132

QUANTITATIVE AGRICULTURAL ANALYSIS

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of these sugars. The values for the specific rotation (for 20-per
cent solutions) of sucrose, dextrose and levulose are +06.5,
+53.1 and —93.3, respectively. The mixture of dextrose and
levulose has a specific rotation, for these concentrations, of
— 20.57, which is practically the mean of the separate values for
the two sugars. Because of the change in the direction of rota-
tion with this conversion of sucrose, the reaction is known as one
of "inversion" and the resulting mixture of sugars is called
"invert sugar."

Cane Sugar.—Sucrose can be determined by a single polar-
ization only in case no other active .substance is present in the
solution. In case either dextrose or invert sugar is present a
polarization before and after the inversion of cane sugar gives the
necessary data for the calculation of sucrose by the modified
Clerget formula. This formula is derived from the following
considerations:

From the values for [a]™*, given above:
f* f* P\
_^. x 26 = 83.9. Therefore 83.9 gm is the normal weight

(International) for invert sugar.
Prom the equation for inversion: 26 gm of sucrose yields

27.37
27.37 gm of invert sugar and this is 00 n- = 0.3262 of the normal

weight. (HerssfolcFs value, 0.3266 is now generally used.)

If the normal weight of sample (based upon sucrose) has been
used for making the solution for the direct polarisation (P), then
each per cent of sucrose in the sample will give a rotation of
+ 1° (International scale) before the inversion awl —0.3266° after
inversion. Therefore the change of rotation (P — I) would be
1.3266° for each per cent of sucrose. If # = per cent of sucrose,

o P-I - m

1.3260" (l)

This is for a temperature of 20° and it is found that between 0°
and 20° the left rotation of invert sugar produced from the
sucrose of a normal solution decreases 0.005° for each per cent,
for 1° 0. rise in temperature. Thin is chiefly duo to a decrease
in the rotatory power of levulose. At 0° C. the formula would
then read:

P- J

1.4266

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