To do the flocculation assay, using 5g/l of kaolin suspension at the pH of 6-7 is recommended. The kaolin suspension in this pH is easily settled and precipitates, so I don't know how to prepare a sample for zeta potential analysis which should be an stable colloidal sample.
Nasrin Keshani
Thermodynamic and kinetic (sedimentation) stability of kaolin depends on the particle size. To increase the fraction of small particles, you can grind them in a mill or disperse them in an ultrasonic disperser. If they then quickly settle, then disperse them in water and fill a long glass cylinder with the suspension. After some settling, collect the suspension with the smallest fraction from the top. It or any other fraction from this cylinder can be used to measure the zeta potential.
Nasrin Keshani Zeta potential without an associated pH value is practically meaningless. The fact that your material is settling indicates that the size is large and you may have agglomerates forming as the zeta potential may be close to 0 mV. This is the time when coagulation/precipitation is statistically most likely. Have you carried out a Stokes’ Law calculation? As Yuri Mirgorod states you can allow the system to settle and measure the supernatant containing the smaller suspended particles. If this is the same chemistry as the settled particles then your zeta potential measurement is likely to be fine. However, you need to examine a range of pH’s in order to have a good estimate of the isoelectric point.
The international standards also recommend a concentration ladder for the measurements.
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