Analysis of potentiometric titrations of heterogeneous natural polyelectrolytes in terms of counterion condensation theory: application to humic acid

A model, developed within the framework of the counterion condensation theo ry of linear polyelectrolytes, is presented in this paper to describe the a cid-base properties of linear polyelectrolytes, consisting of several types of functional ionizable groups. This formalism has been successfully appli ed to Fluka humic acid under salt-free conditions, as well as in the presen ce of supporting simple 1:1 salt (KNO3) at three different concentrations. As part of this approach, the charge density of the humic acid is obtained from the activity coefficient measurements of potassium counterions at diff erent humic acid concentrations at a constant degree of dissociation of the polyelectrolyte. The humic acid average charge density was found to be 0.8 0 +/- 0.05. Using the present model, we are able to satisfactorily describe the experimental data obtained from acid-base potentiometric titrations. F our main functional groups making up the polymer are determined through the ir fractional abundances (X-i) and intrinsic pK (pK(i)(0)) values. The frac tional abundances remained constant and independent of the ionic strength, indicating that the humic acid constitution does not depend on the concentr ation of excess salts. The pK(i)(0) values show a small change with ionic s trength, which can be explained by the polyelectrolytic behavior of the sol ution. (C) 2000 Elsevier Science B.V. All rights reserved.