Determination of Carboxyl Groups

Abstract

Carboxyl groups reportedly have been detected in native lignin, specifically in milled wood lignin, by infrared spectroscopy (Ekman and Lindberg 1960). However, the collective findings of a large number of structural studies indicate that if such groups are in fact present in native lignin, their concentration is extremely small. On the other hand, when native lignin is subjected to chemical (e.g., pulping and bleaching) or biological treatments, significant amounts of carboxyl groups are frequently detected. This effect is particularly pronounced in oxidative treatments where the resulting rupture of phenolic rings in the lignin gives rise to carboxyl-containing entities. Quantitative measurement of carboxyl groups may therefore provide information regarding the degree to which the lignin has been degraded or otherwise modified as the result of a specific chemical or biological treatment. The carboxyl content of a modified lignin may also provide information relating to its solubility characteristics and, for pulps, to strength properties.