Turbidimetry in Particle Size Analysis

Abstract The size of particles in a suspension can be estimated by measuring the turbidity of the suspension. Turbidity measures the attenuation of a beam of light traveling through the suspension caused by the scattering and absorption of light by the particles. The amount of scattering and absorption depends on the size of the particles and their concentration in the suspension. Most of the reported applications in the literature use turbidimetry for the estimation of the particle size distribution (PSD) or an average size for particles with diameters ranging from 0.035 to 50 µm. Turbidimetric techniques are low cost, experimentally simple, fast, nondestructive and require no calibration. They are robust methods with very good reproducibility and can be used either off‐ or on‐line, in a laboratory or a plant environment. They have been used for particle or droplet size determination in a variety of diverse systems such as latex particles, silver bromide sols and cow's milk. Most of the time common ultraviolet/visible (UV/VIS) spectrophotometers are utilized for the turbidity measurements. Certain modifications may be necessary on these instruments depending on the size of the particles. The turbidity of the suspension is measured and the size of the particles or the PSD is estimated utilizing light scattering theory. For spherical isotropic homogeneous particles and absence of multiple scattering this theory is well established. The extension to nonspherical and inhomogeneous particles is a field of active research yielding numerous publications. A controversy existed in the literature regarding the capability of turbidimetry to provide the full PSD but has now been resolved. For polydisperse suspensions, for certain combinations of particle sizes and optical properties, the method cannot provide the full PSD, but only an average of the PSD. This limitation is simply a result of the light scattering patterns and it is expected theoretically. In some cases it can be overcome by simply altering the optical properties of the suspension (i.e. choosing the proper wavelengths for the turbidity measurements or a medium with a different refractive index to suspend the particles). To make the best use of the method and interpret the results in a meaningful way one should be aware of this limitation and work with it. When this is done, excellent results can be achieved. The advantages of the method and a good knowledge of its capabilities make it a very robust choice for many applications. The numerous reports from the successful application of the method are a testimony to this.