352 Dr. C. R. A. Wright on the Determination of 



Experiments with Oxidizable Electrodes. 



91. Suppose that dilute sulphuric acid is electrolysed with 

 copper electrodes so that a gramme-equivalent of copper 

 sulphate is formed at the + electrode for one of hydrogen 

 evolved at the — electrode. In accordance with the general 

 theorem (§ 61), 



,=E 1 + [2{(l-n)H}-2(^)-2(H)] % J, 



the value of e will be 



6 = E 1 + {(l-9i 1 )H 1 -n 1 A 1 -H 3 } % J ? 



where H 3 is the heat evolved during the production of a gramme- 

 equivalent of copper sulphate from copper, ordinary free 

 oxygen, and dilute sulphuric acid ; for in this case £(H) is 

 equal to H 3 + (l — ?2 2 )H 2 — n 2 h 2 (the n 2 , h 2 , and H 2 referring 

 to oxygen, and n 1 , h ly and H^ to hydrogen), the diffusion 

 effect of the copper sulphate solution accumulating round the 

 + electrode being left out of sight. 



Writing E 3 for H 3 ^J, 



^=E 1 -E 3 + {(l-n 1 )H 1 -7 i A} % J. 

 Of course, if some other metal be used instead of copper, 

 e. g. zinc, the same formula will apply, only the value of E 3 

 will be different. 



If, instead of dilute sulphuric acid, a metallic salt, e. g. zinc 

 or copper sulphate, be electrolysed, the same formula will 

 hold, E x now representing the E.M.F. corresponding to the 

 work done in decomposing the salt electrolysed into oxygen, 

 metal, and dilute sulphuric acid, and E 3 , as before, being the 

 E.M.F. representing the work gained in synthesizing the salt 

 produced by the solution of the + electrode from metal, 

 oxygen, and dilute sulphuric acid ; since 7z l = when a solid 

 metal is produced, 



«=E 1 -E 3 + (l-» 1 )H 1% J. 



Taking into account the diffusive effect due to the strength- 

 ening of the metallic salt-solution round the + electrode and 

 its weakening round the — electrode [which, according to the 

 results of Moser (Wied. Annalen,iii. p. 216), H. F. Weber 

 (Phil. Mag. [5] viii. pp. 487 and 523), andHelmholtz (Wied. 

 Annalen, iii. p. 201), may amount to at least some few 

 hundredths of a volt under certain circumstances], this 

 formula becomes 



^=E 1 -E 3 + {(l-n l )H 1 + //} % J, 



where h' is the heat corresponding to the diffusive effect of the 

 concentrated and weak solutions per gramme-equivalent of 

 electrolyte decomposed ; // is of + sign, since the results of 



