OTHER PROPERTIES OF ELECTROLYTIC SOLUTIONS 281 



It will be observed that in the more dilute solutions the diffusion 

 coefficient is the greater, the greater the conductance of the electrolyte. 

 Thus, at 0.01 normal, the diffusion coefficient of HC1 is 2.324, of KOH 

 1.903, of KC1 1.460, and of LiCl 1.000. As the concentration increases, 

 the diffusion coefficient in the more dilute solutions decreases. This may 

 be accounted for if we assume that as the concentration increases the 

 ionization decreases, and that the diffusion coefficient of the neutral 

 molecules is smaller than that of the ions. At higher concentrations the 

 influence of viscosity change must be taken into account. In the case 

 of most salts, the viscosity increases with increasing concentration, and 

 it is to be expected that, owing to this factor, there will be a decrease in 

 the diffusion coefficient at higher concentrations. The increase in the 

 value of the diffusion coefficient at very high concentrations cannot be 

 accounted for in this way. If, however, the ions are hydrated, then it is 

 not improbable that at the higher concentrations, where the number of 

 salt molecules becomes comparable with that of the number of water 

 molecules, the degree of hydration of the ions decreases, as a result of 

 which their mobilities may be expected to increase. 



Of particular significance are the results obtained by Arrhenius 8 

 for the diffusion of electrolytes in the presence of other electrolytes. If 

 the diffusing electrolyte has a rapidly and a slowly moving ion, the dif- 

 fusion of the rapidly moving ion is hindered, owing to the drag exerted 

 upon it by the charge on the more slowly moving ion. If, now, another 

 electrolyte is added, the rate of diffusion of the first electrolyte will be in- 

 creased, since the diffusion of the oppositely charged ion may be compen- 

 sated by the diffusion of another ion in the opposite direction. For exam- 

 ple, the diffusion coefficient of a 0.52 N solution of HC1 in water at 12 

 is 2.09, while that of the same electrolyte in 3.43 N solution of NH 4 C1 is 

 4.67, and in a 0.375 N solution of KC1 3.89. Evidently, on adding am- 

 monium chloride to the hydrochloric acid solution, the rate of diffusion 

 is greatly increased due to the fact that the motion of the Cl~ ions in the 

 direction of the concentration gradient is compensated by a motion of the 

 NH 4 + ions in the opposite direction. This phenomenon is quite general, 

 as may be seen from Table CXI. 



The influence of the added electrolyte on the diffusion coefficient is 

 extremely marked. For example, the addition of 0.028 N KC1 to a 1.04 N 

 solution of HC1 raises the diffusion coefficient from a value of 2.09 to 2.27, 

 or approximately ten per cent. Effects such as these afford perhaps the 

 strongest grounds we have for believing that electrolytes are ionized. 

 On the other hand, they do not enable us to determine to what extent 



Arrhenius, Ztschr. /. phyg. Chem. 10, 51 CL892). 



