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 is the 
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 cc. of a mixture CuSO, , 9O,, began to be cloudy on 
addition of 43.88. v ee ‘caustic potash. 
43. O7 66 “ce 
43.99 is * ‘ 
44.86 iss oe Ce 
43.92 = mean. 
Thus, in these determinations, the difference between the greatest 
and least values would be about .3//. 
A second set of determinations is added: 
5 ec. of a mixture CuSO, + H,SO,, began to become cloudy 
on addition of 28.94 ec. of decinor mal caustic potash. 
28.91 as 
98.99 (a3 ia3 ce a3 
28.95 = mean 
In this case, the difference between the greatest and _ least 
values is about .277/. 
