The Reaction of Phloroglucinol in the Presence of Acid with Ligin-Containing Materials

UV-visible diffuse reflectance spectrometry has been used to monitor the reaction of phloroglucinol (in the presence of hydrochloric acid) with high yield pulps. When papers made from such pulps are irradiated, coniferaldehyde residues are generated as judged by their reaction with phloroglucinol/HCl. Treatment of such papers with sodium borohydride reduces the coniferaldehyde residues and consequently no colour reaction with phloroglucinol/HCl is observed. Irradiation of these borohydride-reduced papers leads to the generation of coniferaldehyde entities as detected by phloroglucinol/HCl. Similar results were obtained using a thin section of abies wood. Irradiation of papers made from high yield pulps destroys coniferaldehyde residues on the one hand and generates them from other sources on the other hand. Examination of papers made from the morphological components of poplar showed that it is impossible to quantify the amount of lignin present by this colour test method. Thioacidolysis analysis of the fractions shows for all of them a higher proportion of syringyl than guaiacyl beta-O-4 aryl ether linkages. In contrast, photocleavage of the beta-O-4 aryl ether linkages is more important for the guaiacyl structures. This is in accordance with the observed higher sensitivity to UV light of softwood pulps compared with hardwood pulps. Also, the thioacidolysis experiments indicate that UV irradiation of the fractions maintains a stationary concentration of the coniferaldehyde structures.