Kappa number
The kappa number is a parameter of the pulp industry that describes the content of residual lignin . It records the amount of easily oxidizable double bonds in the pulp . For the determination, a cellulose sample is mixed with an excess of a solution of potassium permanganate at room temperature in acidic conditions. After a short reaction time, the excess is titrated back using iodometry . Under these conditions, double bonds and aromatic (phenolic) structures of all lignin components still present on the cellulose , as well as resins and hexenuronic acids, react . The latter are found in large quantities in hardwoodContains sulphate pulp . The kappa number is used to estimate the intensity of the digestion and the need for bleaching chemicals. In pulp mills, online devices measure the kappa number for process control.
| Kappa number | Residual lignin (%) | Pulp hardness |
|---|---|---|
| 3-7 | 0.5-1.0 | extra soft |
| 11-15 | 1.5-2.5 | soft |
| 23-31 | 3.0-4.0 | normal |
| 46-54 | 6.0-7.0 | hard |
| 54-69 | 7.0-9.0 | very hard |