The interaction of Cu(II) ion with humic acid

Abstract

The interaction of Cu(II) ion with a humic acid gel was carefully examined as a function of ionic strength and degree of dissociation to test the utility and validity of the fundamental approach developed by the authors for the unambiguous analysis of the protolytic and metal-ion complexation properties of such organic acid substances. Several of the equations developed earlier have been useful for this purpose. Two complexed species, CuS 2, where S is believed to be an oxygen atom accessible from ether, aldehyde, ketone, or phenolic groups contained in the three-dimensional array of aromatic rings and their side chains which determine the structure of humic acid, and (RCOOCu +) ν are believed to be the only Cu(II)-bound entities formed. The stability constant of the more strongly complexed CuS 2 is approximately 1 × 10 4; the formation constant of (RCOOCU +) ν is ∼18, a value not too different from the formation constant that has been reported for this particular species when formed by the interaction of Cu(II) with mononuclear carboxylic acid molecules. The sizable variation in p K (HA) ν app of three orders of magnitude, attributed initially to the heterogeneity of humic acid samples, was found to be more likely a consequence of differences in the accessibility of the functional units (RCOOH) ν, (RCOO −) ν, and (RCOOCu +) ν in these condensed systems.