136 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



effects of the solution. As the concentration decreases, the minimum 

 point is displaced toward higher pressures, and the curves approach one 

 another. Thus the curves at 0.1 N and 0.01 N differ but little. This is 

 due to the fact that below a concentration of 0.1 N the ionization of the 

 electrolyte is so great and the concentration so low that the viscosity 

 effects could not differ materially from those in pure water. At lower 

 concentrations, namely at 10~ 3 N and 10~ 4 N, the minimum disappears 

 and the pressure effect becomes very large, the curves becoming the 

 steeper, the lower the concentration of the solution. This divergence of 

 the curves at very low concentrations is due to the effect of pressure on 

 the conductance of the solvent medium; namely, to the fourth term in 

 Equation 43. In the limit, these curves approach the dotted curve shown 

 in the figure, which is that of the solvent medium. 



We have still to consider the case in which the third term of Equation 

 43 becomes an effective factor. This will obviously be the case with 

 solutions of weak electrolytes. The ionization of an electrolyte, if the 

 mass-action law holds and this is in general the case with weak electro- 

 lytes in aqueous solutions is determined by the value of its ionization 

 constant K. According to the Planck equation, we have: 



(44&) d log __ Az; 



dp RF 



According to Tammann, the value of At> is negative, so that as the result 

 of pressure increase the value of the ionization constant K increases and 

 with it the value of the ionization y. In the case of weak electrolytes, 

 at intermediate concentrations and lower temperatures, the first three 

 terms of Equation 43 have the same sign, and consequently the resist- 

 ance of solutions of weak electrolytes should decrease with increasing 

 pressure much more largely than that of solutions of strong electrolytes 

 under otherwise the same conditions, and the decrease should be the 

 greater the weaker the electrolyte and the greater the value of Ai>. The 

 first investigations in this direction were carried out by Fanjung. 26 

 Measurements on 0.1 N acetic acid were carried out by Tammann up to 

 pressures of approximately 4000 kilograms per square centimeter. In 



R v 



the following table are given values of the ratio -5-*- for acetic acid 



Hp=i 



at 20.14 . 27 



In the case of ammonia, which has approximately the same ionization 

 constant as acetic acid, the pressure effect is even greater than in that of 



28 Fanjung, Ztachr. 1. pTiys. Chem. 14. 673 (1894). 

 "Tammann, Wied. Ann. 69 t 770 (1899). 



