THE CONDUCTANCE OF SOLUTIONS VISCOSITIES 129 



effect, however, is not the only one involved. As Tammann has shown, 20 

 the conductance-pressure coefficient is the resultant sum of four effects; 

 namely, the volume change of the solution due to pressure change, the 

 change in the mobility of the ions due to the viscosity change of the 

 solution, the change in the ionization of the electrolyte, and finally the 

 change in the conductance of the solvent medium, which, as a rule, is due 

 to a small quantity of electrolyte present as impurity. The conductance- 

 pressure coefficient, therefore, is given by the equation: 



X Y' Ap 



where X is the conductance of the solution due to the electrolyte, V that 

 due to the solvent medium, y is the ionization of the electrolyte and y' 

 that of the solvent medium, and cp is the ionic resistance; that is to say, 

 the reciprocal of the ionic mobility. In the equation, therefore, the first 

 term of the right-hand member measures the conductance change due 

 to the volume decrease of the solution; the second term measures the 

 conductance change due to the viscosity change of the solution; the third, 

 the conductance change due to the ionization change of the electrolyte; 

 and the last term, the conductance change due to the ionization change 

 of the solvent medium. By suitably choosing the condition of the solu- 

 tion, it is possible to minimize the value of various of the terms enter- 

 ing into this equation, and thus make apparent the effect of the various 

 factors on the conductance of the solution due to pressure change. 



Let us examine first the typical form of the conductance-pressure 

 curves in the case of aqueous solutions of 0.01 N KC1. In Figure 24 21 

 are represented values of the ratio of the resistance of the solution, 7? p , 

 under a pressure of p kilograms per square centimeter to the resistance 

 Rp=i under a pressure of one atmosphere at a series of temperatures. It 

 will be observed that as the pressure increases the resistance of the solu- 

 tion decreases initially. As the temperature rises, the value of the 

 decrease due to a given pressure change diminishes. At high pressures 

 the isotherms exhibit a minimum. The higher the temperature, the lower 

 the pressure at which the minimum occurs. It is evident that at suffi- 

 ciently high temperatures the minimum will disappear and the resistance 

 of the solution will increase throughout with increasing pressure. This 

 has been found to be the case with strong electrolytes, such as sodium 

 chloride in aqueous solution. 



In solutions of strong binary electrolytes, the ionization at a con- 

 centration of 0.01 N is so high that but little change is to be expected in 



" Tammann, Ztschr. f. pTiys. CJiem. 27, 458 (1898). 

 "Korber, Ztschr. f. phys. Chem. 67, 222 (1909). 



