Alternative Method for Carboxyl Group Determination in Humic Substances

A new precipitation titration method, employing cetyltrimethylammonium (CTA + ), is proposed for analysis of carboxyl groups of humic substances in solution. The method was tested with polyphenol-polycarboxylic acids standard solutions: recovery factors ranged from 0.94 to 1.09. Results obtained by performing precipitation of humic acids samples at pH = 9.8 and at 7.0 are compared with those obtained by the Ca-acetate method. The difference between the number of COOH per unit weight of humic preparation calculated by precipitation with CTA + at pH = 7.0 and by the Ca-acetate exchange reaction ranged between 1 and 45%. In most samples overestimation of COOH groups by the Ca-acetate method appears to be higher than the mean relative error of the CTA + precipitation method (5%).