The Kappa number is an indication of the residual lignin content or bleachability of wood pulp by a standardized analysis method.
[1] ISO 302 is applicable to all kinds of chemical and semi-chemical pulps and gives a Kappa number in the range of 1–100.
The Kappa number is a measurement of standard potassium permanganate solution that the pulp will consume.
Since the amount of bleach needed is related to the lignin content of the pulp, the Kappa number can be used to monitor the effectiveness of the lignin-extraction phase of the pulping process.
It is approximately proportional to the residual lignin content of the pulp.