Methods of Analysis 233 



SO., fumes come off. The material is now washed into a casserole, 

 moistened witli sulphuric and nitric acids and digested at a 

 low heat for at least one hour. The heat is then increased until 

 SO;, fumes are again driven off. The mass is moistened with 

 three to four times its bulk of distilled water and digested at a 

 gentle heat from one to two hours, filtered hot and then tlie fil- 

 trate and washings evaporated almost to dryness, thus driving off 

 the excess of sulphuric acid. The resulting residue is taken up 

 with hot water and again filtered to separate the solution from 

 precipitated calcium sulphate. This evaporation and filtration 

 may have to be repeated one, two or three times in order to get 

 the solution sufficiently free from calcium sulphate. The final 

 filtrate, which contains the copper, is then diluted to about 150 

 to 200 c.c. in a tall beaker, a small quantity of hydrochloric acid 

 is added and hydrogen sulphide passed until the solution is 

 thoroughly saturated. During the hydrogen sulphide precipi- 

 tation there should be no nitric acid or nitrates present in the 

 solution. A large quantity of organic matter is also disadvan- 

 tageous and may be avoided by evaporating the solution several 

 times to dryness with nitric and sulphuric acids, finishing finally 

 witli an evaporation with sulphuric acid alone in order to drive 

 oft' all tracts of nitric acid. 



The precipitate from the treatment with hydrogen sulphide is 

 filtered off, washed with water saturated Avith hydrogen sulphide 

 and digested with a small quantity (2 to 5 c.c.) of nitric acid 

 in a casserole. The digestion should be begun cold and the heat 

 gradually increased. If the digestion is begun at a high tempera- 

 ture the sulphur formed by the decomposition of the copper 

 sulphide will form a film of molten sulphur around the' granules 

 of copper sulphide, and this tends to prevent their solution in 

 nitric acid. The precipitate after digestion in nitric acid should 

 be a clear green or else a yellow. If tliere is any trace of dark 

 color, brown or black, it means that either organic matter has 

 been precipitated with the copper sulphide precipitate, which is 

 extremely unlikely, or else that the above-mentioned sulphur film 

 lias formed around some of the particles of copper sulphide 

 preventing their solution in the nitric acid. If the latter be 

 the case, the determination ma.y still be saved by placing the 

 precipitate in a platinum dish and heating over a gentle flame 

 until the sulphur is volatilized. The residue of copper .sulphide 

 or of copper oxide may then be digested in nitric acid. The 

 digestions in nitric acid should not be carried to a heat high 

 enough to decompose the copper nitrate formed by the solution 

 of copper sulphide. 



After digestion in nitric acid and the evaporation of any 

 large excess of nitric acid, the residue is taken up in hot water, 

 acidified to contain 2-4 per cent nitric acid and filtered into 

 a large platinum dish, i^ to I/2 c.c. of sulphuric acid is added, 

 and the solution electrolyzed with a voltage of from 2 to 21/) 



