158 Conductivities and Viscosities in Pure and in Mixed Solvents. 



the fact that the sodium salt crystallizes with 12 molecules of water, 

 indicating great hydration in solution; while the potassium salt has no 

 water of crystallization, which indicates only slightly hydrated ions 

 in solution. The ammonium chromate is somewhat similar to the 

 unhydrated potassium salt mentioned above, in that it carries no water 

 of crystallization, and would therefore be expected to show greater 

 conductivity than the corresponding hydrated salts of sodium and 

 lithium. The high values of ^ for potassium ferricyanide is largely 

 due to the great number of ions yielded by this compound. The work 

 of Getman and Bassett 1 indicates the production of 6 ions in solutions 

 of this salt. By comparing the conductivities of the chromates of 

 ammonia and the alkali metals, it is found that they stand in the fol- 

 lowing order: 



Potassium chromate > ammonium chromate > sodium chromate > 

 lithium chromate. 



Ammonium compounds, as a rule, show higher conductivity than 

 the corresponding potassium salts. This does not seem to be true in 

 the case of the chromates. The smaller conductivities of lithium 

 compounds when compared with compounds of sodium, is usually 

 attributed to the greater hydration of the lithium ion in solution, as 

 indicated by the greater tendency of lithium salts to crystallize with 

 water. 1 The chromate of lithium, however, crystallizes with 1 mole- 

 cule of water, while the chromate of sodium contains 10 molecules. 

 The work of Jones and Bassett 2 has shown that many substances have 

 greater hydrating power than is indicated by the water of crystalli- 

 zation contained in them. Such may be the case with lithium chromate. 



Rubidium iodide shows a higher conductivity than the iodides of 

 the other alkali metals. Knowing that rubidium has a greater atomic 

 volume than sodium or potassium, we might expect the conductivity to 

 be lowered by a decrease in the velocity of the ions, due to their volume 

 and mass. It should, however, be remembered that the hydrating 

 power of these compounds of the alkali metals decreases with increasing 

 atomic volume. Just as sodium salts are less hydrated than lithium, so 

 rubidium compounds would be expected to hydrate less than potassium. 

 Thus, an apparent exception is explained by the theory of hydration. 



DISSOCIATIONS. 



As a means of determining the dissociation of salts in solution, the 

 conductivity method is of great service, but it is far from perfect. 

 Hydrolysis, hydration, and polymerization all militate against obtaining 

 a true value for fj, K . Since most salts show one or more of the above- 

 named phenomena, it is certain that dissociations calculated from con- 

 ductivity data are in most cases simply close approximations. 



Carnegie Inst. Wash. Pub. No. 60, p. 46 (1907). 2 Amer. Chem. Journ., 33, 562 (1905). 



