266 



CHEMICAL TESTS AND ANALYSES 



VOLUME OF MILK CORRESPONDING TO A LACTOSE DOUBLE NORMAL 



WEIGHT 



Low's VOLUMETRIC METHOD MODIFIED 



PREPARATION OF REAGENTS 



"(a) Copper sulphate solution. Dissolve 34.639 grams of. 

 CuSO 4 .5H 2 O in water and dilute to 500 c.c. 



(b) Alkaline tartrate solution. Dissolve 173 grams of Ro- 

 chelle salts and 50 grams of sodium hydroxid in water and dilute 

 to 500 c.c. 



(c) Mixed solution. Mix equal volumes of solutions (a) and 

 (b) immediately before use. 



(d) Standardisation of the thiosulphate solution. Prepare a 

 solution of sodium thiosulphate, dissolving 24.659 grams of pure 

 crystals to 1,000 c.c. Weigh 6.36 grams copper foil. Dissolve by 

 warming in minimum amount of nitric acid and water required. 

 Boil to expel the red fumes, add 160 c.c. strong bromine water and 

 boil until the bromine is thoroughly expelled. Remove from the 

 heat and add a slight excess of strong ammonium hydroxid ; 223 c.c. 

 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 partial 

 precipitation. Now add a slight excess of strong acetic acid (100 

 to 130 c.c. of 80 per cent acid) and boil for a minute. Cool to 

 room temperature and dilute to 1,000 c.c. Titrate a known amount 

 (10 to 15 c.c.) of the copper solution, to which 10 c.c. of a 25 per 

 cent solution of pure potassium iodid has been added, with the 

 thiosulphate solution until the brown tinge has become weak, then 

 add sufficient starch liquor to produce a marked blue coloration. 



