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QUANTITATIVE AGRICULTURAL ANALYSIS

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Most transparent media permit plane-polarized light to pass
through them unchanged but there are certain crystals and
solutions that possess the remarkable property of rotating the
plane of polarization to the right or left. Further, this is a
quantitative property and the magnitude and direction of the
angle of rotation is a specific property of the substance itself,
a given solvent being understood in the case of solutions. This
property undergoes a definite change of value with definite
changes in temperature and in the case of solutions it varies
directly with the concentration. This last is a very important
consideration and it will be readily seen that if an instrument
can be constructed for measuring the angle of rotation, this will
serve as a means for the quantitative determination of optically
active substances in solution.

Substances that rotate the plane of polarization to the right
are dextro-rotatory while those that rotate to the left are laevo-
rotatory. The angle of rotation varies according to the wave
length of the light that is used and it therefore becomes
necessary to use monochromatic light in order to have a
definite, measurable rotation. Sodium light is generally used
for this purpose.

Specific Rotatory Power.—The angle of rotation which would
be produced by a column of solution 1 dm long and containing I gm
of the active substance in each cubic centimeter is known as the
"specific rotatory power" or "specific rotation" of. a given
active substance. This is, in a sense, a hypothetical figure as
few solutions could be made of so great a concentration; however,
the specific rotation can be calculated from the results of measure-
ments on more dilute solutions. The specific rotation, at 20°
and for the D-line of sodium light, is expressed by the symbol
LaJ D -

For a solution of an active substance in an inactive solvent,
such as water, the angle, a, of rotation is approximately in
proportion to the concentration, which may be expressed as
grams of active substance in 100 cc of solution. This relation
does not hold strictly, on account of changes in electrolytic
dissociation (ionization), molecular association (polymeriza-
tion), hydrolysis, or hydration, with changes in concentration,
where any of these factors apply to a given solution. In many

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