Formation and chemical structures of acid-soluble lignin I: sulfuric acid treatment time and acid-soluble lignin content of hardwood

To elucidate the formation mechanism of acid-soluble lignin (ASL) formed in the Klason lignin determination, beech wood meals were treated with sulfuric acid (SA) under various conditions, and the ASL solution was extracted with CHCl3. The results indicated the following: (1) wood components yielding ASL are dissolved in 72% SA during the initial stage: (2) the quantity of ASL is highest during the initial stage, then decreases with prolonged time of 72% SA treatment and finally reaches a constant value; (3) soluble lignin prepared by 72% SA treatment and subsequent standing in 3% SA again yield insoluble Klason lignin and ASL after boiling in 3% SAI and (4) about half the amount of ASL is dissolved in CHCl3. The foregoing suggest that wood components yielding ASL are dissolved in 72% SA at the beginning and finally change to ASL after being subjected to depolymerization, hydrolysis, and other reactions. ASL may thus be composed of low-molecular-weight degradation products and hydrophilic derivatives of lignin.