Chapter: Pharmaceutical Drug Analysis: Polarimetry
Nephelometry and Turbidimetry: Theory
THEORY
In short, turbidimetry
is the measurement of the degree of attenuation of a radiant beam incident on
particles suspended in a medium, the measurement being made in the directly
transmitted beam. Thus, turbidity (T) may be expressed as :
.......................(a)
where, T =
Turbidity,
l = Length of dispersion through
which the light passes,
Io = Intensity of incident light,
It = Intensity of transmitted light, and
n = Refractive index of the
dispersion medium.
The International
Pharmacopoeia describes Turbidance
(S) as—‘a measure of the
light-scattering effect of suspended
particles’ ; and Turbidity (r) as—‘a measure of the decrease in incident beam
intensity per unit length of a given suspension’.
Nephelometry exclusively refers to the measurement
of the light scattered by suspended particles at right angles (perpendicular)
to the incident beam.
Turbidimetry or nephelometry may be employed judiciously
for the measurement of precipitates produced by the interaction of very dilute
solutions of reagents, or other particular matter, for instance : concentration
of colloidal dispersion of organic and inorganic compounds and suspensions of
bacterials cells (microbial assays).
It is, however, pertinent to mention here that in order
to achieve the prime objective of obtaining fairly reproducible analytical
results and absolutely consistent results the following experimental parameters
may be observed strictly with regard to the production of suspensions of
reasonably uniform characteristic fea-tures, namely :
(i) the
extremely dilute suspensions of bacterial cells may be employed to encounter
the problems caused due to birefringence,
(ii) the
concentrations of the two ions that combine to yield the respective
precipitate, besides the ratio of the concentrations in the solutions that are
mixed,
(iii) the
procedural details including the order and the rate of mixing,
(iv) the
amounts of other salts and substances present e.g., the protective colloids such as : dextrin, gelatin, gum
arabic ; and
(v) the temperature.
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