Coulter Counter

Counter Coulter (electrical sensing zone ), which is recently developed method for the determination of particle size, is based on conductivity measurement.

Sample Preparation for Coulter Counter

Powder samples are dispersed in an electrolyte to form a very dilute suspension, which is usually subjected to ultrasonic agitation, for a period, to break up any particle aggregates. A dispersant may also be added to aid particle deaggregation.

The Principle of Coulter Counter Method

The testing tube is surrounded by a highly conducting electrolyte solution (i.e., about 0.1 M ionic strength), with particles dispersed in it. There is a small aperture on that tube, with two electrodes set at each side.

Solution flow is initiated by the height difference between the liquid surfaces, which is caused by applying vacuum to the tube. When the particles pass through the aperture, the instant change in resistance leads to voltage pulses, the extent of which is proportional to particle volume. These pulses are amplified and counted electronically, the number of the particles and the particle size distribution is displayed on the computer. To obtain the size distribution over a large diameter range, making a change to the aperture size may be necessary.

Particle Size Range of the Coulter Counter

A typical particle size range from about 0.6 to 120 μm can be measured by the Coulter counter method, depending on how many apertures and tubes are used in the apparatus. There are some problems associated with this method. When multiple apertures and tubes are used, large particles may block the pathway or more than one particle may pass through the aperture at one time; particles smaller than the detection limit passing through the aperture may not be detected; and nonuniformity of the electrical field may lead to the abnormal voltage pulses.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier
  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd

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