Dissociation of phenolic groups in kraft lignin at elevated temperatures

The dissociation of the phenolic groups in a polydisperse, low molecular we ight kraft lignin (Indulin AT) was studied in alkaline aqueous solutions in the temperature interval 21-70 degrees C, using a UV-spectrophorometric me thod. It was found that at a constant concentration of hydroxide ions, the degree of dissociation was decreasing when the temperature was elevated. Di ssociation curves and apparent pK(0) values were also calculated for the po lydisperse sample at the same conditions, using the van't Hoff and the Pois son-Boltzmann equations. At degrees of dissociation exceeding alpha approxi mate to 0.4, the outcome of the theoretical approach showed to be in good a greement with the experimentally obtained results. Furthermore, calculation s were performed for different molecular weights of kraft lignin and from t his it was found that the apparent pK(0) is shifted to higher values by inc reasing molecular weight, due to an increased electrostatic attraction of t he hydrogen ions, which is arising from a less curved surface. Predictions of the dissociation behavior at temperatures reached in the kra ft process were performed and under these conditions, higher molecular weig ht lignin fragments seem never to reach the point of complete dissociation. It was also found that an increase in temperature results in phase separati on in kraft lignin solutions with high ionic strengths and pH values close to the pK(a) of the phenolic groups.