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isoelectric point[¦ī·sō·i′lek·trik ′pȯint]
(point of zero charge), the state of the surface of a body or of a particle in the dispersed phase in contact with an electrolytic solution; it is characterized by an equal number of positive and negative charges in the adsorption layer. Here the zeta potential is equal to zero. Colloidal systems stabilized by electrolytes are unstable at the isoelectric points—that is, they break down as a result of the adhesion of particles in the dispersed phase. The electrically neutral state of amphoteric electrolytes (ampholytes) that contain separate acid and base groups is also called the isoelectric point. A specific pH value corresponds to the isoelectric point of every ampholyte. At the isoelectric point the molecules of ampholyte, like colloidal particles, lose the ability to shift directionally in an electrical field. The swelling, solubility, and viscosity of solutions and many other characteristics of ampholytes, particularly macromolecular ampholytes, assume extreme values as the isoelectric point is approached.