Characterization of acid/base properties for a bleached wood fiber as influenced by ionic salt medium.

In this work, the acid/base properties of a fully bleached softwood kraft fiber have been determined at 25 degrees C and at a series of ionic strengths (10-600mM Na(Cl)). The measurements have been performed as a series of potentiometric titrations between pH 2 and 6, above which unstable readings were recorded. Equilibrium models, containing intrinsic dissociation constants and corresponding specific capacitance values were evaluated al six different ionic strengths and showed, in all cases, that data could be explained with one equilibrium reaction, drop COOH reversible arrow drop COO + H+. They also showed that the concentration of drop COOH constantly corresponded to 31.10(-3) moles/kg dry fiber. With regard to the influence of ionic medium, data show an increasing acid strength with increasing ionic strengths and an increase in electrostatic influence with decreasing ionic strengths. This latter trend is broken at 0.01 M ionic strength, at which it is suggested that the fiber swelling, which causes the distances between acidic groups to increase, becomes important.