Dissociation of Compounds in Water. 477 



a monovalent anion and a monovalent kation. Each ion con- 

 tains only one element, with the exception of the kation NH 4 , 

 which resembles so closely the alkali elements. This is the 

 simplest kind of dissociation, and regularities which appear 

 with these salts might not be found when the molecules are 

 more complicated and yield more complex ions. It may be 

 said in advance that such regularities do exist in general, and 

 will be seen from an examination of the following compounds. 



Potassium Sulphate. — The molecules of potassium sulphate 

 dissociate into two univalent kations and one bivalent anion. 

 The molecules here, as w T ith other compounds which give 

 three ions, may dissociate at first into K and KS0 4 , and then 

 with increase in dilution the ion KS0 4 dissociate into K and 

 S0 4 . The freezing-point method is not capable of deciding 

 this point, since it has to do with the quantity and not the 

 quality of the ions present. This problem must be solved by 

 a study of some property of the particular ions, electrical or 

 otherwise. 



The complete dissociation of potassium sulphate into three 

 ions is less than the complete dissociation of potassium chloride 

 for all the dilutions measured. This difference increases 

 rapidly with the concentration. At O'Ol n it is about 8 per 

 cent., while at 0*1 n it is about 20 per cent. This rapid 

 decrease in dissociation with increase in concentration for 

 potassium sulphate is seen in curve 4, Plate IV., which 

 rapidly rises from the abscissa and approaches curve 3. 



The comparison between the isotonic coefficients from 

 osmotic pressure and the molecular lowering of the freezing- 

 point for this and other salts has been made by de Vries^. 



Barium Chloride. — The molecules of barium chloride, like 

 the molecules of potassium sulphate, dissociate into three ions, 

 but into one bivalent kation and two univalent anions. The 

 complete dissociation here is less than for the alkali chlorides, 

 and this difference also increases with the concentration. The 

 decrease in dissociation with increase in concentration is less 

 rapid than for potassium sulphate. 



Magnesium Sulphate. — The molecules of this salt dissociate 

 into two ions, a bivalent kation and a bivalent anion. This 

 differs from the alkali chlorides, which also yield two ions, in 

 that both ions in the former case are bivalent, in the latter 

 univalent. It also differs from the two salts just considered, 

 which give three ions, one bivalent and two univalent. 



The compounds thus far considered can be regarded as 

 typical representatives of four different ways in which mole- 

 cules dissociate into ions : — 



* de Vries, Zeits. phys, Chem. ii. p. 427. 



