160 



Prof. J. Walker. 



[Feb. 3, 



Of the amphoteric substances measured by Winkelblech* all show 

 a ratio k b /K at least equal to 100. In order then that the expression 



ji u should become small in comparison with unity, the concentration of 



the unionised substance (which is roughly equal to the total concentra- 

 tion for the compounds he investigated) must be of the order 10~ 4 , 

 that is, v must be of the order 10,000. Solutions of feeble electrolytes 

 having this dilution are practically beyond our ordinary means of 

 measurement of electrical conductivity, so that we may conclude that at 

 customary dilutions the amphoteric electrolytes studied by Winkelblech 

 cannot give a dissociation constant when the values of the conductivity 

 are treated in the ordinary way. This conclusion is in accordance with 

 AVinkelblech's electrical measurements, from which he endeavoured in 

 five instances to calculate a dissociation constant. The values he 

 obtained at different dilutions varied for each substance from 20 to 50 

 per cent. 



In order to exhibit the effect of the presence of even feebly marked 

 basic character in an amphoteric acid, I have calculated the concen- 

 trations of the positive ions H + and HX + for substances possessing the 

 acid constant k a = 10~ 5 and basic constants 1*2 x 10 -14 , 1*2 x 10~ 13 , 

 1*2 x 10" 12 , and 1*2 x 10 _n respectively, the corresponding ratios h^jK 

 at 25° being 1, 10, 100, and 1000. The calculation was made for the 

 most part by approximation, it being assumed in the first instance that 

 the total concentration was equal to the concentration of the unionised 

 substance. A second approximation in which the value of u obtained 

 from the first calculation was adopted usually sufficed. If a great 

 many dilutions have to be calculated, the employment of graphical 

 methods may effect a saving of time. 



In the following table all values of a and d have been multiplied by 

 10 : *. For comparison, the values for fo/K = 0, i.e., for a simple acid 

 have been added. 



V. 



Tc b \K 



= 0. 



Te b \K 



= 1. 





= 10. 





= 100. 



Tc b \K^ 



-- 1000. 



a. 



d. 



a. 



d. 



a. 



d. 



a. 



d. 



a. 



d. 



1 



316 









224 



223 



95 '3 



943 



31 5 



3050 



9-99 



9091 



10 



100 









95 



8 -5 



70 5 



69 -5 



30 1 



291 



9-94 



904 



100 



31 



•2 







31 



3 



29 -7 



2-9 



22 -1 



21 



9'49 



86 



1000 



9 



'5 







9 5 



o-o 



9 4 



o-i 



9-06 



0-8 



6 79 



6 



The values of a for a given dilution fall off as 7c& increases, and that 

 * Loc. ext., p. 587. 



