Characterization of acid/base properties for bleached softwood fibers as influenced by ionic salt medium

In this work, the acid/base properties of fully bleached softwood kraft fibers were determined at 25 degrees C and at a series of ionic strengths (10-600 mM Na(Cl)). The measurements have been performed as a series of potentiometric titrations between -log[H+] 2 and 6, above which unstable readings were recorded. Equilibrium models, containing intrinsic dissociation constants and corresponding specific capacitance values were evaluated at six different ionic strengths and showed, in all cases, that the data could be explained with one equilibrium reaction, =COOH reversible arrow=COO-+H+. They also showed that the concentration of =COOH was independent of ionic strength and corresponded to 31.10(-3) moles/kg dry fiber. With regard to the influence of the ionic medium, the measurements showed an increasing dissociation constant with increasing ionic strength and on an increase in electrostatic influence with decreasing ionic strength. A thermodynamic intrinsic dissociation constant, valid at infinite dilution, was evaluated by using the Davies equation.