422 



On the Theory of Solution Pressure. 



into which formula it is only the pressures of the metal ions 

 at the two electrodes which enter. This result, however, does 

 not follow except on the law of mass-action. 



A further question arises as to the effect of the presence of 

 any other salts in the solution in addition to the silver nitrate. 

 It is clear, however, that so long as these are not reversible 

 with respect to the electrodes, they can be cut off from 

 exercising any pressure on the partitions X, Y, and Z by 

 partitions impermeable to these salts, although permeable to 

 silver nitrate, and the cycle can be carried out as before 

 without alteration of the final result. It follows that all sub- 

 stances in solution which are not deposited on an electrode 

 or otherwise removed from the solution to an indefinite extent 

 by the passage of a current, however small, should produce 

 no effect on the potential step there *. 



It will be seen from the above that Nernst's formula 

 depends for its validity only on the osmotic pressure 

 and dissociation theory, and on the reversibility of the 

 electrolytic solution and deposition of the metal at the 

 electrode. It is, as Dr. Lehfeldt pointed out f , independent 

 of the nature of any special theory of the formation of the 

 potential step at the electrode, so long as this admits this 

 reversibility. On the other hand, it is very difficult to imagine 

 a reversible equilibrium at the surface of the eleatrode which 

 shall not be a dynamical one like all other chemical equilibria, 

 that is, an equilibrium consisting in ions going into solution 

 from the metal, and being deposited from the solution onto 

 the metal, at equal rates ; and this in itself is JSTernst's theory 

 of solution pressure. For in this case the tendency of the 

 ions to go into solution may clearly be considered as due to a 

 " pressure of solution " (quite independently of the assump- 

 tion — necessary for the application of a cycle to a single 

 electrode — that it can be represented by ions existing at a 

 pressure in the metal differentiable from the rest by a semi- 

 permeable partition). The value of this solution pressure 

 would obviously be equal to the osmotic pressure of ions which 



* The importance of this deduction lies in the fact that it forms a 

 general thermodynamical rule by which it may be judged whether or not 

 the addition of any particular class of ions will affect the potential-differ- 

 ence at the electrode. Thus the addition of zinc nitrate to one side of a 

 silver-nitrate concentration cell does not affect the E.M.F., as zinc is 

 not deposited indefinitely by a current onto silver ; on the other hand, 

 the addition of mercurous nitrate produces an effect, as a current 

 will deposit mercury from this cell. It also explains why in general the 

 negative ion does not affect the potential step, as has been shown to be 

 mostly the case by Neumann (Zeit. Phijs. Chem. xiv. p. 229, 1894). 



f Loc. cit. 



