DETERMINATION OF CARBOXYL AND NONCONJUGATED CARBONYL GROUPS IN DISSOLVED AND RESIDUAL LIGNINS BY IR SPECTROSCOPY

FTIR spectroscopy is a well known method for elucidating the structure s of lignin samples. Determination of the ratio between the content of C=O groups and aromatic structures in lignin samples is an important aid to monitoring the degree of delignification during different pulpi ng and bleaching procedures. In the present work a semiquantitative me thod is described in which calibration mixtures with eight different r atios of tartaric acid to milled wood lignin are prepared in order to account for the different responses of the aromatic band at 1510cm(-1) and the C=O band at 1740cm(-1). From the calibration function obtaine d it is possible to calculate the results in mmoles of C=O/g aromatic lignin. The method is tested on residual lignins and spent liquor lign ins from pulps with different degrees of delignification.