VI. GENERAL METHODS FOR THE ANALYSIS OF FOODS AND 

 FEEDING STUFFS. 



Estimation of Reduced Copper. 



[Page 51.] 

 Low's VOLUMETRIC METHOD, MODIFIED.** PROVISIONAL. 



(a) STANDARDIZATION OP THE THIO8ULPHATE SOLUTION. 



Prepare a solution of sodium ihiosulphate containing 19 grams of pure crystals to 

 1,000 cc. Weigh accurately about 0.2 gram of pure copper foil and place in a flask 

 of 250 cc capacity. Dissolve by warming with 5 cc of a mixture of equal volumes 

 of strong nitric acid and water. Dilute to 50 cc, boil to expel the red fumes, add 5 cc 

 strong bromin water, and boil until the bromin is thoroughly expelled. Remove from 

 the heat and add a slight excess of strong ammonium hydroxid 7 cc is about the right 

 amount. Again boil until the excess of ammonia is expelled, as shown by a change 

 of color of the liquid, and a partial precipitation. Now add a slight excess of strong 

 acetic acid (3 or 4 cc of 80 per cent acid) and boil for a minute. Cool to room temper- 

 ature and add 10 cc of a solution of pure potassium iodid containing 300 grams of 

 potassium iodid to 1,000 cc. Titrate at once with the thiosulphate solution until the 

 brown tinge has become weak, then add sufficient starch liquor to produce a marked 

 blue coloration. Continue the titration cautiously until the color due to free iodin 

 has entirely vanished. The blue color changes toward the end to a faint lilac. If at 

 this point the thiosulphate be added drop by drop and a little time be allowed for com- 

 plete reaction after each addition there is no difficulty in determining the end point 

 within a single drop. One cubic centimeter of the thiosulphate solution will be found 

 to correspond to about 0.005 gram of copper. 



(6) DETERMINATION OP COPPER. 



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 (1:1) poured under the 

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

 glass and gooch free of copper; 50 cc of water will be sufficient. Boil to. expel red fumes, 

 add 5 cc of bromin water, boil off the bromin, and proceed exactly as in standardizing 

 the thiosulphate. 



(d) UNIFORM METHOD FOR DETERMINING REDUCING SUGARS IN GENERAL (MUNSON 

 AND WALKER). & PROVISIONAL. 



[Page 42.] 



(1) PREPARATION OF SOLUTIONS AND ASBESTOS. 



(a) Solutions. Use solutions (a), (6), and (c) as given on page 42, under Soxhlet's 

 modification of Fehling's solution. 



(&) Asbestos. Prepare the asbestos, which should be the amphibole variety, by 

 first digesting with 1:3 hydrochloric acid for two or three days. Wash free from acid 



J. Amer. Chem. Soc., 1902, 24: 1082. 



Ibid., 1906, 28: 663 ; 1907, 29: 541. 

 241 



