NEPHELOMETRY AND TURBIDIMETRY
When light is passed through moderately stable suspensions, a portion of the incident radiant energy is dissipated by virtue of the absorption, refraction, and reflection, whereas the remaining portion gets transmitted. It is quite evident that the optical characteristics of each suspension shall alter according to the concentration of the dispersed phase. In fact, the measurement of the intensity of the transmitted light through such suspensions vis-a-vis the concentration of the dispersed phase serves as the basis of turbidimetric analysis.
In another situation when the aforesaid suspension is viewed at 90° (i.e., right angles) to the direction of the incident light (Figure 20.1) the system appears opalescent on account of the reflection of light from the particle of the suspension. This scattering of light is termed as the Tyndall effect. The observed opalescence or cloudiness is the net result caused by irregularly and diffusely reflected light from the suspension. Consequently, the ultimate measurement of the intensity of the scattered light as a true representation of the actual concentration of the dispersed phase forms the basis of nephelometric analysis (derived from Greek : nephele-means cloud). It is found to be most sensitive and effective specially in the case of very dilute suspensions having a concentration not greater than 100 mg L–1. However, it is interesting to observe that the technique of turbidimetric analysis resembles that of flame photometry ; and nephelometric analysis to that of fluorimetry.