AND COPPER SULPHATE LINDSAY. 207 



The ordinary methods of acid titration are, of course, unavail- 

 able in this case, for not only does the copper sulphate itself 

 affect alkalimetric indicators, but the sulphate is precipitated as 

 hydroxide, by the base used for titration. The latter fact isth e 

 one used in the method of titration which was employed. 



Standard caustic potash solution is added from a burette to 

 the mixture, with constant stirring, until the solution just begins 

 to become cloudy, owing to the beginning of the precipitation of 

 the hydroxide of copper. I found that, using this precipitating 

 point as an indicator, very good determinations of the acid 

 present could be obtained, and would suggest that copper sul- 

 phate might be used as an indicator in the determination of free 

 sulphuric acid, in cases where the ordinary indicators are of 

 no use. 



The following results of an analysis will show with what 

 accuracy the determination of this precipitating point could be 

 ascertained : 



5 c.c. of a mixture CuS0 4 + H 2 S0 4 , began to be cloudy on 

 addition of 43.88 c.c. decinormal caustic potash. 

 43.97 

 43.99 

 44.86 



43.92 = mean. 



Tnus, in these determinations, the difference between the greatest 

 and least values would be about .3%. 



A second set of determinations is added : 



5 c.c. of a mixture CuSO 4 + H 2 SO^, began to become cloudy 

 on addition of 28 94 c.c. of decinormal caustic potash. 

 28.91 

 28.99 



28.95 = mean 



In this case, the difference between the greatest and least 

 values is about .27%. 



