O. Folin and W. Denis 823 
II. Colorimetric Method for the Determination of Lactose. 
Dehn and Hartman? have already described. a colorimetric 
picrate method for the determination of the sugar in milk. Our 
procedure is materially simpler, and it gives, we believe, as ac- 
curate results as it is possible to obtain by a colorimetric picrate 
method. 
The method is as follows: With an accurate 2 cc. pipette trans- 
fer 2 cc. of milk to a 100 cc. volumetric flask previously half 
filled with saturated picric acid solution. Fill to the mark with 
saturated picrie acid solution, shake, and filter. Transfer 5 ce. 
of the filtrate to a large test-tube or 100 ce. volumetric flask, 
add 15 cc. of saturated picric acid solution and 3 cc. of 20 per 
cent sodium carbonate solution. Mix and place ina boiling water 
bath for 15 minutes. Cool and dilute to a volume of 100 cc. 
The standard consists of a solution of lactose in saturated picric 
acid solution. For use with cow’s milk this solution should 
contain 5 mg. of lactose in 20 ce. For use with human milk it is 
necessary to employ two standards; namely, one containing 5 
and the other containing 7 mg. of lactose in 20 ce. To 20 cc. of 
the standard add 8 cc. of the sodium carbonate solution and 
heat simultaneously with the milk filtrate. Cool, dilute to 100 
ec., and compare the colors in the usual manner in a suitable 
colorimeter. 
According to our experience accurate results cannot be uni- 
formly obtained except by heating the standard simultaneously 
with the unknown and it is therefore absolutely necessary to pre- 
pare a new standard for each set of determinations. Nor can the 
color values of the standard and the unknown be more than 20 
per cent apart. 
In the following table are given a few figures representing 
parallel determinations of lactose in milk made by the colorimetric 
and by the titration methods. 
2 Dehn, W., and Hartman, F. A., J. Am. Chem. Soc., 1914, xxxvi, 404. 
