On the Cooking Mechanism of Wood. XVII. Further Investigations on the Conductometric Titrations of Lignin Models with Phenolic Hydroxyl Groups and a Brief Theory of the Conductometric Titration of Phenols

Abstract

Abstract 1. Conductometric titrations of the model compounds IV, V, VI and VII provided reconfirmation of the previously reported findings that simple guaiacol-type phenolic hydroxyl group I was conductometrically titratable, while non-guaiacyl phenolic hydroxyl group belonging to type II was conductometrically untitratable. 2. It was thus further confirmed, that the separate estimation of both type-I and -II phenolic hydroxyl groups of lignosulfonic acid by the method of conductometric titration is possible in combination with the ultraviolet absorption spectra taken simultaneously with the conductometric titration without any dilution of the solution. 3. As we have now considerable accumulation of facts supporting the existence of o,o′-diphenol type phenolic hydroxyl group in lignin, three water-soluble model compounds VIII, IX and X were titrated conductometrically. Phenolic hydroxyl groups of IX and X were found to be conductometrically titratable. Of the two phenolic hydroxyl groups of the model compound VIII only one was found to be titratable. 4. A brief theoretical consideration of the conductometric titration of extremely weak acid (phenol) having pK values larger than 9 were made. As is evident from Fig. 12, which shows theoretical conductometric titration curves of phenols having pK values 9∼12, one can calculate the equivalence-point in so far as the pK values of the phenols are smaller than 10; it is, however, impossible to find the equivalence-point when pK values are larger than 10. As the calculated titration curve for pK 12 is nearly linear, it is expected that the pK value of phenolic hydroxyl group of type II is 12 or larger.