138 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



coefficient of weak electrolytes as a function of their concentration at a 

 pressure of 500 kg./cm. 2 The curve actually corresponds very closely with 

 that of acetic acid in water at this pressure. As Tammann has shown, 

 it follows from the Planck equation that at low concentrations and for 

 relatively small values of the constant K the ionization change, due to 

 increasing pressure, increases with increasing concentration, until a prac- 

 tically constant value is reached. The conductance-pressure coeffi- 

 cient increases with increasing concentration of the weak electrolyte 

 up to a concentration of about 10~ 3 normal for electrolytes whose con- 

 stant is below 10"*. At higher concentrations the ionization change due 

 to pressure change remains practically constant. However, at higher 



concentrations the value of - -r decreases, while the value of T^ 



v Ap q) Ap 



decreases and ultimately changes sign, as follows from Cohen's observa- 

 tions on the viscosity of aqueous salt solutions. Therefore, the con- 

 ductance-concentration curves, and consequently the curves for the coeffi- 

 cient, exhibit a very flat maximum. In the case of solutions of strong 



electrolytes, the term --T? has inappreciable values at concentrations 



below 10~ 2 normal, and has only very small values at much higher con- 

 centrations. In dilute solutions, therefore, the pressure coefficient has 

 very nearly a constant value, independent of concentration. At higher 

 concentrations, however, the value of the coefficient decreases, owing to 



the diminution in the value of ~T and owing to an ultimate change in 



the sign of the viscosity effect at higher concentrations of the electrolyte, 

 as was found by Cohen. Electrolytes of intermediate strength exhibit 

 a type of curve intermediate between these two extreme types, as repre- 

 sented by Curve 2. In this case the value of the coefficient increases 

 with increasing concentration of the solution at lower concentrations 

 owing to the increasing ionization of the electrolyte. Ultimately, how- 

 ever, the effect of the viscosity change makes itself felt, the curve passes 

 through a maximum, and thereafter falls with increasing concentration. 

 At very low concentrations the viscosity-pressure coefficient has actually 

 been found to increase and approach large values due to the effect of the 

 fourth term in the right-hand member of Equation 43. This increase 

 in the coefficient, as was shown by Tammann, 29 is due to the increased 

 ionization of the solvent medium. 



The limiting value which the coefficient ^-r- approaches at low con- 



Tammann, Zt&cTvr. f. phys. Chem. 27, 464 (1898). 



