Change of apparent acidity constant of indicators in polyelectrolyte solutions

Abstract

The change of apparent pK and visible spectra of pH indicators produced by polyelectrolytes has been studied. Appreciable changes are observed when indicators have opposite charge to that of the polyions. The effect of polyelectrolyte concentration, molecular weight of the macroions, and addition of simple salts and dioxan to the solutions are reported. The observed phenomena may be attributed to the “polyelectrolyte effect” due to the large charge density on the polymeric chains. There are however non-electrostatic interactions contributing to the changes in indicator properties; stacking of dye molecules may be ruled out.

The relevance of these studies in the field of ionic reactions in the presence of macroions and their relationship with the so called protein error, introduced in colorimetric pH determinations by the presence of proteins or colloids, is discussed.