62 METHODS OF ANALYSIS. 



tate the separation of the precipitated sulphids. Dry the precipitate and 

 insoluble ash residue, and then fuse in a porcelain crucible with a mixture 

 consisting of 1 gram each of sodium carbonate, potassium carbonate, and 

 sulphur. Dissolve the fused mass with hot water and filter. Sulphids of lead 

 and copper remain upon the filter. Acidify the filtrate with acetic acid to 

 precipitate the tin sulphid. Collect the tin sulphid upon a filter. Wash 

 thoroughly and then dissolve by the aid of heat in a concentrated solution of 

 ferric chlorid. The reduced iron salt is then titrated with potassium dichro- 

 mate. One cubic centimeter of tenth-normal potassium dichromate equals 

 0.00295 gram of tin. The determination of the tin by igniting and weighing as 

 stannic oxid is unreliable, owing to the precipitation of appreciable amounts of 

 silica dissolved by the mixed carbonates from the porcelain crucible. Deter- 

 mine the copper and lead, which remain as insoluble sulphids after the fusion, 

 and the zinc, which remains in the original filtrate, according to the following 

 method : 



(b) MUNSON'S METHOD. 



Treat 100 grams of the moist sample after evaporating to dryness, or 25 grams 

 of the dry sample, in a 4-inch porcelain evaporating dish with sufficient con- 

 centrated sulphuric acid to thoroughly carbonize the mass. Gently heat over 

 a Bunsen burner until all danger of foaming is past, which will require not more 

 than three minutes ; then transfer the dish to a muffle and keep it at a low red 

 heat until all organic matter is destroyed. It is occasionally found necessary to 

 add a few drops of nitric acid to completely destroy organic matter. When 

 the material is completely ashed, allow the dish to cool ; add 25 cc of hydro- 

 chloric acid (1:8) and evaporate on a water bath to dryness; take up with 

 water and acidify with two or three drops of hydrochloric acid. Transfer to a 

 beaker without filtering and treat with hydrogen sulphid. After heating upon a 

 water bath for a few minutes the precipitate and the insoluble residue are col- 

 lected upon a filter. The precipitate and residue may contain sulphids of tin, 

 lead, and copper, and oxid of tin ; the filtrate will contain any zinc that is 

 present. 



Fuse the sulphid precipitate and insoluble ash residue with about 3 grams of 

 caustic soda in a silver crucible for a half hour to render soluble any insoluble 

 tin compounds. Dissolve the mass with hot water and slightly acidify with 

 hydrochloric acid. Again treat with hydrogen sulphid without filtering. By 

 this treatment all the tin is thrown down as sulphid with the sulphids of copper 

 and lead. Collect the precipitate upon a filter and wash thoroughly with hot 

 water. The filtrate may be rejected. To separate the tin sulphid from those of 

 copper and lead, wash several times upon the filter with separate portions of 10 

 cc. of strong boiling ammonium sulphid. Usually 50 cc of ammonium sulphid 

 will be found sufficient to completely dissolve all tin sulphid, but portions of 

 the filtrate should be tested to prove this point. Acidify with hydrochloric 

 acid to precipitate the tin sulphid, which, after standing for a few moments, is 

 collected upon an ashless filter, ignited, and weighed as stannic oxid. 



Treat the insoluble residue remaining from the ammonium sulphid washing 

 with nitric acid, filter, wash, nearly neutralize the excess of mineral acid with 

 ammonium hydroxld, and add ammonium acetate, as there is usually a small 

 amount of iron present. If an iron salt is precipitated, filter, wash, and divide 

 the filtrate for the determinations of copper and lead. In the absence of lead, 

 copper may be determined electrolytically, or it may be titrated with potassium 

 cyanid. Unless added as a coloring agent, copper will seldom be present in 

 sufficient quantity to warrant its determination. 



