816 



0,49. Flade^) gives for the active potential in 0.1 n H,SO, —0,32, 

 Fredenhagen^) in ?i H^SO^ — 0,48. Muthmann and Fraunberger ') 

 found for the potential of chromium according to Goldschmidt, which 

 had been activated by cathodic polarisation in potassium hj'droxide 

 -0,64 in uKCl 



The given values vary accordingly from — 0,32 to — 0,64. 

 FoERSTER'') takes as potential — 0,48. The "Messungen elektromo- 

 torischer Kriifte'") give —0,6. 



In view of the great differences in these values it seemed desirable 

 once more to determine the potential of equilibrium of chromium. 

 Metallic chromium can probably only be in real equilibrium with 

 a solution of a chromous salt, which contains a very small quantity 

 of chromic salt. 



This follows from the value of the chromous-chromic-potential, which 

 according to Marzucchelli 'j amounts to — 0,36, according to Forbes 

 and Richter ') to — 0,40, when it is assumed that the equilibrium 

 potential of chromium is about — 0,5. It is, therefore, required for 

 the determination of the [)Otential of equilibrium to examine chro- 

 mium electrodes which are in contact with a solution of a chromous 

 salt. For this was chosen chromous sulphate, as it is easy to prepare. 



4. Preparation of chromoussulphate. 



For the preparation of chromoussulphate a solution of 200 gr. of 

 chromicsulphate in 300 gr. of water and 100 gr. of sulphuric acid 

 was reduced at an amalgamated lead cathode at 15° with a current 

 density of 15 amp. /dm'. ^). A blue solution of chromoussulphate is 

 then formed, from which part of the salt is deposited in solid state. 

 To be able to collect this without its coming into contact with the 

 air, the reduction was carried out in a porous cup, in the bottom 

 of which there was a hole closed by a rubber stopper. When the 

 reduction was completed, the porous cup was placed in tube 1 of 

 the apparatus drawn in figure 1. This tube was filled with pure 

 carbonic acid through A or B. Then the stopper was pushed out 

 of the bottom of the porous cup, so that the liquid and the crystals 



1) Zeitschr. f. physik. Chemie 88, 569, (1014). 



3) Ibid. 63, 1. (1908). 



8) Sitzungsber. Bayr. Akad. 34, 201, (1904). 



*) Elektrochemie wassriger Lösungen, Leipzig 1915, 191. 



6) Abh. d D. Bunsengesellschaft N^. 5, 8, (Halle 1911, 1915). 

 «) Zeitschr, f. Elektrochemie 11, 882, (1905). 



7) Journ. Amer. Ghem. Soc. 39, 1140, (1917). 



^) FoERSTER, Electrochemie wassr. Lösungen, Leipzig 1915, 532. 



