Humic substances acid-base properties

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. 

Nbt only do the elemental composition, Mn, and aromaticity of a humic substance constrain the concentrations of acidic functional groups but they also give a crude picture of the spatial distribution of dipolar groups and formal charges in the vicinity of an acidic functional group. Such information is especially useful in evaluating the structural soundness of the multitude of models that are used to describe the acid-base properties of humic substances. 

Gustafsson, J.P, Modehng the acid-base properties and metal complexation of humic substances with the Stockholm humic model, J. Colloid Interf. Sci., 244,102, 2001. 

The potentiometric titration curves of HA are usually broad and ill-defined, reflecting the varying properties of acidic groups on HA [23,24]. Wilson and Kinney [25] investigated the acid-base properties of lake water and marine humic substances and determined a two-step dissociation process. Schnitzer and... 

Humic substances influence soil properties to a degree out of proportion to then-small percentage in soil. They strongly bind metals, and serve to hold micronutrient metal ions in soil. Because of their acid-base character, humic substances serve as buffers in soil. The water-holding capacity of soil is significantly increased by humic substances. These materials also stabilize aggregates of soil particles, and increase the sorption of organic compounds by soil. 

Tipping, E., and M. A. Hurley. 1988. A model of solid—solution interactions in acidic organic soils, based on the complexation properties of humic substances. Journal of Soil Science 39 505-519. 

Bowles E. C., Antweiler R. C., and MacCarthy P. (1989) Acid-base titrations and hydrolysis of fulvic acid from the Suwannee River. In Humic Substances in the Suwannee River, GA Interaction, Properties, and Proposed Structures, USGS Open File Report 87-557 (eds. R. C. Averett, J. A. Leenheer, D. M. McKnight, and K. A. Thorn), United States Government Printing Office, Washington, DC, pp. 209-229. 

Humic substances, humic and fulvic acids, are essentially a mixture of compounds of different molecular weights. The total number of base-titratable groups is in the range of 10-20 meq per gram of carbon. Chelation by neighboring carboxyl and phenolic groups is the major mode of metal complexation. Compounds such as malonic acid, phthalic acid, salicylic acid, and catechol serve as convenient monomeric model compounds for estimating the coordi-native properties of humic substances. 

Research on the chemical properties of humic substances was extended by the Swedish investigator Berzelius (1839). One of his main contributions was the isolation of two light-yellow-colored humic substances from mineral waters and a slimy mud rich in iron oxides. They were obtained from the mud by extraction with base (KOH), which was then treated with acetic acid containing copper acetate. A brown precipitate was obtained ctilled copper apocrenate. When the extract was neutralized, another precipitate was obtained, called copper crenate. The free acids, apocrenic and crenic acids, were then brought into solution by decomposition of the copper complexes with alkali. These newly described humic substances were examined in considerable detail, including isolation, elementary composition, and properties of their metal complexes (Al, Fe, Cu, Pb, Mn, etc). 

Techniques used to separate humic substances from either a dissolved or sedimentary matrix rely on their acid-base solubility properties. In some of the separation techniques, it is likely that some nonhumic material accompanies the humic substances during the purification processes—to an extent... 

Examples of successful fractionations of aquatic humic substances where only one fractionation mechanism was operative include utilization of the hydrophobic properties of XAD resins (Mantoura and Riley, 1975 Aiken et al., 1979), hydrogen bonding of weak-acid functionalities of humic constituents to weak-base anion-exchange resins (Kim et al., 1976), and use of ion-exchange celluloses for ion-exchange fractionation of aquatic humic substances without hydrophobic matrix adsorption (Sirotkina et al., 1974). These examples of successful fractionations demonstrate the potential for chromatography of aquatic humic substances when fractionations are designed carefully to avoid undesirable interactions. 

In a thorough paper that discusses many of the important properties of multiligand mixtures in the context of acid-base equilibria. Gamble (1970) has provided a direct method for estimating the distribution of pKa values in humic substances. Gamble assumes that a continuous distribution of nonidentical binding sites exists in humic substances and that the stoichiometric... 

Humic substances have been arbitrarily divided into three diverse groups of compounds on the basis of their solubility in dilute acid and dilute base, Fulvic acids are soluble in both dilute acid (pH 1) and dilute base. Humic acids are soluble in dilute base but are precipitated by dilute acid (pH 1). Humin is insoluble in both dilute acid and dilute base. Some authorities divide humic substances into only two groups the humic acids and the fulvic acids, stating that the humins have the same characteristics as humic acids but that their solubility in base is hindered because they are associated with clay minerals in natural waters.The gross chemical and physical properties of humic acids and fulvic acids are presented in Table 5-12. It is the fulvic acid fraction that appears to... 

Humic substances are contained extensively throughout the environment (e.g., soils, sediments, surface waters, oceans, groundwaters, and swamps). These substances originate from the decomposition of animal and plant tissue with the variety of their chemical and physical properties dependent on their origin and age. Designation of the substances into chemical classes is not possible due to their complexity, even though research efforts to characterize them have been conducted for more than 200 years [96]. As a result, classification is based on the solubilities of the substances. Humic acids are only soluble at pH values >2 and fulvic acids are soluble at all pH values. 

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