GLUCOSE. 31 



Again treat with hydrogen sulpliid without filtering. By this treat- 

 ment 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 nitrate 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 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 an ashless filter, ignited, and 

 weighed as stannic oxid. 



Treat the residue insoluble in ammonium sulphid 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 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. 



Precipitate lead with potassium chromatc in an acetic acid solution, 

 and weigh upon a tared filter as lead chromate. 



Evaporate the filtrates from the hydrogen sulphid precipitate to 

 about 60 cc; add bromin witer to oxidize the iron salts and any 

 remaining hydrogen sulphid. Boil off the excess of bromin and, 

 unless the solution is distinctly yellow, add a few drops of concentrated 

 solution of ferric chlorid to make it so. Nearly neutralize the mineral 

 acid with ammonia, and add ammonium acetate to precipitate iron 

 phosphate and excess of iron. Filter and thoroughly wash the pre- 

 cipitate. To the filtrate, made distinctly acid with acetic acid and 

 boiled, add hydrogen sulphid to precipitate zinc. Unless the zinc sul- 

 phid comes down white it should be dissolved, again treated with 

 ammonium acetate to remove traces of iron, and reprecipitated as 

 sulphid. Finally collect the zinc sulphid upon an ashless filter, ignite, 

 and weigh as zinc oxid. 



ADDED SUBSTANCES. 

 GLUCOSE. 



The substitution of glucose for the more expensive sugars is exten- 

 sively practiced in the manufacture of various food materials, and in 

 no class of foods is there better opportunity offered for this substitu- 

 tion than in fruit products. Of 214 samples of commercial fruit prod- 

 ucts examined, 110 samples contained glucose. The amount used varies 



