176 QUANTITATIVE ANALYSIS. 



of pure rubber tubing. Catch the filtrate, which must be per- 

 fectly clear, in a clean beaker, then wash the precipitate with 

 hot water until the last wash- water is no longer made turbid 

 by silver nitrate solution. Dry the filter with its contents 

 (after it has been filled once with alcohol and once with ether), 

 place it in a platinum or porcelain crucible, put the cover 

 partly on and heat, at first gently and then more strongly, 

 until the contents of the crucible appear pure white. Let 

 cool in the desiccator, weigh, heat once more, cool, and weigh 

 again. Heating with the blast-lamp is not advisable. Make 

 two determinations. 233.46 parts by weight of barium sul- 

 phate correspond to 80 parts of S0 3 . Copper sulphate con- 

 tains 32.03 per cent. S0 3 . 



2. DETERMINATION OF COPPER IN COPPER SULPHATE AS 

 CUPRIC OXIDE. 



Dissolve about 0.8 to 1 g. (accurately weighed) in 80 to 

 100 cc. of water in a porcelain dish, heat until it begins to 

 boil, take away the flame and add, with constant stirring, 

 dilute sodium hydroxide solution to alkaline reaction. Con- 

 tinue the heating on the water-bath or cautiously on the wire 

 gauze until the precipitate has become entirely black, let 

 settle, decant off the supernatant fluid through an ash-free 

 filter, cover the copper oxide with water, heat again, let set- 

 tle, decant again, etc., and repeat this operation once or twice 

 more. Finally place the entire precipitate on the filter, 

 wash thoroughly with hot water (the last wash-water must 

 not become turbid when treated with hydrochloric acid and 

 barium chloride solution), dry, and heat the precipitate, 

 together with the filter, in an open porcelain crucible, at first 

 gently and then strongly. In case some of the copper oxide 

 sticks so firmly to the dish that it cannot be removed by the 

 glass rod, dissolve it in a. few drops of nitric acid, evaporate 

 the solution on the water-bath in the porcelain crucible 



