



i% = *%[1 + O-O26(0-18°)], 



in which 0*026 is the temperature-coefficient of diffusion 

 assumed by Wiedeburg. 



Experimental Part. 



Determination of Time required till Hydrogen appears in the 

 Electrolysis of an acid solution of Copper Sulpliate. 



A first series of experiments was performed to ascertain the 

 time required till hydrogen first began to appear on electro- 

 lysing the mixtures of copper sulphate and sulphuric acid 

 arranged in the preceding table, the solutions electrolysed 

 being contained in cylindrical vessels, bounded at the top 

 by horizontal cathodes. The values obtained are compared 

 with the limits calculated by the formulas given in the 

 preceding table. Had hydrogen ever appeared before the 

 lowest limit of time was reached, this would have proved 

 conclusively that it was given off before the concentration 

 of the copper at the electrode had gone down to zero,- 

 The upper limit could not be exceeded unless there were 

 some considerable error in the experimental numbers em- 

 ployed. 



Preliminary experiments were first carried out to study 

 the subject qualitatively. It was found that on electrolysing 

 the solutions with low current densities, horizontal cathodes 

 being employed, which could be lit up for observation, at 

 first only copper was deposited, after a certain time, however, 

 varying greatly with the current density, the total surface 



