Donnan Equilibria in Wood-Alkali Interactions. Part I. Quantitative Determination of Carboxyl-, Carboxyl Ester and Phenolic Hydroxyl Groups

Abstract

Abstract The functional groups in wood that may become ionized by the action of sodium hydroxide consist of carboxyl, phenolic hydroxyl and carbohydrate hydroxyl groups. The extent of ionization is dependent on the pH of the solution and causes the establishment of a Donnan equilibrium between the outer solution phase and that associated with the solid phase. Analytical techniques were developed to determine the carboxyl and phenolic hydroxyl contents of extractive-free mildly saponified hemlock and cottonwood meals using equilibration in the pH range of 12 to 12.5. The determined carboxyl contents, 0.158 and 0.127 meq/g OD wood, respectively, were confirmed by conductometric titrations of wood meal samples methylated with diazomethane prior to saponification with NaOH. Further confirmation was obtained from conductometric titrations of saponified and HCl-washed samples with NaHCO3. The last-mentioned method was also applied to unsaponified hemlock and cottonwood samples to obtain their free carboxyl contents, 0.031 and 0.066 meq/g, respectively. The phenolic hydroxyl contents determined on borohydride-reduced wood samples were 0.260 and 0.147 meq/g, respectively. They were used to estimate the percentages of phenolic units present in in situ lignins. The results obtained suggest that hemlock lignin contains 15.0%, and cottonwood lignin, 14.2% of phenolic C6C3 units.