Determination of carbonyl groups of six round robin lignins by modified oximation and FTIR spectroscopy

The carbonyl group content of six round robin lignins, which were distributed in the scientific community 1990-1991, has been determined by a modified oximation. In this technique, the reaction occurs at higher temperature (80 degrees C) than usual (25 degrees C) and the oximating mixture contains besides hydroxylamine hydrochloride also triethanolamine (TEA) in excess. The latter shifts the chemical equilibrium towards a fully oximated product. The portion of TEA not consumed in oximation is titrated with HCl either by potentiometric titration to pH 3.3 or by high frequency conductometric titration. Both titration techniques give similar results. The modified oximation yields essentially higher CO contents than the traditional one. A correction technique is presented which is necessary for lignins containing carboxyl groups. The corrected CO contents show an excellent correlation (r(2)= 0.99) with the normalized area of the total carbonyl region of the FTIR spectra. This measure, obtained through division of the integral between 1840 and 1550cm(-1) by the absorbance at 1600cm(-1), allows the reproduction of the results of CO group determination by modified oximation. Several correlation equations are presented and problems of CO group determination in lignins are discussed.