821 



electrolytic chromium assumes under all circumstances in CrSO^, as 

 the real potential of equilibrium of chromium. 



It is, however, possible that the potential of — 0,55 V. {Ei, = 



— . — 0,27) does not correspond with the equilibrium Crmetai,^ Crgöi. 

 but with another equilibrium. 



In the first place the chromium in these solutions of Cr" might 

 assume the potential corresponding with the equilibrium Cr'^^Cr" -j- ©• 

 To examine this, in a number of measurements by the side of the 

 chromium electrodes a tin electrode was brought into the liquid 

 which is best suited for the determination of the chromous-chromic 

 potential according to Mazzucchelli. These tin electrodes showed 

 on an average a potential of — 0.68, hence Eh = — ^^0.40, the 

 chromous-chromic potential lying at Eh = — 0.36 according to Mazzuc- 

 chelli. When we take into consideration that the solutions used 

 contain much more chromous than chromic, the agreement may be 

 called satisfactory. If we wanted to consider the end potential of 



— 0.55, which the chromium electrode gets, as the chromous-chromic 

 potential, we should have to assume that through insufficient 

 establishment of the equilibrium with respect to chromium the 

 chromous-chromic potential is 0,13 V more positive than with respect 

 to tin. This is not impossible, because also with respect to a platinum 

 electrode the chromous-chromic potential is about 0.16 V more positive 

 than with respect to mercury ^). 



The second possibility is this, that we have to do here with a 

 hydrogen potential, that therefore not the chromium itself, but the 

 hydrogen dissolved in it, is electromotively active. When the chromium 

 at its surface has a hydrogen charge of one atmosphere, a hydrogen 

 ion concentration in the solution of about lO^-^ would be required 

 for a potential of — 0.55 V. Such a hydrogen ion concentration 

 could, probably, occur in a solution of chromoussulphate which is 

 saturated with chromous hydroxide. 



There is, however, still another potential to be observed for 

 chromium electrodes, which must have a special meaning, and this 

 renders it difticult to assume that the end-potential at — 0.55 is the 

 potential of equilibrium. 



When newly chromated electrodes are brought into a very feebly 

 acid solution of chromous-sulphate, they present a potential lying in 

 the neighbouihood of — 0.7 to — 0.8 V. These values vary pretty 

 considerably. For chromium that had been deposited from chromic 

 sulphate and chromic acid, 20 "/o of the observations lay below 



— 0,8, 65 7„ between — 0.8 and — 0.7. and 15 7„ above — 0.7. 

 ^) Forbes and Richter, Journ. Amer Ghem. Soc. 39, 1144, (1917). 



