823 



hydrogen electrode. The greater the overvol tage tor liydrogen 

 generation is at the metal, the neaier the potential will be to that 

 of the metal. This appears e.g. clearlj^ for amalgamated zinc, pure 

 zinc, and zinc in contact with platinum in hydrochloric acid. 

 The first is most strongly negative, the last least so. A metal can, 

 therefore, for this reason never possess a too negative potential. 



There is, however, another cause which can render the potential 

 too negative, viz. complex formation by the hydrochloric acid, in 

 consequence of which the concentration of the elementary metal ions 

 is kept at a small value. This is the case for zinc : in strong hydro- 

 chloric acid this has a much too negative potential. Thus also the 

 potential of — 0,84, which chromium presents in 25 7o hydrochloric 

 acid, can be too negative through complex formation. For 6 "/o ^"d 

 1,5 7o hydrochloric acid this will be observed to a much smaller 

 degree, so that the value — 0,75 is certainly not too negative. The 

 same value of — 0,75 V is found when chromium amalgam, prepared 

 according to Férée ^) is brought into chromous sulphate. After some 

 days also these amalgam electrodes assume the end-potential — 0,55 V. 

 Pyrophorous chromium, according to Férée, was slightly less 

 negative, viz. — 0,70 V. 



Hence it follows from the above that electrolytic chromium and 

 chromium amalgam in chromous sulphate, just as chrouiium of 

 GoLDscHMEDT in hydrocliloric acid, present a potential of al)out 

 — 0,75 V or — 0,47 V with respect to the hydrogen electrode. 

 This value is in agreement with that found by Neumann, Rathert 

 and Fredenhagen, and also with that assumed by Foerster. 



This potential, which might be called the active potential, will 

 probably lie not far from the potential of equilibrium. 



This can only be reached when the metal contains a sufficient 

 quantity of hydrogen, which is present in the metal for electrolytic 

 chromium, and which is generated in acids by the metal for 

 chromium of Goldschmidt. The hydrogen must, therefore, catalyti- 

 cally promote the establishment of the equilibrium here. That 

 the potential runs back to — 0,55 V or — 0,27 V after some days 

 with respect to the hydrogen electrode, may be explained so that the 

 metal loses so much hydrogen that the hydrogen pressure corresponds 

 with one atmosphere. This quantity of hydrogen will then be too 

 small to serve as catalyst for the establishment of the equilibriuui, 

 and this end-potential will be a hydrogen potential, not a chromium 

 potential, as was already suggested as a possibility on p. 822. 



Amsterdam, October 1917. Chemical Lab. of the Univ. 



1) Compt. Rend. 121, 822, (1895). 



