Isoelectric Point

ISOELECTRIC POINT of a colloid, the reaction (pH) at which the potential difference between the amphoteric colloid and the solution becomes equal to zero. Amphoteric colloids represent substances that can form salt-like compounds with both acids and alkalis. In an acidic environment at a sufficiently high concentration of hydrogen ions, the acid dissociation of these colloids is suppressed, and they combine with the anion of the added acid. The colloidal particle behaves in this case as a cation and acquires a positive charge. Similarly, in an alkaline reaction, its charge becomes negative. At some intermediate pH, the colloid has neither a positive nor a negative charge and consists predominantly of undissociated molecules (or forms an equal number of colloidal cations and anions). This intermediate pH has been named the isoelectric point of the colloid. Depending on whether acidic or alkaline dissociation predominates in a given amphoteric colloid, its isoelectric point is shifted toward the acidic or alkaline side from neutrality. The sign of the charge of a suspended particle can be judged by the direction of its movement in an electric field. The following table gives the isoelectric point of some biocolloids. Colloid 5.4

Thus, under physiological conditions, at a tissue and body fluid pH > 7.0, the majority of biocolloids are located in the alkaline zone relative to their isoelectric point. Similarly, cell suspensions such as erythrocytes, bacterial cells, and others have been investigated. For erythrocytes in a nonelectrolyte solution, the isoelectric point proved to be at pH 4.6–4.7; for bacterial cells, it corresponds to a more acidic reaction (around pH 3.0). However, the given figures refer only to the colloids of the cell membrane. Various colloidal components of cells and tissues can differ greatly from one another in this respect. The recharging of colloids can also be produced by other ions, especially multivalent ones, opposite in sign to the colloidal charge. In particular, electronegative colloids are easily recharged by salts of lanthanum, thorium, and the like. In this case too, at a certain intermediate concentration of ions, the colloidal particles (or cell suspensions) turn out to be completely devoid of charge. Despite the external similarity, however, in this case there is a phenomenon of an entirely different order: not a change in the character of dissociation of the amphoteric colloid, but the neutralization of its charge by the adsorption of the oppositely charged ion. Therefore, it is more correct here to avoid the term "isoelectric point" and speak only of the "isoelectric state" of the colloid. D. Rubinstein.