302 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



measured is smaller than that which would be calculated from the con- 

 centrations of the ions and larger than that calculated from the con- 

 centrations of the un-ionized fraction. One obvious explanation is that 

 the conditions assumed to hold in applying the Nernst equation are 

 not fulfilled, for this equation obviously can apply only to solutions 

 which are sufficiently dilute so that the deviations from ideal systems 

 lie within the experimental error. The behavior of solutions of strong 

 electrolytes clearly shows that this condition is not fulfilled. The 

 Nernst equation, therefore, should not apply. 



On the other hand, it is possible that the ionization measured by 



means of the conductance ratio -r- is not correct. If this is true, the 



A 



concentrations of the ions are not known and it is therefore not possible 

 to calculate the electromotive force of a concentration cell from Equa- 

 tion 95. In this case, we still have to take account of the fact that 

 solutions of strong electrolytes do not fulfill the conditions of dilute 

 solutions. Consequently, it is not possible to calculate from the electro- 

 motive force of concentration cells the concentrations of the ions in 

 solution ; for it may readily be shown, from electromotive force measure- 

 ments, that the law of mass action does not apply to solutions of strong 

 electrolytes and that, consequently, the laws of dilute solution do not 

 apply. The ratios of the concentrations of the ions, therefore, cannot 

 be calculated by means of the Nernst equation. 



It has been suggested that strong electrolytes are completely ionized 

 even at fairly high concentrations. In that case the function J 8 in 

 Equation 96 measures the change in the potential of the electrolyte due 

 to interaction between the ions. Granting this assumption, the function 

 J s has a negative value at relatively low concentrations. With increas- 

 ing concentration the value of J diminishes, passes through a minimum, 

 and thereafter increases, passing through a value and becoming posi- 

 tive at very high concentrations. 28 



A considerable number of measurements have been made on the 

 electromotive force of concentration cells in which other electrolytes have 

 been added to the solution of the electrolyte surrounding one electrode. 

 Poma and Patroni 29 have measured the electromotive force of copper 

 electrodes in solutions of copper salts, to which various electrolytes 

 with a common ion had been added. Poma 30 measured the potential 

 of the hydrogen electrode in acid solutions in the presence of other elec- 



28 The manner in which J varies is discussed further in the next chapter as is also 

 the relation of this function to the activity. 



29 Poma and Patroni, Ztschr. f. phys. Chem. 87, 196 (1914). 

 80 Pojna, Ztschr. /. phys. Chem. 88, 671 (1914), 



