134 QUANTITATIVE AGRICULTURAL

diluted to 100 cc, as usual. Another sample of the normal weight
is clarified and diluted to 200 cc and the filtrate is polarized.

Let P = true polarization of a normal solution,

Pi = polarization of the solution in the 100-cc flask,
I , , ^ P<2 = that of the solution in the 200-cc flask and

I * • \\ v = actual volume of solid precipitate.

f ,, Then

; •>*! /> _ ipo p (

100^ - v W

IS 200 P \ 100 P ^ (

— v

fpr

'ill

I, . ......

12 ~ (100 ~?;)(200 -v)

_ P

l 2~~

-0X200 - 0 "^

(1QO)«P

From Eqs. 3 and 4,

._ _
(100 - ?;)(200 - 0) k ;

Therefore the true rotation is the product of the two readings, divided
by their difference.
The Association of Official Agricultural ChemiHts ha« placed
the arbitrary limit of 1 cc of precipitate from 2(J gm of sample,
for the solution for which no correction need be made.
Determination of Sucrose in a Commercial Syrup. Prepare a solution
of basic lead axsctatc by one of the following methods:
(«) Add 215 gin of neutral lead acetate and 155 gut of litharge (PbO)
to 500 cc of distilled water (or a smaller amount using the saniu proportions)
and boil in a covered dish for 30 minutes. Cool, decant the dear nolution
and determine the specific gravity by means of a hydrometer. Dilute until
the specific gravity is 1.2.5, using recently boiled water.
(k) Make the solution directly from dry basic lead acetate and dilute until
the specific gravity is 1 .25.
Direct Polarization.— Weigh a «mall dish, then drop in and weigh 26.000
grn of the commercial syrup or molasses. Rinse into a 100-cc volumetric
flask with about 50 ec of water and then carefully add basic lead acetate
solution until the sugar solution is decolorized as far as any effect can be
noticed, avoiding an unnecessary excess of the clarifying agent. Dilute to
the mark on the flask, mix thoroughly and filter through a dry filter, rejecting