Comparison of Light Scattering with other Techniques for Particle Size Measurement

Abstract The forward‐light scattering technique was compared with three other well known size analysis techniques in determining the size distribution of a crystalline adsorbent, Silicalite (1–10 μm) in a dilute aqueous suspension. The commercial light scattering equipment Malvern was found to produce size distributions within ± 10% agreement with those obtained by scanning electron microscopy (SEM), electronic image analysis (Quantimet) and the electrical sensing zone method (Coulter Counter). This rather surprising accuracy of the light scattering technique with crystal sizes quite close to the wavelength of the incident light is explained by considering the relative refractive index and the light absorbancy of the crystals in the “anamolous diffraction” regime. The theoretical analysis shows quite clearly that the calculation of a “unique” size distribution from a given set of scattering data is possible only when independently measured values of the optical properties are used. The commercial equipment (Malvern) which is currently in use does not require a priori knowledge of the optical properties. The exceptional accuracy obtained with Silicalite crystals using the Fraunhofer approximation in the anamolous diffraction regime is demonstrated with the aid of the light scattering theory to be due to a rather fortunate combination of the light absorptive nature and the polydispersity of the crystals.