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 poimt 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. 
“A fter washing the precipitated cuprous oxid, cover the gooch 
with a watch glass and dissolve the oxid by means of 3 ¢.c. of warm 
nitric acid (1:1) poured under the watch glass with a pipette. Catch 
the filtrate 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 jactose 
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: 
