A modified ionization difference UV–vis method for fast quantitation of guaiacyl-type phenolic hydroxyl groups in lignin

Abstract

An ionization difference UV–Vis method (Δε-spectrum method) is the most potentially simple method for fast quantitation of phenolic hydroxyl groups (ph-OH) in lignin. However, the underestimated results were calculated from the conventional Δε-spectrum method using one- or two-point wavelengths measurement. In this study, a modified Δε-spectrum method using multi-point wavelengths measurement was developed and the negative absorbance was also considered. Four main typical lignin models, e.g. vanilla alcohol, 5-5 biphenyl, stilbenoid and vanillin, were applied as the guaiacyl-type (G-type) phenolic models for the determination of ph-OH by the modified Δε-spectrum method. The 2-methoxyethanol/water/acetic acid = 8/2/0.2 (V/V/V) was used as the acidic solvent system and the 2-methoxyethanol/0.2 M NaOH solution = 1/9 (V/V) was used as the alkaline solvent system. The ph-OH contents in the spruce milled wood lignin (SMWL) and the spruce Kraft lignin (SKL) were respectively quantified by the modified Δε-spectrum method as 1.078 and 4.348 mmol/g, which were comparable to the counterparts determined by 31P Nuclear Magnetic Resonance Spectroscopy (31P NMR). The results revealed that the modified Δε-spectrum method can provide more accurate and reliable results compared to the conventional method.