142 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



the increasing viscosity effect overbalances the effect of increased ioniza- 

 tion and the curve passes through a maximum. In solutions of weak 



electrolytes the ratio p increases rapidly with increasing concentration 



of the electrolyte, due to increased ionization, and, for very weak electro- 

 lytes, particularly at low temperatures, passes through unity to greater 

 values. Here, again, at sufficiently high concentrations, the curve may 

 pass through a maximum, owing to the ultimate predominance of the 

 viscosity effect. From his measurements, making the assumption that 

 the Planck equation holds as well as certain other assumptions, Schmidt 



has calculated the value of , the viscosity ratio, due to pressure, for 



potassium iodide, sodium iodide and tetramethylammonium iodide in 

 alcohol. He found that this ratio increases markedly with the pressure. 

 In the case of potassium iodide and sodium iodide the increase is very 

 nearly the same, being from 1.0 to 2.34 for 0.02 N solutions and a pres- 

 sure change from 1 to 3000 atmospheres. In the case of tetramethyl- 

 ammonium iodide the ratio increases somewhat more than for the 



other two electrolytes measured. This indicates that the viscosity effect 

 in alcohol, similar to that in water, is a property of the ions. It appears, 

 however, that the effects in the case of different ions are much more 

 nearly the same in non-aqueous solutions than in water. This is as 

 might be expected, since in solvents of low dielectric constant the ionic 

 conductances themselves differ much less than in water. Schmidt has 

 also calculated the value of the ionization y at different pressures and 

 has found that the ionization increases with increasing pressure. 



That the pressure effect is intimately related to the viscosity of the 

 solution is clearly indicated by the fact that the order of the effects in 

 different solvents corresponds to the order of the viscosities of these 

 solvents. The higher the viscosity of the solvent, the greater is the ratio 



r> for a given pressure change. In the majority of solvents Schmidt 



found that this ratio might be expressed as a function of the pressure by 

 the equation: 



R V 



(45) log =. pp } 



where |3 is a constant. This equation was found to be particularly ap- 

 plicable at higher temperatures. In other cases it was necessary to add 



