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 :
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.