There are old pubs (1,2) measured the pKa values of cellulose weak acids by fitting line onto experimental data taken from potentiometric titration. I did the same potentiometric titration and collected the data and fitted the line and its working good in the pH range from 4 to 8 but its not giving me a good fit from 8-10. From the fitted line info, I got 2 pKa values, one was around 5 and the other was 8. pKa value of 5 makes sense and can be related to the carboxylic acid of wood but for the phenols I should get pKa around 10 and not 8 I guess! Has someone had the same issue ? and can anyone explain this ? and if you know any practical method for determining pKa values of wood weak acids and phenols I appreciate it if you share it with me. Thanks
1-Zhu, Tony QinYong. Studies on the surface charge of eucalypt pulps by potentiometric titration. CRC Publications Committee, 1995.
2-Laine, Janne, et al. "Potentiometric titration of unbleached kraft cellulose fibre surfaces." Colloids and Surfaces A: Physicochemical and Engineering Aspects 88.2 (1994): 277-287.
Dear Mohammad,
The pK values of about 8 are usual for phenols containing conjugated carbonyl groups (for example vanillin with pK 7.4) and for corresponding lignin structural units. You can observe such structures easily in titration curves. The compounds with higher pK (10 or more) do not demonstrate the obvious jumps on titration curves due to hydrolysis оf corresponding phenolate anions. You can find more information in our publications attached.
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