112 CONDUCTIVITY AND VISCOSITY OF SOLUTIONS 



of their difficult solubility in this solvent. However, their great solu- 

 bility in water made it possible to obtain quite concentrated solutions 

 in the majority of the mixed solvents, even in that containing 90 per 

 cent acetone. Only in one instance, that of N/2 rubidium chloride 

 in 75 per cent acetone, was a solution obtained which was non-miscible 

 with the solvent at 20, and at 15 a homogeneous solution was obtained. 

 Table 31 contains the values found. 



From our previous work on these salts in glycerol and water, we 

 should naturally expect to find instances of negative viscosity in acetone- 

 water mixtures. However, the peculiarity of acetone as a solvent at 

 once makes itself evident. Except in those mixtures containing the 

 larger percentage of water, it will be noted that these salts increase the 

 viscosity of the various solvents. Jones and Veazey had already noted 

 this phenomenon in the case of potassium sulphocyanate, but the nega- 

 tive effect produced by rubidium salts is so great in other solvents that 

 the two classes of salts can hardly be regarded as comparable. 



A glance at the tables will show that rubidium iodide and nitrate, the 

 two salts found to give the greatest viscosity lowering in glycerol-water 

 and their mixtures, produce a marked increase at all dilutions in the 

 viscosity of the solvents up to the 50 per cent acetone mixture. Beyond 

 this point the fluidity curve (fig. 46) for the salts crosses that of the 

 solvent, and a negative viscosity effect becomes apparent in the mix- 

 tures containing the lower percentages of acetone. The 50 per cent 

 mixture is apparently very close to the transition-point, since certain 

 dilutions apparently increase the viscosity of the solvent, while others 

 lower it. It would seem that in mixtures from 100 per cent to 50 per 

 cent acetone the molecular volume of the dissolved salt is smaller than 

 the molecular aggregates of the solvents; and in the other mixtures, 

 larger. The salts lower the viscosity of pure water, because according 

 to Jones and Veazey 's theory their molecular volumes are greater than 

 the complexes of the solvent. On the addition of acetone having appar- 

 ently much larger molecular complexes, this negative viscosity effect 

 becomes less and less with increasing percentage of acetone, until we 

 reach a mixture in which the two factors balance one another. This 

 point is in the neighborhood of the 50 per cent mixture. By still 

 further increasing the percentage of acetone, the aggregates of the 

 solvent exceed the molecules and ions of the solute in size and a 

 positive viscosity effect results. 



Associated with each table of viscosities and fluidities is a corre- 

 sponding table of temperature coefficients. Their relations to those of 

 conductivity are taken up in the discussion of that phase of the work, 

 which immediately follows. 



It was found by Jones and Veazey 1 that the curves expressing the 

 fluidity of varying mixtures of acetone and water are almost exactly 



J Amer. Chem. Journ., 37, 405 (1907). 



