142 HYDRATES IN AQUEOUS SOLUTION. 



The total amount of water combined with the dissolved substance, in both 

 cases, increases with the concentration, as we should expect. The number 

 of molecules of water combined with one molecule of the dissolved sub- 

 stance, in the case of glycerol. increases with a fair degree of regularity, with 

 the dilution of the solution. 



A number of organic acids were also studied. These include acetic, 

 oxalic, succinic, tartaric, and citric acids. The results are recorded in tables 

 94 to 98 and the conductivity data plotted in curves, fig. 63. None of the 

 organic acids studied in this investigation show any appreciable tendency 

 to combine with water in solution. Some exhibit a marked tendency to 

 undergo polymerization in aqueous solution. 



GENERAL RELATIONS. 



It must be remembered that the problem of calculating even the approx- 

 imate composition of the hydrates existing in solution is difficult. Some 

 of these difficulties have already been mentioned. Certain assumptions 

 must be made that are only approximately true. This is the case especially 

 with the law of Raoult, which probably does not hold rigidly for concentrated 

 solutions. Further, it has already been pointed out that the conductivity 

 method is only a rough measure of dissociation in concentrated solutions. 

 Taking all of the difficulties into account, we are still of the opinion that 

 we can get a fairly good idea as to the composition of the hydrates formed 

 by the various electrolytes, in aqueous solutions of different concentrations. 



An examination of the results recorded in this monograph will show that 

 the amount of water held in combination by the dissolved substance in- 

 creases as the concentration of the solution increases. This is, of course, 

 what would be expected in terms of mass action. 



The number of molecules of water in combination with one molecule 

 of the dissolved substance, frequently increases from the most concentrated 

 to the most dilute solution, as with magnesium chloride and bromide, man- 

 ganese chloride, nickel chloride, and copper chloride. 



With some substances the number of molecules of water held in combina- 

 tion by one molecule of the dissolved substance may pass through a well- 

 defined maximum as the dilution is increased. In other cases, the number 

 of molecules of water held in combination by one molecule of the dissolved 

 substance may reach a maximum value as the dilution is increased; this 

 maximum value may then remain practically constant with further increase 

 in the dilution. Examples of all of these conditions are to be found in the 

 substances discussed in this monograph. 



An examination of the curves, figs. 46 to 52, will show that they are all 

 of the same general type. These express the relation between the total 

 amount of water in combination, and the concentration of the solution. 

 This resemblance in type is what we should expect, because the total amount 



