It is well known that the conductometric titration of ligninsulphonic acid with strong base shows the existence of weakly acidic group, which has been suggested to be phenolic group, beside sulphonic group.
However, it was found that if ligninsulphonic acid was titrated with weak base, ammonia or dicyclohexylamine, only smaller amount of weakly acidic group was detected than the case with strong base.This weakly acidic group detected with weak base was studied on model substances.
As the results, it was found the pKa value of this group was below eight, and it was suggested this group was carboxylic group.
For that reason, the true value of phenolic group was shown by the difference between the weakly acidic value of strong base titration and that of weak base.