During oxygen delignification of kraft pulp the alkalinity has a decisive influence upon the selectivity, defined as viscosity at a given kappa number. Using 0.2% pulp consistency in order to achieve constant pH-levels a minimum selectivity was reached at pH 13, corresponding to 0.1 M NaOH. At higher pH-levels the selectivity increased and the yield decreased. By lowering the pH the selectivity and the yield increased. With Na2CO3 as active alkali the selectivity increased with decreasing alkalinity.
Additions of large amounts of magnesium converted the alkali to Mg(OH)