CATION-ANION INTERACTIONS WITH SOIL HUMIC SUBSTANCES (COUNTERION CONDENSATION, DONNAN THEORY, CHLORIDE ION-SELECTIVE ELECTRODE)

Investigations were conducted on the interference of humic substances on Cl('-) determination using the Cl('-) ion-selective electrode. Both humic and fulvic acids interfered with Cl('-) analysis by complexing Ag('+), thereby reducing the activity of the potential determining ion. Known ligands were found to react in a similar manner. Selectivity coefficients (k(,Cl,HA)) for humic substances ranged from 3.86 x 10('-5) to 1.01 x 10('-4) ml mg('-1) at pH 6.5. Data obtained from electrode response curves were also used to calculate apparent stability constants for the binding of Ag('+) by humic substances: log K's at low Ag('+) saturation ranged from 3.35 to 4.46 at pH 6.5. Evidence was obtained indicating that N containing functional groups (i.e., NH(,2)) played an important role in Ag('+) complexation.; Donnan and counterion condensation theories were used to model the distribution of ions (specifically K('+) and Cl('-)) in a Donnan system containing humic acid. A general equation was derived to determine Donnan distribution of ions with variable valence at any pH and ionic strength. For soil humic acids, Donnan ratios for K('+) (r(,D)('K)) were greater than for Cl('-) (r(,D)('Cl)), indicating that a greater amount of K('+) was present in the interior solution than would be expected for an ideal Donnan system. The data were analyzed on the basis that the non-ideal behavior was due to: (i) lower activity coefficients for the ions in the interior, as compared to the exterior solutions, and (ii) counterion condensation of K('+) on the humic polymer. Whereas the latter could not be dismissed entirely, the major cause of the unequal Donnan ratios appeared to be due to a difference in the activity coefficients.; Inferences on charge spacings and the configuration of humic acids in solution were made from experimental data on Cl('-) exclusion. For three soil humic acids, charge spacings ranged from 3 to 28 (ANGSTROM); for a lignite humic acid, the range was 10 to 200 (ANGSTROM). A rather constant charge spacing over the pH range from 3.5 to 6.8 supported the hypothesis that humic acid is a flexible molecule in solution.