728 HARMED 



ART. M 



Since their calculation of 7 is numerically a very close approxi- 

 mation, it justifies their initial assumption of complete disso- 

 ciation of strong electrolytes. Even a conservative estimate of 

 this theory will convince us that by far the larger part of the 

 deviation factor, 7, is due to interionic forces in the case of 

 strong electrolytes in media of high dielectric constant, such as 

 water. It would be far beyond the purpose of the present dis- 

 cussion to develop this theory and its many ramifications, but 

 the knowledge that m is an ionic concentration or very nearly 

 so in the case of strong electrolytes permits us to develop the 

 possibilities of the study of reversible cells to a considerable 

 extent without any sacrifice in accuracy. 



We shall now sketch briefly some developments which 

 illustrate the more recent means of obtaining valuable data 

 regarding strong electrolytes, weak electrolytes, and ampholytes 

 from reversible cell measurements. To assure exactness, we 

 shall omit measurements of all cells with liquid junctions since 

 these all involve an undefinable and physically meaningless 

 hquid junction potential.* 



(1) The Activity Coefficients of Strong Electrolytes 



We have already shown how the change in chemical potential 

 of hydrochloric acid in passing from a solution at one concen- 

 tration to a solution at another concentration may be measured 

 by a cell without Uquid junction. For the change 



CA(m2) ^CA{mi), 

 we have, according to equation (42), 



- Ar = (m' - m") = Rt log ^—^Tr (44) 



etc dA 



If we adopt the convention that a positive electromotive force 

 accompanies a decrease in thermodynamic potential, we obtain 

 from equation (39) 



*Harned, J. Physical Chem., 30, 433 (1926). Taylor, /. Physical 

 Chem., 31, 1478 (1927). Guggenheim, /. Phtjsical Chem., 33, 842 (1929); 

 34, 1540 (1930). 



