27-29] Estimation of Sulphuric Acid 23 



ESTIMATION OF SULPHURIC ACID (S0 3 ) 



27. Substance used.— Copper sulphate, CuSO4.5H.2O. 

 Method employed.— The S0 3 is precipitated by barium 



chloride and weighed as barium sulphate. 



Weigh out about *5 gram of pure crystallised copper sul- 

 phate on a watch glass. Transfer to a 10-oz. beaker, and 

 dissolve as in the previous estimation, using HC1 instead of 

 H 2 S0 4 to clear the solution. 



28. Precipitation. — Add to the liquid in the beaker 

 about 20 c.c. of ammonium chloride solution and bring it to 

 the boil over a Bunsen. The reason for adding this NH 4 C1 is 

 that barium sulphate is thrown down in a more granular form 

 in the presence of this salt than is otherwise the case. This 

 minimises the risk of the precipitate being so fine as to pass 

 through the pores of the filter paper, which is the chief 

 difficulty with this estimation. As soon as the solution boils 

 add excess of boiling barium chloride solution. 1 Cover the 

 beaker with a watch glass, and boil for about half-a-minute ; 

 then allow the precipitate to settle completely. Add another 

 drop of barium chloride to make sure that no sulphate re- 

 mains in solution. Should a further precipitate be caused, the 

 liquid must be stirred, reheated, and a further quantity of 

 boiling barium chloride added. 



If the above directions have been followed exactly, the 

 barium sulphate will have settled in about ten minutes. 



29. Washing. — The precipitate should be washed about 



five times by decantation (paragraph 12), then transferred to 



the filter and washed with hot water from the wash bottle 



until a few drops of the filtrate no longer give a cloudiness 



when tested with sulphuric acid. The test tube in which this 



1 For the estimation of sulphuric acid the ordinary barium chloride 

 solution is not very satisfactory : a saturated solution should be used. 



