220 CONDUCTIVITY AND VISCOSITY IN MIXED SOLVENTS. 



alcohols, however, the molecules are quite complex, as is shown by the sur- 

 face-tension method of Ramsay and Shields. The water has thus broken 

 down the complex molecules of alcohol into the simpler molecules. 



A more direct line of evidence bearing upon this problem has been furnished 

 by the work of Jones and Murray. They worked with mixtures of water, 

 formic acid, and acetic acid each with the other. It will be recognized that 

 these are all associated liquids. The molecular weight of each of these liquids 

 dissolved in each of the other two was determined by the freezing-point 

 method. It was found that the molecular weight was smaller the more dilute 

 the solution, and even in the most concentrated solutions that could be 

 studied, the molecular weight was always much less than the molecular weight 

 of the substance when in the pure homogeneous condition. This showed 

 beyond question that the action of an associated liquid is to diminish the 

 association of another associated liquid. 



The above explanation to account for the conductivity minimum in the 

 mixed solvents is, then, undoubtedly an important factor. We shall, however, 

 see that this is only one factor in conditioning the existence of this minimum. 

 The investigation by Carroll included the same solvents that had been used 

 by Lindsay, i. e., water, methyl alcohol, ethyl alcohol, and binary mixtures 

 of these solvents; and in addition acetic acid was also used. The electro- 

 lytes employed by Carroll are cadmium iodide, sodium iodide, calcium nitrate, 

 hydrochloric acid, and sodium acetate, in mixtures of acetic acid and water. 

 The minimum in the molecular conductivity was found for cadmium iodide, 

 sodium iodide, and hydrochloric acid, in mixtures of methyl alcohol and water. 

 The dissociation of sodium iodide, potassium iodide, and potassium bromide 

 in a 50 per cent mixture of methyl alcohol and water was determined directly, 

 and was found to be apparently slightly greater than in water alone at the 

 same dilution. 



The question as to the cause of the minimum in conductivity was then taken 

 up. It was shown that there was a parallelism between the conductivity 

 minimum and the minimum in the fluidity of the solvent. It was further 

 shown that both minima are more pronounced at lower temperatures, and that 

 both occur at approximately the same points. Again, the effect of rise in 

 temperature is the same upon both minima, i. e., a shift towards the mixture 

 containing a greater per cent of alcohol. 



From a quantitative study of these two classes of phenomena, the con- 

 clusion was drawn that the decrease in conductivity in electrolytes when 

 dissolved in binary mixtures of various alcohols and water, which is frequently 

 accompanied by a well-defined minimum in conductivity, is due largely to 

 the diminution in the fluidity of the solvent which takes place when the two 

 solvents are mixed. This diminishes the velocity with which the ions move, 

 and, consequently, diminishes the conductivity. 



