Critical comparison and validation of methods for determination of anionic groups in pulp fibres

Anionic or acidic groups in pulp fibres are important for many interactions in pulping and papermaking. The total amount of anionic groups can be determined by acid-base titration, as well as different sorption and ion-exchange techniques. In this study, different methods for determination of total anionic groups were compared on a broad selection of pulps. Conductometric titration with NaOH and methylene blue sorption gave very similar results on all pulps, and both methods had a good repeatability. The methylene blue technique is especially attractive because of its simplicity. Polyelectrolyte adsorption with polybrene generally gave higher readings, was less repeatable, and was also a very laborious method. Uronic acids, including hexenuronic acid, determined by enzymatic hydrolysis and HPLC (High Pressure Liquid Chromatography), represented only 20-60% of the total anionic groups in the kraft pulps. Uronic acids analysed by acid methanolysis and GC (Gas Chromatography) usually represents 70-100% of the anionic groups in mechanical pulps.