Vni] FOODS AND FEEDING STUFFS 97 



until the bromin is completely driven off. Remove from the heat and add a slight 

 excess of strong ammonium hydroxid (about 7 cc. is required). Again boil until 

 the excess of ammonia is expelled, as shown by a change of color of the liquid, 

 and a partial precipitation. Then add a slight excess of strong acetic acid (3 or 4 

 cc. of 80% acid) and boil for a minute. Cool to room temperature and add 10 cc. 

 of 30% potassium iodid solution. Titrate at once with the thiosulphate solution 

 until the brown tinge has become weak, then add sufficient starch indicator [VII, 

 3 (a)l to produce a marked blue coloration. Continue the titration cautiously 

 until the color due to free iodin has entirely vanished. The blue color changes to- 

 ward the end to a faint lilac. If at this point the thiosulphate be added drop by drop 

 and a little time allowed for complete reaction after each addition, there is no diffi- 

 culty in determining the end point within a single drop. One cc. of the thiosulphate 

 solution will be found to correspond to about 0.005 gram of copper. 



29 



DETERMINATION. 



After washing the precipitated cuprous oxid, cover the Gooch with a watch 

 glass and dissolve the oxid by means of 5 cc. of warm nitric acid (1 to 1) poured under 

 the watch glass with a pipette. Catch the filtrate in a 250 cc. flask, wash the 

 watch glass and Gooch free of copper, using about 50 cc. of water. Boil to expel 

 red fumes, add 5 cc. of bromin water, boil off the bromin, and proceed exactly as 

 in 28. 



30 ///. Volumetric Permanganate Method. — Tentative. 



Filter and wash the cuprous oxid as directed under 25. Transfer the asbestos 

 film to the beaker, add about 30 cc. of hot water, and beat the precipitate and asbes- 

 tos thoroughly. Rinse the crucible with 50 cc. of a hot saturated solution of ferric 

 sulphate in 20% sulphuric acid, receiving the rinsings in the beaker containit)g the 

 precipitate. After the cuprous oxid is dissolved, wash the solution into a large 

 Erlenmeyer flask and immediately titrate with a standard solution of potassium 

 permanganate, 1 cc. of which should he equivalent to 0.010 gram of copper. Stand- 

 ardize this solution by making 6 or more determinations with the same sugar solu- 

 tion, titrating one half of the precipitates obtained, and determining the copper in 

 the others by electrolysis. The average weight of copper obtained by electrolysis, 

 divided by the average number of cc. of permanganate solution required for the 

 titrations, gives the weight of copper equivalent to 1 cc. of the standard perman- 

 ganate solution. A solution standardized with iron or oxalic acid will give too low 

 a result. 



31 IV. Electrolytic Deposition from Sulphuric Acid Solution. — Tentative. 



Filter the cuprous oxid in a Gooch, wash the beaker and the precipitate thoroughly 

 with hot water without transferring the precipitate to the filter. Wash the asbestos 

 film and the adhering cuprous oxid into the beaker by means of hot dilute nitric acid. 

 After the copper is all in solution, refilter through a thin film of asbestos in a Gooch 

 and wash thoroughly with hot water. Add 10 cc. of sulphuric acid (1 to 4), and 

 evaporate the filtrate on the steam bath until the copper salt has largely crystal- 

 lized. Heat carefully on a hot plate or over asbestos until the evolution of white 

 fumes shows that the excess of nitric acid is removed. Add 8-10 drops of nitric acid 

 (sp. gr. 1.42) and rinse into a 100-125 cc. platinum dish. Deposit the copper by 



