Stability of many dispersions and emulsions are often related to pH of the solution. There is semi-empirical rule stating that system would be stable beyond 2 pH units vicinity of iso-electric point. Definition of the iso-electric point can be found in ISO Standard ISO 13099, Parts 1 and 3: 2012.Colloidal systems – Methods for Zeta potential determination.
Iso-electric point can be briefly defined as pH value at which zeta potential of the system equals 0. Precision of the zeta potential measurement is critical for high accuracy of the iso-electric point determination. In this sense Electroacoustic method has big advantage over other methods of zeta potential characterization. Precision of zeta potential measurements with models DT-310 and DT-1202 is ± 0.1 mV.
There was detail study of various methods suitable for characterizing iso-electric points conducted by international group:
Hackley, V., Patton, J., Lum, L., Wasche, R.J., Naito, M., Abe, H., Hotta, Y., and Pendse, H. “Analysis of the iso-electric point in moderately concentrated alumina suspensions using electroacoustic and streaming potential methods”, J. of Dispersion Science and Technology, 23, 5, 601-617 (2002)
This study established that our instruments can determine iso-electric point with accuracy of 0.1 pH unit.
It would be important to stress that determination of iso-electric point does not require calibration of electroacoustic device. Phase of electroacoustic signal rotates 180 degrees at this pH. This is an absolute definition of the iso-electric point.
Figure below illustrates results of pH titrations for alumina slurry at 4% vl and rutile slurry at 7% vl.
Iso-electric point of rutile is around pH=3.8, whereas for alumina it is pH=9.3.