Binding by humic substances

Tipping E., 1994, WHAM - a chemical equilibrium model and computer code for waters, sediments and soils incorporating a discrete site/electrostatic model of ion-binding by humic substances. Computers and Geosciences 20,973-1023. 

Tipping, E. and Hurley, M. A. (1992). A unifying model of cation binding by humic substances, Geochim. Cosmochim. Acta, 56, 3627-3641. 

Tipping E. 2002. Cation Binding by Humic Substances. Cambridge Cambridge University Press. 

Milne, C. J., Kinniburgh, D. G, and Tipping, E. (2001). Generic NICA-Donnan model parameters for proton binding by humic substances. Environ. Sci. Technol. 35, 2049-2059. 

Tipping E., Rey-Castro C., Bryan S. E., and Hamilton-Taylor J. (2002) Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation. Geochim. Cosmochim. Acta 66, 3211-3224. 

Tipping, Edward. Cation Binding by Humic Substances. 1st ed. Cambridge Environmental Chemistry Series, no. 12. New York Cambridge University Press, 2002. 

Until recently, most mathematical models that were used to describe proton binding by humic substances were far too simple, and, consequently, served only as empirical curve-fitting equations. More recently, several authors have used mathematical models that treat humic substances as having... 

This chapter examines the fundamental structural properties of organic molecules that directly or indirectly affect pKa values of organic acids, making it possible to estimate theoretical upper limits for concentrations of common acidic functional groups, to assess methods of quantitative analysis of acidic functional groups in humic substances, and to examine several models that have been proposed for the description of proton binding by humic substances. Some of the concepts that appear obvious in the context of this chapter have been included for the simple reason that the literature on acidity of humic substances reveals that these points have frequently been either misunderstood or neglected. 

Whenever an organic acid contains two or more chemically identical (i.e., stereochemically equivalent) functional groups, statistical factors that originate in the entropy of formation of the acid and/or its conjugate base contribute to the variation of thermodynamic dissociation constants with the degree of dissociation of the acid. Such statistical effects are implicitly included in equations that are often used to describe acid-base equilibria in synthetic and natural polymers. Because those equations have frequently been applied to proton binding by humic substances, a brief discussion of statistical ef-... 

EQUILIBRIUM MODELS OF PROTON BINDING BY HUMIC SUBSTANCES... 

Many mathematical models have been used to describe proton binding by humic substances however, in every case, the models were initially developed for other purposes, often for the description of proton binding by proteins, acidic polymers, ion exchange resins, and so on. The assumptions and approximations that were inherent in the original models have often been overlooked or forgotten when those models are applied to humic substances. In this section, several common models are examined to evaluate their applicability to humic substances, with due consideration for the complexity of this mixture of nonidentical organic acids. 

The literature on proton binding by humic substances indicates that statistical effects, delocalization effects, and, probably most importantly, the effects of dipolar groups on the acidity of a functional group have generally been ignored. An attempt has been made in this chapter to provide the reader with a rather detailed discussion of the nature of substituent effects on the dissociation constants of organic acids. Statistical, electrostatic, and delocalization effects have been treated separately. 

Tipping, E. (1993a) Modelling the competition between alkaline earth cations and trace metal species for binding by humic substances. Environmental Science and Technology 27, 520-9. 

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