Potentiometric Titration of Unbleached Kraft Cellulose Fiber Surfaces

The surface properties of unbleached kraft cellulose fibres in the pH range 2-8 have been investigated using a high precision potentiometric titration method. The pulps were digested in various ways, varying both digestion time (level of residual lignin) and the amount of excess alkali. Titrations were made at constant ionic strength (0.1 M NaCl). The results were evaluated by assuming the existence of different types of acidic groups in the surface and adjusting their intrinsic stability constants and the capacitance of the electrical double layer to obtain the best possible fit to experimental data. Fitting was done using the error square sum minimising program FITEQL, version 2.0. Distribution diagrams were calculated with the program SOLGASWATER. It was found that the amount of acidic groups decreases with increasing digestion time, while the amount of excess alkali is not important. The pulps contain two types of acidic groups that dissociate in the pH range 2.5-7.5 (intrinsic stability constants pK almost-equal-to 3.4 and pK almost-equal-to 5.5). The pK value of the stronger acid agrees with what could be expected for uronic acids in hemicellulose (xylan), while the amount of the weaker acid correlates directly with the amount of lignin in the pulps.