Impact of Residual Extractives on Lignin Determination in Kraft Pulps

The amount of residual extractives in kraft pulps and oxygen‐delignified pulps and their impact on the usual methods of lignin determinations were investigated. The lignin analyses investigated were kappa number and Klason lignin in conjunction with acid‐soluble lignin. The species investigated were loblolly pine, white birch, and aspen. In the hardwoods, significant amounts of extractives remained after kraft pulping and oxygen delignification. These residual extractives in the hardwood pulps had an impact on the lignin determination, more so on the acid lignin method than kappa number. For an accurate lignin determination, solvent extraction of hardwood kraft and oxygen‐delignified pulp is needed, but not for softwood pulps.