Determination of the Sodium bound by Poly-acids

Abstract

THE fact is now widely accepted^1–3 that some part of the sodium in animal tissues occurs not in the form of free sodium ion but, rather, in an osmotically inactive form. The structures on which the binding occurs, as well as the amount of the sodium so bound, remain to be determined. A number of substances are capable of binding the sodium ion and some of them, such as the acid mucopolysaccharides and nucleic acids, belong to the group of poly-acids—polymers with at least one acidic group in the monomer unit. Sodium as well as other positively charged ions are attracted into the neighbourhood of the fixed negatively charged acidic groups by the strong electrostatic fields present, and their apparent activity coefficient is thus considerably lowered in comparison with that in corresponding solutions of sodium salts of simple mineral acids. This type of binding is called “site-binding”⁴.