214 QUANTITATIVE AGRICULTURAL ANALYSIS

A still more accurate method involves the double dilution,
discussed on page 133.

The normal weight for lactose, 32.90 gm (see page 131), is^too
small a quantity of milk for convenient accurate determinations
and it is customary to use twice this amount or 65.80 gm. In
order that the sample of milk may be measured instead of weighed
the following table may be used and a special pipette like Fig-
49 will be found convenient.

TABLE XV.—VOLUME OF MILK FOR LACTOSE DETERMINATION_____

Volume of milk (cc) for a lactose

Specific gravity of milk double normal weight (International

scale)

1.024
64.26

1.025
64.20

1.026
64.13

1.027
64.07

1.028
64.01

1.029
63.94

1 . 030
63.88

1.031
63.82

1.032
63.76

1.033
63.70

1.034
63.64

1.035
63.58

1 . 036
63.51

Determination of Lactose.—Prepare acid mercuric iodide solution as
follows:
Dissolve 33.2 gm of potassium iodide, 13.5 gm of mercuric chloride and
20 cc of glacial acetic acid in 640 cc of water.
Determine the specific gravity of milk by means of a sensitive hydrometer
or a picnometer. Refer to the table and measure, at the temperature at
which the specific gravity was taken, the quantity of milk indicated in the
table above, the sample having been mixed thoroughly immediately before
making both measurements. The milk is run into a volumetric flask,
graduated at 102.6 cc. Add 30 cc of acid mercuric iodide solution, dilute
to the mark on the flask, mix well and allow the precipitate to settle. Filter
through a dry filter, rejecting the first 25 cc of the filtrate and receiving the
remainder in a dry flask. Polarize in a 200-mm tube, having the solution at
a temperature of 20°. The reading on the sugar scale is to be divided by