Figure shows a schematic diagram of Coulter counter equipment used for determining the particle volume. The equipment is applicable to materials of particle size ranging between 0.5-1000μm.

Principle

The principle involved in the working of this instrument is that when the particles suspended in a conducting liquid passes through a small orifice (having electrodes on either side), the electrical resistance changes. A known volume of dilute suspension is pumped through the orifice. If the suspension is sufficiently dilute, the particles pass through the orifice one at a time. A constant voltage is applied between both the electrodes for the generation of current.

Procedure

During the process, a particle passes through the orifice displacing its own volume of electrolyte. This increases the resistance between the two electrodes. The change in resistance, which is related to the particle volume, causes a voltage pulse that is amplified and fed to a pulse-height analyser calibrated in terms of particle size.

Coulter counter is one of the suitable instruments for electronically recording the voltage pulses within two threshold values of the analyser. By changing the threshold settings and counting the number of particles in a constant sample size, the particle-size distribution can be obtained. This instrument is used for counting the particles at the rate of 4000 per second, and hence both gross counts and particle-size distributions are achieved rapidly. The data obtained is easily converted from volume distribution to weight distribution.

Uses

It is widely used to study the effect of antimicrobial agents on microbial growth, and to examine the growth and dissolution of particle in formulations.

Advantages

1. The process completes in a very short period of time (less than 30 seconds).
2. It is possible to count large number of particles by using this method.
3. Operator variability is prevented as the aperture is automatic.

Disadvantages

1. Before measuring the particle volume, the material is suspended in a conducting liquid, i.e., electrolyte.
2. Accumulation or aggregation of particles produces false results.