204 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



ductance of the sulphocyanate ion is markedly greater than that of the 

 iodide ion. 



It is evident that, applied to solutions in non-aqueous solvents, the 

 theory of Born and Lorenz meets with great difficulties. The values of 

 the ion conductances in different solvents are in much better agreement 

 with the assumption that the differences in the ion velocities of the 

 simpler ions are primarily due to the size and nature of the complexes 

 formed between the ion and the solvent medium. An ion will, in gen- 

 eral, exhibit a low conductance in a medium with which it has a great 

 tendency to form stable complexes. On the other hand, in media with 

 which the tendency to form complexes is less pronounced, its conductance 

 will be relatively high. Thus, the silver and ammonium ions in ammonia 

 have a relatively low conductance; while in acetone, sodium and lithium 

 have a relatively high conductance. The properties of solutions of these 

 salts in the solvents mentioned indicate a high solvation in ammonia 

 solutions and a low solvation in acetone solutions. 



It has been demonstrated by means of transference measurements in 

 aqueous solutions that the alkali metal and hydrogen ions are hydrated. 

 While the absolute degree of hydration remains uncertain, it is probably 

 safe to assume that the hydration of the hydrogen ion is not less than 

 unity, which requires a hydration in the neighborhood of 5 for the 

 potassium ion and 14 for the lithium ion. 



As we have seen in Chapter V, the relation between the conductance, 

 i.e., the speed of an ion, and the viscosity of the medium through which 

 it moves is anything but simple. The conductance of ions of small 

 dimensions is not proportional to the fluidity of the ionizing solvent. 

 On the addition of a second non-ionic component, the conductance change 

 of the ion is approximately proportional to the fluidity change of the 

 medium only when the molecules of the added substance are small. 

 When the dimensions of the molecules of the added substance are large 

 compared with those of the ions, the conductance change is invariably 

 smaller than the fluidity change. 



Finally, Dummer 2e has measured the diffusion coefficients of a num- 

 ber of organic solvents of varying molecular volume in one another and 

 compared his results with one another by means of the Einstein-Stokes 

 equation. The molecular dimensions found for the same substance in 

 different solvents do not agree well with one another. The Einstein- 

 Stokes equation should be applied with caution to systems of particles 

 of molecular dimensions. 



Dummer ,Ztschr. f. anorg. Ohem. 109, 31 (1919). Compare also, oholm, Medd K 

 Vetens-Akad's Nobehnstitut, Nos. 23, 24 and 26 (1912). 



