212 



HYDRATES IN AQUEOUS SOLUTION. 



volume of water added so greatly increases the volume of the solution as to 

 change the concentration of the cobalt chloride itself, quite independently of 

 any change in the amount of hydration of this salt. In other words, the 

 colors of the two solutions can not readily be made the same under com- 

 parable conditions, when the concentrations differ very widely. 



The matter under consideration was carefully tested for some of the solu- 

 tions of cobalt chloride and aluminium chloride as follows: The lower 

 compartment of a double-cell was filled with the less concentrated solution 

 of a chosen pair. A measured volume (4 cc.) of the more concentrated 

 solution was placed in the upper compartment of the cell. Then pure 

 water was run into the upper solution from a burette, until the color in the 

 two compartments appeared the same. The upper solution was of course 

 made homogeneous. The volumes of water which were both insufficient 

 and too great to dilute the more concentrated solution to the color of the less 

 concentrated were noted, as well as the volume of water which was necessary 

 to produce approximate equality of color. The mean of several trials was 

 taken. In some cases the solutions were examined with the Hilger spectro- 

 scope. In this way it was possible to obtain data which are at least of the 

 right order of magnitude. The results are given in table 108. The first 

 column (cj gives the concentration of the solution to which the water was 

 added. The second column (c 2 ) gives in the same line the concentration of 

 the solution which was used as the standard for color. The difference in the 

 concentrations (c t c 2 ) is shown by the third column. The volume (v) of 

 water necessary to produce equality of color is given in the fourth column. 

 The fifth column gives unit volume of water per unit volume of solution, 

 per unit concentration of aluminium chloride, and per unit change in concen- 

 tration, i. e., 



v 



4ci (ci c 2 )' 

 TABLE 108. 



The numbers in the last column show a definite decrease as the concen- 

 tration of the dehydrating agent increases. 



