The contemporary distribution of cations between bleached softwood fibres and the suspension liquid, as a function of -log[H+], ionic strength and temperature

The distribution of combinations of metal ions between a water volume confined to the fully bleached softwood kraft fibre (i.e. the Donnan volume) and the surrounding water volume was studied at 25 degreesC and 90 degreesC. Three metal ion systems were studied Na+- K+- Mg2+- Ca2+, Na+- Cu2+ and Na+- Al3+, as series of batch experiments in the -log[H+] range 2-9, and in all cases the contemporary metal ion distributions could be predicted by the Donnan ion exchange theory. To enable the Donnan modelling at elevated temperature, the acid/base properties of the fully bleached softwood fibre were determined at 70 degreesC and 90 degreesC. These measurements were performed as series of potentiometric titrations at a constant ionic strength of 100 mM Na(Cl). Data were collected between -log[H+] 2 and 6, above which unstable readings were recorded. At both temperatures, one single equilibrium reaction, drop COOH reversible arrow drop COO-+H+, with an intrinsic dissociation constant and a specific capacitance value, was sufficient to model data. Using the van't Hoffs equation, the enthalpy for the acid/base reaction of the Fibre material was estimated to -3kJ . mol(-1). This value is very similar to literature values for monocarboxylic acids