The Influence of pH on Zeta Potential

  • The most important factor that affects zeta potential is pH
  • A zeta potential value quoted without a definition of it's environment (pH, ionic strength, concentration of any additives) is a meaningless number
Imagine a particle in suspension with a negative zeta potential
  • If more alkali is added to this suspension then the particles tend to acquire more negative charge
  • If acid is added to this suspension then a point will be reached where the charge will be neutralized
  • Further addition of acid will cause a build up of positive charge
  • In general, a zeta potential versus pH curve will be positive at low pH and lower or negative at high pH
  • There may be a point where the curve passes through zero zeta potential
  • This point is called the isoelectric point and is very important from a practical consideration
  • It is normally the point where the colloidal system is least stable

In the above example it can be seen that if the dispersion pH is below 4 or above 8 there is sufficient charge to confer stability. However if the pH of the system is between 4 and 8 the dispersion may be unstable. This is most likely to be the case at around pH 6 (the isoelectric point)

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