20 carhart: concentration cells 



ELECTRO-CHEMISTRY.— Thermodynamics of concentration 

 cells. Henry S. Carhart. Communicated by E. B. Rosa. 

 To appear in the Bulletin of the Bureau of Standards. 



dA 

 The equation A = H-\- may be Considered as a very general 



dT 



expression of the laws of thermodynamics for isothermal processes. 



For the E. M. F. of a voltaic cell, this may be put in the more 



specific form of the Helmholtz equation E = - + T — . 

 P H nF dT 



We may emphazise the two following cases : first, when H, the 



change in internal energy, is a constant. dA/dT or dE/dT is 



then zero, and the relation between A or E and the absolute 



temperature T is linear; second, an examination of Nernst's 



expressions for A and H in terms of the integral powers of T. 



These are according to Nernst 



A=Ao-bT 2 -±cT*-. . . 

 H=H Q +bT 2 +cT s + . . . 



Nernst makes the coefficient of the first power of T in the expres- 

 sion for A necessarily equal to zero. It is easily shown, however, 

 that this coefficient is not zero from a mathematical point of view. 



We have made an experimental investigation of the E. M. F. 

 of various concentration cells with amalgams of different concen- 

 tration as the electrodes. In every case a linear relation is estab- 

 lished between the E. M. F. and temperature. The expression 

 for the E then has the form E =E h -\-aT, where E h is the E. M. F. 

 due to the internal change of energy H. The coefficient a is not 

 zero as Nernst assumed it to be. 



For the zinc amalgams used the equation expressing the E. M. F. 

 is E= -0. 001455+0. 000030847 7 



and the following table shows how nearly the E. M. F. as com- 

 puted from this equation correspond with the observed ones : 



