CANNED VEGETABLES. 53 



about 50 cc of hydrochloric; acrid (1:3) and heat to boiling or upon a steam bath for 

 a half hour. Nearly neutralize the acid with sodium hydroxid dilute to 150 cc with 

 water, precipitate with hydrogen sulphid, and filter, after heating fora few moments 

 upon a steam bath to facilitate 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 one 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 dis- 

 solve by the aid of heat in a concentrated solution of ferric chlorid. The reduced 

 iron salt is then titrated with potassium dichromate. a One cc of decinormal potas- 

 sium dichromate equals 0.00295 grams of tin. The determination of the tin by ignit- 

 ing and weighing as stannic oxid was found to be unreliable, owing to the precipitation 

 of appreciable amounts of silica that was dissolved by the mixed carbonates from the 

 porcelain crucible. Determine the copper and lead, which remain as insoluble 

 sulphids after the fusion, and the zinc, which remains in the original filtrate, 

 according to the scheme described under the following method. 



(b) MUNSON'S METHOD. 



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

 dry sample in a four-inch porcelain evaporating dish with sufficient concentrated sul- 

 phuric acid to thoroughly carbonize the mass. Usually from 10 to 15 cc are sufficient 

 for this purpose. 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 

 muhMe furnace 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 hydrochloric; acid (1 to 8) and evaporate on a water bath to dryriess; 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 three grams of caustic 

 soda in a silver crucible fora 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 pre- 

 cipitate 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 sul- 

 phid. Usually 50 cc of the ammonium sulphid will be found sufficient to completely 

 dissolve all tin sulphid; but portions of the filtrate should be tested to make sure of 

 this point. The filtrate is then made acid with hydrochloric acid to precipitate the 

 tin sulphid, which, after standing for a few moments, is collected upon anashless fil- 

 ter, ignited, and weighed as stannic oxid. 



Treat the insoluble residue remaining from the ammonium sulphid washing with 

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

 and add ammonium acetate, as there is usually a small amount of iron present. If 

 any iron salt precipitates, filter, wash and divide the filtrate for the determinations 

 of copper and lead. In the absence of lead, copper may be determined electrolytic- 

 ally, or it may he titrated with potassium cyanid. Unless added as a coloring agent, 

 copper will seldom he present in sufficient quantity to warrant its determination. 



'Suttnii, Voliimrtnr Analysis, sth i-d., p. 



