Acid-base properties of oxygen-delignified kraft pulps

The acid-base properties of oxygen-delignified kraft pulps have been investigated in 0.02 and 0.10 mol/L (Na)Cl-media at 25 degreesC. The measurements have been performed with high precision potentiometric titrations in the pH-range of 3 to 5.5. The acid-base properties were ascribed to fibre-bound carboxylate groups. The acid-base reaction is affected by the charge arising in the fibre when the acid groups are deprotonated, and can be described by an intrinsic constant, beta (s)(1,1(intr.)) valid for an uncharged surface, and a capacitance of the electrical double layer, representing the influence of the surface charge. Two investigated softwood pulps were found to have similar acid-base properties having log beta (s)(1,1(intr.))-values of 3.20 and 3.25 respectively in 0.02 mol/L (Na)Cl, while the investigated hardwood pulp had a log beta (s)(1,1(intr.))-value of 3.45. When the strength of the ionic medium increased, the acid strength of the fibre-bound carboxylic acid groups increased. The influence of the charged groups on the acid-base properties is affected by the ionic strength and the distance between the acid groups on the fibre. The charged groups had the smallest influence on the acid-base properties in the hardwood, even though the amount of acid groups per unit weight was approximately twice that in the softwood.