20 Conductivities and Viscosities in Pure and in Mixed Solvents. 



As is now well established, the dielectric constant of the solvent is 

 a measure of its own association, and therefore a measure of its 

 dissociating power for electrolytes. Of the common solvents, water 

 has the highest dielectric constant, and is the best dissociant. Forma- 

 mid, however, has a higher dielectric constant than water, as will be 

 shown by referring to the table of physical constants of the two 

 solvents (table 5), and should therefore be a better dissociant, which 

 in our discussion will be pointed out to be the fact. 



PURIFICATION OF THE SOLVENT. 



It might be well to state at the outset, that neither a clean melting- 

 point nor a constant boiling-point is a sufficient criterion for con- 

 ductivity purposes as to the purity of the solvent, since it has been 

 found by ourselves and others, that a constant-boiling liquid such as 

 formamid, may be separated into fractions of widely different specific 

 conductivities. That this is true is due, no doubt, in this particular 

 case, to the fact that minute quantities of the products of hydrolysis, 

 such as would have no measurable effect on the apparent boiling- 

 point of the liquid, on account of the high dissociating power of the 

 formamid, produce a marked increase in its conductivity. 



The chief criterion, therefore, in judging of the purity of the solvent 

 used in this investigation was its specific conductivity. The material 

 with which we started was obtained from Kahlbaum, and had a specific 

 conductivity of about 674 X 10~ 5 , or about that of tap-water. Samples 

 obtained from Bender and Hobein, from Schuchardt, and from Hoffman 

 and Kropf proved from the conductivity standpoint to be little or no 

 better than the above. 



As has already been mentioned, formamid is hygroscopic, forms a 

 true solution with water, and subsequently undergoes slow hydrolysis 

 into ammonium formate. The first problem, therefore, that must be 

 solved, was the removal of any dissolved water not already acted on; 

 second, the removal of the products of hydrolysis already present. 

 The method of fractional distillation made it necessary to design and 

 construct suitable apparatus for distillation in comparatively high 

 vacuo. 



A third problem presented itself in connection with the preservation 

 and subsequent manipulation of the solvent and of solutions in it, in 

 such a manner as to incur minimum exposure to moisture. In addition, 

 the expense of the solvent made necessary the recovery of it from 

 solutions of salts in this solvent, with the least possible loss by decom- 

 position. 



The removal of dissolved water was finally effected by the use of 

 carefully dehydrated sodium sulphate. After testing a number of 

 other dehydrating agents, such as magnesium sulphate, calcium chlor- 

 ide, sulphuric acid in vacuo, etc., it was found that sodium sulphate 



