GENERAL DISCUSSION OF RESULTS. 203 



same electrolytes would give such abnormally great lowerings of the freezing-point 

 of water. After this fact was brought to light in my laboratory, the question arose 

 whether we can use the freezing-point method as an accurate measure of dissociation 

 at any dilution. To test this point Dr. Pearce carried out the elaborate investiga- 

 tion recorded in this monograph. The dissociations of a large number of electro- 

 lytes were measured by the freezing-point method, and the results were compared 

 with those obtained by the conductivity method. For this purpose the freezing- 

 point method must be improved as much as possible and all sources of error reduced 

 to a minimum. The results recorded in this chapter are to be regarded as among the 

 most reliable freezing-point measurements that have been made. Similarly, the 

 conductivity results are among the most accurate that had been obtained up to that 

 time. The conductivity method and apparatus have subsequently been somewhat 

 improved, yet the agreement between the values obtained by Pearce and those of 

 subsequent work 1 are, in general, very good. This investigation, in which the dis- 

 sociation was measured by both the freezing-point and the conductivity methods, 

 ought to show to what extent each method is influenced by the hydration of the ions. 

 It ought also to throw some light upon the effect of hydration on the relative veloc- 

 ities of the ions. 



It would seem on a priori grounds that hydration ought to affect the freezing- 

 point method more than the conductivity method as a means of measuring electro- 

 lytic dissociation. If a part of the water is combined with the dissolved substance, 

 this is removed from the field of action as far as solvent is concerned. Freezing- 

 point lowering is an arithmetical property. It depends upon the ratio between 

 the number of molecules of the dissolved substance and those of the solvent. Freez- 

 ing-point lowering would, therefore, be affected directly by any loss in solvent water 

 due to a combination of a part of the water with the substance dissolved in it. 



The conductivity method of measuring dissociation would also be affected by water 

 of hydration. The hydrated ions would have greater mass than the unhydrated, 

 and their velocities would, therefore, be less. Hydration would also have some 

 effect on the viscosity of the solution. Taking all of these factors into account, it 

 would seem that hydration would affect the freezing-point method more than the 

 conductivity method, and dissociation as measured by freezing-point lowering ought 

 to have slightly higher values than as measured by conductivity. We shall see 

 whether or not this is the case. 



Pearce also determined the freezing-point lowerings produced by concentrated 

 solutions of the electrolytes with which he worked, and from the results calculated 

 the approximate compositions of the hydrates formed by these substances at the 

 various dilutions. Pearce worked especially with strongly hydrated substances, 

 such as the chlorides of calcium, strontium, barium, and magnesium, and with the 

 nitrates of these same elements. From the results with these substances it seemed 

 that dissociation as measured by the freezing-point method was slightly greater 

 than as measured by conductivity. This work was extended to the chloride and 

 nitrate of cobalt, the chloride and nitrate of copper, and nickel nitrate. From these 

 results it would seem that the molecular hydration and the total amount of combined 



'Carnegie Institution of Washington Publication No. 170. 



