I hope you are referring to Zeta Potential , which is a measure of electro-kinetic potential in colloidal dispersion to determine the stability of the of colloids . Electrophoresis is used for determining zeta potential . In general , the values were expressed in [mV] since it involves potential difference in the electrodes , the range of values will be differentiated from < +/- 5 ( in stable ) to >= +/- 61 ( high stable ). This analysis will be highly useful to determine the stability of colloids especially in chemical and drug sectors.