Shepard — New Solutiojifor the Copper Voltameter. 51 



for this is that a copper plate oxidizes rapidly when wet with 

 a solution of neutral copper sulphate and exposed to the air. 

 This was accomplished by removing the voltameter plates as a 

 whole from the solution and dipping at once into water con- 

 taining a few drops of sulphuric acid. They were then rinsed 

 in clean water, dried with filter paper and over a flame. 



Gray's solution was first tried for comparison. Very good 

 deposits were obtained using a current density up to about *03 



— — — ; but the deposits would sometimes differ asmuch as 

 cm 2 ' r 



one per cent in the two voltameters. 



Oettel's solution gave fine deposits for low current densities. 

 When, however, the current density was increased the plates 

 blackened badly and copper was found in the form of powder 

 on the bottom of the jars. The highest current density per- 

 missible with this solution was *02 „ — . 



cm 2 



Then the solution with NH 4 C1 prepared as above was investi- 

 gated in the same manner. For low current densities it gave 

 as good results as had been obtained with either Gray's or 

 Oettel's solution, but, in contradistinction to these, the results 

 continued to be satisfactory when these limits were widely 

 surpassed. With a current of 20 amperes the density was 

 increased, by raising the plates so as to lessen the immersed 



surfaces to a value even more than *05 — —„ — . Notwith- 



cm 



standing this very material increase in the demands upon the 



voltameter, the deposits still remained perfectly satisfactory. 



With 30 amperes and a current density of '045 — the 



solution still gave results differing by less than one per cent. 



The voltameters were then filled, one with Gray's or Oettel's 

 solution and the other with the new solution. When employed 

 for low current densities the solutions in all cases gave deposits 

 agreeing within the limits of error imposed by the methods 

 employed. 



With regard to the constancy of the apparent electro- 

 chemical equivalent when a high current density was used 

 with this solution, no exact statement can be made, as the abso- 

 lute measurement of the current was not attempted. In the 

 first part of this investigation a Weston ampere-meter was 

 included in the circuit, the error of which the writer has reason 

 to believe was less than one per cent. Assuming the indica- 

 tions of this instrument to give the true values of the current, 

 the electro-chemical equivalent obtained from the new solution 

 was fairly constant. 



