METHODS OF ANALYSIS 33 



not boil too long after dense fumes of H 2 S0 4 have started, or some 

 small amounts of arsenic may be volatilized. 



Allow the casserole to cool and add 40 cc. of cold water and 

 10 cc. of HC1. Some tartaric acid should be added, also, if anti- 

 mony is to be determined. Boil to dissolve all soluble matter. 



If much gangue is present, filter; if not, wash into a No. 3 Grif- 

 fin lipped beaker, using warm water, and reduce to a colorless so- 

 lution with a mixture of one part of ammonium bi-sulphite and two 

 parts of strong ammonia. The reduction is best made by adding 

 the ammoniacal sulphite solution drop by drop with constant stir- 

 ring, waiting for the ppt. formed to dissolve after each addition. 

 Do not add any more sulphite than that necessary to reduce to 

 colorless stage. Add a little more HC1 in case the hydrates formed 

 do not dissolve. If there is much gold, selenium, or tellurium in 

 the ore, these metals will be ppt. by the excess of sulphurous acid 

 and darken the solution; so, if after the solution is nearly colorless 

 this darkening occurs, no more sulphite need be added. 



Boil the solution a few minutes until there is no apparent 

 odor of sulphurous acid, and then while still warm pass in a lively 

 current of H 2 S gas for about 15 minutes, or until the precipitate 

 gathers together and the super-natant liquid does not appear 

 murky. It is safer to pass the gas through for a longer period of 

 time; but, in case of hurry, after some experience, the point may be 

 told almost with certainty by inspection. 



Filter the ppt. sulphides througb an 11 cm. filter paper and 

 wash the sulphides all on to the paper with water. Wash out the 

 iron salts. Test the filtrate with H 2 S gas. 



Put the paper containing the sulphides into a 4-oz. distillation 

 flask, the arm from the neck of which is bent down at the end, so 

 as to connect with a 12-inch Liebig condenser set vertically. If 

 the sulphides are too bulky to wrap in the paper and put into the 

 flask, pierce the filter and wash most of the ppt. through the fun- 

 nel into the flask, using a minimum wash of 1 volume of HC1 and 

 1 volume of water, used in a wash bottle with a Bunsen valve. 



Remove the paper and put it into the flask, then pour through 

 the funnel, to wash it, 50 cc. of the cupric chloride solution. Always 

 pour the chloride solution in through a funnel reaching below the 

 opening at the side of the neck, so as to avoid getting copper into 

 the distillate. 



A thermometer through a rubber stopper is inserted in the 



