CHEMICAL TESTS AND ANALYSES 349 



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 complete 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 .00636 grams of copper." 



Determination of Copper. 



"After washing. the precipitated cuprous oxid, cover the gooch 

 with a watch glass and dissolve the oxid by means of 5 c.c. of warm 

 nitric acid (1 :1) poured under the watch glass with a pipette. Catch 

 the nitrate in a flask of 250 c.c. capacity, wash watch glass and 

 gooch free of copper; 50 c.c. of water will be sufficient. Boil to 

 expel red fumes, add 5 c.c. of bromine water, boil off the bromine 

 and proceed exactly as in standardizing the thiosulphate." 



Determination of Lactose. 



Place 50 c.c. of the mixed copper reagent in a beaker and heat 

 to the boiling point. While boiling briskly add 100 c.c. of the lactose 

 solution containing not more than 0.300 grams of lactose and boil 

 for six minutes. Filter immediately through asbestos and wash. 

 Obtain the weight of lactose equivalent to the weight of copper 

 found from the following table: 



