Humic substances acidic functional groups

Perdue, E. M. 1985. Acidic functional groups of humic substances. In Humic Substances in Soil, Sediment and Water Geochemistry, Isolation, and Characterization, (G. R. Aiken,... 

Ritchie, J. D., and Perdue, E. M. (2008). Analytical constraints on acidic functional groups in humic substances. Org. Geochem. 39, 783-799. 

Oiganic colloids (mainly humic substances) also possess large specific surfaces and are generally negatively charged due to the acidic functional groups that tend to dissociate or polarize. Examples of such groups include carboxyls (R-COOH), quinones (-Ar=0), phenols (-Ar-OH), and enols (R-CH=CH-OH). (See Section 5.4.2 for a more complete discussion of humic substances.)... 

The technique of titration calorimetry has been successfully used to determine the nature and abundances of a variety of acidic functional groups in proteins ( ). Several investigators have made rather limited efforts to use titration calorimetry to study humic substances, usually as a method to determine the cation exchange capacity or titratable acidity of humic substances (23, 24). Choppin and Kullberg ( ) have recently used titration calorimetry to determine the enthalpies of neutralization of acidic functional groups in humic substances and have combined that data with pH titration data to obtain AG, AH, and AS values... 

Because of the considerable potential of titration calorimetry as an analytical technique for characterization of the acidic functional groups of humic substances, our studies have been extended to river water humic substances. In this paper, results are presented for the thermochemical characterization of the acidic functional groups of river water humic substances from two quite different river systems 1) the Satilla River in southeastern Georgia, and 2) the Williamson River in southern Oregon. 

Concentrations of Acidic Functional Groups in River Water Humic Substances. The results of the total acidity, carboxyl... 

Table I. Concentrations of acidic functional groups in river water humic substances and soil humic acid.

Perdue E. M., Reuter J. H., and Ghosal M. (1980) The operational nature of acidic functional group analyses and its impact on mathematical descriptions of acid-base equilibria in humic substances. Geochim. Cosmochim. Acta 44, 1841-1851. 

Relative to soil humic substances, humic substances from Lake Celyn, Wales, and fulvic acids from lakes near Mt. St. Helens contain larger amounts of reactive acidic functional groups (especially carboxyl groups). The reason for this is not known. In Lake Celyn, 24% of the humic acid carbon is carboxyl and 40% is aromatic, suggesting that the Lake Celyn humic acids are largely of terrestrial origin (M. A. Wilson et al., 1981a). 

To properly describe the acid-base properties of humic substances, it is essential that (1) the identification and quantification of acidic functional groups be accomplished in a rigorous, reproducible manner and (2) the range of pKa values that exists in humic substances be described by a suitable model that is as rigorous as possible. Despite years of research, neither of these objectives has been satisfactorily met. Consequently, it is not generally possible to compare results obtained by different scientists if different methods and/or humic samples have been used. 

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. 

Both the abundances and the values of acidic functional groups in humic substances are constrained and controlled by the compositional and structural properties of these substances. It is therefore important to be familiar with the principal structural properties that determine the acidities of organic acids. In this section of the chapter, compositional constraints on the abundances of acidic functional groups and a general discussion of statistical, electrostatic, and delocalization effects on the acidities of organic acids are presented. Specific applications to humic substances are given in the following sections of the chapter. 

There is clearly some overlap of pKa ranges for the classes of acidic functional groups. While the distribution of pK values in humic substances... 

---