Isoelectric Point
The pH at which a molecule carries no net electrical charge.
Formula / equation
pI = (pKa₁ + pKa₂)/2
Unit
Dimensionless
In depth
At its isoelectric point a molecule does not migrate in an electric field and its solubility is at a minimum, since there is no charge to promote solvation. Both facts are exploited to separate and purify proteins by isoelectric focusing and by selective precipitation.
Examples in the real world
Glycine has pI 5.97; casein precipitates from milk at pH 4.6, which is how cheese-making begins.