Humic substances general discussion

Because humic substances generally contain low amounts of nitrogen and sulfur (see next section), the remaining discussion will focus on the properties of oxygen acids (carboxylic acids, phenols, alcohols) and active methylene compounds. 

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 fact that a humic substance is not a pure compound, but is a heterogenous mixture of many compounds with generally similar chemical properties, places an important constraint on all these characterization methods. Examples of the multicomponent mixture problem are presented in the chapters discussing interpretation of elemental analysis, determination of molecular weight, analysis of potentiometric data, and interpretation of infrared and other spectroscopic data. 

Most ultracentrifuge studies of humic substances have used the sedimentation-velocity method. Flaig et al. (1975) have reviewed this work. In general, the results of this work indicated that all the solutions studied were polydisperse as we have discussed previously, in a polydisperse system, the ordinary sedimentation-velocity equation cannot be applied because each different particle size has a different diffusion coefficient and different sedimentation constant. 

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. 

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. 

A critical examination of chemical and physical methods of investigation reveals that they are generally confined to simple systems, and, once a mixture is involved, one s ability to interpret the data is extremely limited. These complications are aggravated in the case of humic substances which are the epitome of molecular complexity as evidenced by the results of separation attempts to date. The limitations imposed on the interpretability of elemental analysis data and hydrolytic data are discussed by Steelink in Chapter 18 of this book. 

Felbeck Jr. (1965a) summarizes at some length the hypotheses that have been suggested on the structure of humic substances. Attention is given to the ideas of Thiele and Kettner (1953), Flaig (1959, 1960, 1964), Kononova (1961), Swaby and Ladd (1962) and Felbeck Jr. (1965a,b). These hypotheses cannot be discussed here in detail. Felbeck s general summary of the various ideas follows. 

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