Determination of phenolic hydroxyl groups in lignin by combined use of H-1NMR and UV spectroscopy

Two independent spectroscopic methods are presented and compared for the qu antitation of the phenolic hydroxyl groups in lignins. The combined informa tion is used to further elucidate the character of the lignin samples exami ned. The UV method is based on the difference of the spectroscopic properti es of the ionised and the nonionised phenol. The method using H-1 NMR spect roscopy is based on the exchange of phenolic protons in D2O. The difference in integrated proton intensities in the sample dissolved in DMSO and the s ample with additional 20% D2O is proportional to the phenolic protons. The method based on UV spectroscopy uses differences in the maxima close to 300 nm and 350 nm of the sample dissolved in alkali and the neutral sample. Th e results using the two independent methods are in agreement for milled woo d lignin, for kraft lignin and for model compounds carrying one aromatic hy droxyl group. For modified lignins and for model compounds with more than o ne aromatic hydroxyl group, the UV method fives too low values for phenolic hydroxyl groups. The combined results obtained by the two methods however provide information of the total amount of the phenolic groups and of the n ature of the phenolic structure formed by the lignin refining.