72 



ELECTRICAL CONDUCTIVITIES, ETC. 



The vessels used for holding the solutions were of Jena glass, which had been 

 treated for months to remove all soluble matter. The increase in conductivity in 

 the heated solutions could, therefore, not have been due to matter dissolved from 

 the glass vessels. It will be seen that the solutions which had been heated had 

 higher conductivity than those which had not. This is especially true of the solu- 

 tion which had been heated to 90. This was undoubtedly due to hydrolytic dis- 

 sociation of the salt into acid and base, and these did not completely recombine on 

 cooling the solution to the initial temperature. 



The same process was repeated, using a solution of chromium chloride which was 

 w/512, heating one part to 50, another to 65, still another to 90, then cooling all 

 down to room temperature, and measuring the conductivities at the following tem- 

 peratures : 



Chromium Chloride. 



The results for the more dilute solutions are of the same general character as those 

 for the more concentrated. 



To throw some light on the length of time required for the acid and base to recom- 

 bine, the following experiment was carried out: The w/512 solution of chromium 

 chloride, which had been heated to 90, was cooled to room temperature and allowed 

 to stand for 20 days. It was then warmed to 35, and its conductivity determined. 

 The value found was 508, while the value found shortly after heating was 559.9. 

 The unheated solution gave a conductivity of 487.4. It is thus obvious that in 20 

 days the hydrolysis had not all disappeared. 



A similar experiment with w/512 chromium chloride, which had been heated to 

 90, cooled to room temperature and allowed to stand 20 days, and then warmed to 

 65 and its conductivity determined, gave the value 885. The conductivity shortly 

 after heating was 915.1. The conductivity of the unheated solution was 842.5. 

 This shows that the dehydrolysis, in this case, was not complete even after the solu- 

 tion had stood for 20 days. We propose to study these changes quantitatively in 

 the near future, and see how long it requires for the completion of the dehydrolysis, 

 in the cases especially of those salts which are strongly hydrolyzed. 



The bearing of these facts on the purification of salts by recrystallization from 

 water is important. The usual method of purification, by preparing a saturated 

 solution at a higher temperature and then lowering the temperature and allowing the; 

 salt to crystallize is open to objection, especially for those salts which are strongly 

 hydrolyzed by water. It has been supposed that when the solution in question was 

 cooled down, the free acid and free base recombined. This work shows that such is 

 not the case. There remains in the solution, for a long time, some free acid ; and when 

 the salt crystallizes from such a solution it is likely to occlude some of the free acid. 



