Ultramicroscopic Technique Microelectrophoresis

This is the most commonly used method since it allows direct observation of the particles using an ultramicroscope (suitable for partides larger than 100 nm). Basically, a dilute suspension is placed in a cell consisting of a thin-walled ( 100 pm) glass tube that is attached to two larger bore tubes with sockets for placing the electrodes. The cell is immersed in a thermostated bath (accurate to +0.1 °C) that contains an attachment for illumination and a microscope objective for observing the particles. It is also possible to use a video camera to observe directly the partides. 

Since the glass walls are charged (usually negative at practical pH measurements), the solution in the cell will, in general, experience electro-osmotic flow. Only where the electro-osmotic flow is zero, i.e. at the stationary levd, can the electrophoretic mobility of the particles be measured. The stationary level is located at a 0.707 of the radius from the centre of the tube or 0.146 of the internal diameter from the wall. 

By focusing the microscope objective at the top and bottom of the walls of the tube, one can easily locate the position of the stationary levels. 

The average particle velocity is measured at the top and bottom stationary levels by averaging at least 20 measurements in each direction (the eye piece of the microscope is fitted with a graticule). 

Several commerdal instruments are available (e.g. Rank Brothers, Bottisham Cambridge England and Pen Kem in USA). 

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Microelectrophoresis is the most common technique for electrokinetic measurements in colloidal systems. Here individual particles can be observed, in their normal environment, under the microscope. Very dilute dispersions can be studied and very small particles, down to about 0.1 pm diameter, can be observed using the dark-field microscope (ultramicroscope). High magnifications allow minimization of observation times, and in polydisperse systems a given size range of particles can be studied to the exclusion of others. 

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