What is the Difference Between Isoelectric and Isoionic Point?

The isoelectric point and isoionic point are related concepts, but they have slightly different meanings. The isoelectric point is defined as the pH at which a molecule has no net charge, meaning that the positive and negative charges balance each other out. In contrast, the isoionic point is defined as the pH at which there is no net charge on the molecule, and the dissociable groups of the substance combine equally and only with hydrogen and hydroxyl ions.

Key differences between isoelectric and isoionic points include:

The isoelectric point is applicable to a wider range of molecules, while the isoionic point only applies to zwitterionic substances in pure water.
The isoelectric point is determined by several factors, such as titration or electrokinetic methods, while the isoionic point can only be determined by an electrokinetic method.
The isoelectric and isoionic points of some soluble proteins may be approximately the same, but for insoluble materials, especially those with a more crystalline nature, the isoelectric and isoionic points may be far apart.

In summary, the isoelectric point refers to the pH at which a molecule has no net charge, while the isoionic point refers to the pH at which a molecule has no net charge and its dissociable groups combine equally with hydrogen and hydroxyl ions. Although the terms are often used interchangeably, they have slightly different meanings and apply to different types of molecules and conditions.