Comparing Laser Diffraction and Electrozone Sensing

	Figure 1: Basic set up of electrozone sensing technique

Electrozone sensing (EZS) was one of the first widely-used methods for particle characterisation. The technique requires the particles within a sample to be dispersed at low concentrations within an electrolyte (conducting) solution. The volume equivalent diameter of particles is then measured by drawing the particles through an aperture, or sensing zone, on either side of which two electrodes are placed (figure 1). Particles passing through the sensing zone displace a volume of electrolyte equal to their size, causing a change in impedance between the electrodes. The intensity of this change is proportional to the particle size, and by counting pulses and measuring pulse heights a particle size distribution can be measured. The range of measurement depends on the size of the orifice. Typically particles with sizes ranging from 2% to 40% of the orifice diameter can be measured within a single measurement.

The comparison between EZS and laser diffraction will depend on the porosity of the particles being measured. To a reasonable approximation, both techniques report the diameter of a volume-equivalent sphere, such that a solid sphere would be measured in the same way using laser diffraction and EZS. However, if a particle of the same overall size but containing pores is measured, then the volume of electrolyte that is displaced during the EZS measurement will be reduced, causing the reported size to be smaller than that reported by laser diffraction.

An example of the differences which can be observed between EZS and laser diffraction is shown in Figure 2. Here the particle size distributions for four grades of a sample containing porous particles have measured by laser diffraction. Each sample grade is defined by the median size reported by EZS. As the grade number decreases, so the size distribution reported by laser diffraction also decreases. The relationship between the two techniques can then be investigated by plotting the median size from EZS against the laser diffraction median size (Figure 3).

Figure 3 shows that the median size reported by EZS is significantly smaller than the median size reported by laser diffraction – this relates to the porous nature of the particles. However, the relationship between the two sets of results in linear, enabling the customer to transfer their methods to laser diffraction in this case.

Figure 2: Laser diffraction particle size distributions of catalyst grades

Figure 3: Comparison of laser diffraction and electrozone sensing median sizes