[ 377 ] 



XXXIV. Electromotive Force and Osmotic Pressure. 

 By R. A. Lehfeldt *. 



IX a communication read before the British Association at 

 Dover f I drew attention to a difficulty in the interpre- 

 tation of the ordinary (logarithmic) expression for the electro- 

 motive force between a metal and solution. The following J 

 is an attempt to supply some explanation of the difficulty 

 there raised. 



(i.) Electromotive Force of a Concentration-cell calculated 

 Tliermodyna mically. 



The simple case of a cell made up of metal : dilute electro- 

 lyte : concentrated electrolyte : metal in which the electrolyte 

 is regarded as completely dissociated, has frequently been 

 treated by the thermodynamic method of the cyclic process 

 and yields, as is well known, the result 



E = RT (1 ~^%g~ 3 , (1) 



re G lY ' 



* Communicated by the Physical Society : read Nov\ 26, 1900. 



t Sept. 1899. Printed in the Phil. Mag. (5) vol. xlviii. pp. 430-433 

 (1899), and Zeits. f. phys. Chem. vol. xxxii. p. 360 (1900). 



X Note. — After reading the above-mentioned short paper at the British 

 Association, it occurred to me that the answer to the question there raised 

 probably lay in the inapplicability of Boyle's law to strong solutions of 

 electrolytes. I then worked out the piece of thermodynamical theory 

 given in the present article, but held it over till the completion of the 

 experiments undertaken. Since then Dr. S. P. Milner has published an 

 interesting communication (Phil. Mag. May 1900) written from th« 

 same point of view, and partly anticipating the theory given below. I 

 haye nevertheless thought it worth while to publish this paper in its 

 original form, as the deduction differs in certain respects from Milnsr's. 

 In thermodynamic arguments about osmotic pressure, it is apparently 

 indispensable to make use of the conception of a semi-permeable partition ; 

 but I think it is highly important to make the assumption of such a par- 

 tition harmonize as closely as possible with experimentally practicable 

 processes. Milner assumes a membrane permeable to anion (N0 3 ), but not 

 to cation (Ag), and then by a reversible cycle deduces an equation similar 

 to (3) below. 



Such a partition would of course set up an electromotiye force between 

 two solutions of different concentrations; but the E.M.F. so obtained is 

 not identical with that of the actual fluid-contact in a concentration-cell, 

 as reckoned by Nernst and Planck. No such partition has yet been 

 discovered, and it is doubtful whether it could exist; and therefore I 

 think doubts may well be cast on processes of reasoning based on its 

 assumption. In my own deduction I have followed out a cycle which 

 appears to me of a more natural kind, and assumed only a partition pos- 

 sessing properties similar to those observed in the case of copper ferro- 

 cvanide. 



" Phil. Mac/. S. 6. Vol. 1. Ko. 4. April 1901. 2 



