396 



(Ml) can be substituted by — ; — - so tliat 



' (OHJ' 



0.058 Lm(OH')' 



or 



0.058 Lm K;^ „. .„. 



E^ _,og.^_^^^.^„-2.8 ... (3) 



From which it appears, that the electrode will beliave as an 

 oxygen or ii\drogen one, l)iit Ihat the electromotive forces will show 

 a constant diti'erence. 



These considerations however are no iielp to ns, for expression (i) 

 which always holds good, requires the potential of the melal to be 

 ver\- negative, because (he concentration of ihe metal ions in a 

 saturated solution of Mg(OH), or Al (0H)3 is ver^' small. The exact 

 converse is observed. 



Ten years ago Kistiakowsky ') calculated the normal potentials 

 neglecting the temperature coeflicieni in the formula of Gibbs-Hei,m- 

 HOLTZ and found with Mn, Fe, Co, Cu and Cd differences between 

 the calculated and experimentally found normal potentials of 

 10—60 m.V.; with Ni, Sn, Pb and Hg differences of 140— 190 

 m.V.; with Ag he found diverences of 310 m.V., and with Tl of 

 360 m.V., whilst the difference with Al was 460 m.V. and with Mg 

 900 m.V. 



As Kistiakowsky found the electromotive force which the calcu- 

 lated for Mg and Al so much higher than that found experimentally, 

 he simply assumed that at the two electrodes in the galvanic cell 

 metal-electrolyte-hydrogen, the reactions 



Mg ^ M;; + 2(9^ 

 and 



do not take place as in other cases, but the following: 



Mg+2 0H;^-^M(OH),g + 2(9 



and 



2(9 + 2H,Oj^^2 0H; +H,^^. 



It should be luiticed that the remarkable assumption is made there, 



1) Z. f. phys. Ghem. 70, 206 (1910). 



