IN MIXTURES OF ACETONE AND WATER. 



113 



parallel to those for the conductivity of potassium sulphocyanate in 

 solution in the same mixtures. The fluidity curve for acetone has a 

 minimum between 37.5 per cent and 50 per cent; and the fluidity for 

 the rubidium halides and nitrate has its minimum in approximately the 

 same position (fig. 46). The conductivity curves, however, of the rubi- 

 dium salts have their minima corresponding to a much greater per- 

 centage of acetone (fig. 47). As has been shown by previous workers, 

 this minimum in fluidity occurs at the position where the breaking 

 down of association of one solvent by the other is greatest. The con- 

 ductivity depends upon the velocities of the ions and the degree of 

 dissociation. The dissociation is least when the association of the 

 solvent is least, and the speed of the ions is least when the fluidity is 

 greatest. Therefore, if these were the only determining factors, the 

 conductivity minima always correspond to the fluidity minima. 



110 



100 



90 



100 87.5 75 62.5 50 37.5 25 12.5 

 Percentage acetone 



FIG. 46. Fluidity of rubidium bromide, 

 solution and solvent. 



70 



75 



62.5 



50 



37.5 



25 



12.5 



Percentage acetone 



FIG. 47. Conductivity and viscosity of rubid- 

 ium iodide in acetone-water at 25. 

 Curve I, ordinates, molecular conductivity. 

 Curve II, ordinates, fluidity. 



Potassium sulphocyanate has a considerable solubility in pure ace- 

 tone (about 20 grams in 100 grams of acetone at 20), whereas the 

 rubidium salts studied are only slightly soluble in pure acetone. There- 

 fore, in the same concentrations, the rubidium salts are nearer satura- 

 tion than potassium sulphocyanate. The percentage dissociation is, 

 therefore, lower in the case of rubidium salts than in that of potassium 

 sulphocyanate. A possible explanation of the shifting of the minimum 

 in the conductivity of rubidium salts towards the greater proportions of 

 acetone, is that the great insolubility in acetone might cause the disso- 



