ELECTROLYTES AND THEIR ACTION 



189 



afterwards through water containing the indicator alone. When used for titration, for 

 which purpose the arrangement is particularly adapted, the acid or alkali is added to 

 the cell containing the indicator alone until the change in colour corresponding to the 

 required concentration in H' ion is obtained. This cell is then observed by light which 



has passed through a depth of the coloured solution equal to that to which the indicator has 

 been added. Acid or alkali is then added to the latter until its colour is the same as that , ot 

 the combination of the coloured solution with the indicator solution in separate vessels, 

 absorption due to the coloured substance is obviously identical in the two cases. 



The table given in Fig. 56, which is extracted from the results of Salm (1906) 

 and of Sorensen (1909), may be useful. With the exception of those indicators 



