Model describing Donnan equilibrium, pH and complexation equilibria in fibre suspensions

A model for the calculation of ionic equilibria and pH in fibre suspensions has been developed. Two extensions to existing Donnan theory-based models are provided: the equations facilitate the calculation of pH and combine Do nnan theory with complexation models. It is assumed that freely mobile ioni c species are Donnan equilibrium distributed between fibres and bulk soluti on and that only hydrogen ions are specifically bound to the fibres. The mo del has been verified by determinations of the acid/base properties and met al ion sorption of hardwood kraft fibres at different ionic strengths, in t he presence of a chelant (EDTA) and by modification of the fibre charge by selective hydrolysis of hexenuronic acids. The stability constants of acid groups in the fibres were determined by potentiometric titration. The amoun ts of acid groups were determined by titration and by analysis of uronic ac ids by enzyme hydrolysis/HPLC and acid hydrolysis/UV spectrophotometry. The model quantitatively predicts ionic equilibria (pH, distribution of ions, complexation) in all the systems investigated.