The contribution to kappa number from hexeneuronic acid groups in pulp xylan

The kappa number of chemical pulps is widely used both in mill operation and in laboratory work as a measure of the degree of delignification in pulping, oxygen delignification, and prebleaching. Recently, it has been shown that the kappa number reflects not only lignin but also carbohydrate structures sensitive to oxidation by permanganate, notably hexeneuronic acid groups linked to xylan. In the present work, the kappa number units originating from hexeneuronic acid groups calculated on a molar basis have been determined in two different ways, viz. by permanganate oxidation of model compounds and by selective elimination of hexeneuronic acid groups from a series of kraft pulps. The results are in good agreement with each other and demonstrate that 10 μmol of hexeneuronic acid correspond to 0.84–0.86 kappa units. From kappa number determinations combined with hydrolysis of the pulp with mercuric acetate, it is possible to calculate the amount of hexeneuronic acid groups present in a pulp.