160 CONDUCTIVITY AND VISCOSITY IN MIXED SOLVENTS. 



and isobutyric acids, when mixed with water, show viscosity maxima. In 

 every case there is a greater decrease in fluidity than would be expected from 

 the law of averages. 



When methyl and ethyl alcohols are mixed, they do not exhibit the same 

 phenomena as is shown in the case of mixtures of organic solvents with water. 

 Arrhenius 1 states that there is no observable change when these alcohols are 

 brought together. 



When acetone is mixed with methyl or ethyl alcohol, the fluidity curve of 

 the mixture is approximately a straight line. The same is true when we have 

 an electrolyte dissolved in mixtures of these solvents. 



From a consideration of the fluidity curves and conductivity curves for 

 lithium bromide in mixtures of methyl alcohol and water, it is quite evident 

 that the minimum of fluidity corresponds to the minimum of conductivity, 

 both generally occurring in the 50 per cent mixtures of the solvents. The 

 drop in fluidity is more pronounced at the lower temperature, and, similarly, 

 the drop in conductivity. In fact, at the higher temperature the minimum 

 of conductivity occurs in the 75 per cent mixture until v = 100, where there is 

 a shifting of the minimum to the 50 per cent mixture. 



In the case of mixtures of ethyl alcohol and water, the minimum of fluidity 

 is in the 50 per cent mixture, and more marked at the lower temperature. 

 The conductivity minimum in this case occurs in the 75 per cent mixture, and 

 is very slight indeed at 25, although the values in each case are much less 

 than we should expect from the law of averages. Thus, we have in both the 

 case of methyl alcohol and water and of ethyl alcohol and water, a tendency 

 for the shifting of the minimum towards the mixture containing the greater 

 per cent of alcohol, whenever we have a rise in temperature. 



Stephan, 2 working with mixtures of ethyl alcohol and water, found that the 

 temperature coefficients of conductivity and of fluidity were very similar. 

 A minimum in his curves was observed, and he proposed the relations 



KH wKH 



k = and k = -. 



?) Wr) 



the first holding for the mixtures up to the minimum point, and the second 

 from that point on. K is the same in both formulae, and is the conductivity 

 of the equivalent aqueous solution of the electrolyte; k is the conductivity 

 in the mixture; H and rj are the viscosity coefficients for water and for the 

 mixture, respectively, w and w' are the per cents of water in the mixture and 

 in the aqueous alcoholic mixtures of minimal fluidity, respectively. Stephan 

 concluded that each ion carries with it molecules of the solvent, and that the 

 ionic friction consists in friction between these molecules and the rest of the 

 solvent. Thus we have very early the idea of ionic hydration introduced 



1 Ztschr. phys. Chem., 1, 285 (1887). 2 Wied. Ann., 17, 673 (1882). 



