1123 



Chromated Pt in CrSO* 



•Cf 



potential 



The only difference is, that the first vertical portion lies at a more 

 positive potential, viz. -j- 0.4 V. 



For chromium on platinum or on gold the shape of the current 



potential line is dilferent. Figure 6 represents the behaviour of chromium 



Fig. 6. on platinum, when the 



strength of the current is 

 quickly increased. F'rom 

 — 0.8 V a slow increase 

 of the strengtli of the cur- 

 rent takes place till a 

 potential of about —0.1 V 

 has been reached. Then 

 the line has a very flat 

 course, till the formation 

 of chromate begins at ■\- ^.1 ^ . When now the strengtli of the 

 current is slowly raised, and we wait every time till the potential 

 has become constant, we find a line with a much flatter course, 

 for which already at 0.1 mA the potential rises to -[-0.7 V. It 

 appears from this, that electrolytic chroniium cannot continue to 

 go into solution at a potential that lies in the neighbourhood of 

 the potential of equilibrium. It may be, that the ascending course 

 that the line exhibits at — 0.8 V iii figure 6, does not corre- 

 spond with the solution of chromium, but with the solution of 

 hydrogen present in the electrolytic chromium. 



Immediately after the current has been broken, the chrondum on 

 platinum or gold is not activated, but presents a potential of -|- 0.7 V. 

 This phenomenon is treated more at length under 2. 



2. The activation of chromium by anodic polarisation in solutions 

 of chroinous sulphate. 



Chromium that has been deposited electrolytically on silver, presents 



like chromium on copper, an active 

 potential after anodic polarisation. 

 Immediately after anodic polarisa- 

 tion the potential is fairly positive. 

 It descends, however, in a few 

 minutes, to a pretty great negative 

 value. In figure 7 the course of 

 the potential has been drawn as 

 function of the time, when the 

 current of 1, 5, 10, 20, and 50 mA 

 is suddenly broken off. The general 



78 



in minutes 



Proceedings Royal Acad. Amsterdam. Vol. XX. 



