_C_
_A_ ----
_P_
can be determined by experiment. The molecular weight can therefore be
calculated from the formula
_K_
_M_ = ----.
_A_
With organic compounds in water the value of _K_ is almost constant.
Brown and Morris[117] report results of their work in extending Raoult’s
investigations of the molecular weight of the carbohydrates. The
process is carried on as follows:
A solution of the carbohydrate is prepared containing a known weight
of the substance in 100 cubic centimeters of water. About 120 cubic
centimeters of the solution are introduced into a thin beaker of about
400 capacity. This beaker is closed with a stopper with three holes.
Through one of these a glass rod for stirring the solution is inserted.
The second perforation carries a delicate thermometer graduated to
0°.05. The temperature is read with a telescope. The beaker is placed
in a mixture of ice and brine at a temperature from 2° to 3° below
the freezing point of the solution. The solution is cooled until its
temperature is from 0°.5 to 1° below the point of congelation. Through
the third aperture in the stopper a small lump of ice taken from a
frozen portion of the same solution, is dropped, causing at once the
freezing process to begin. The liquid is briskly stirred and as the
congelation goes on the temperature rises and finally becomes constant.
The reading is then taken. The depression in the freezing point,
controlled by the strength of the solution, should never be more than
from 1° to 2°.
The molecular weights may also be determined by the boiling points of
their solutions as indicated by the author,[118] Beckmann,[119] Hite,
Orndorff and Cameron.[120]
The method applied to some of the more important carbohydrates gave the
following results: