Average molecular weight, humic

Kiss, G.,Tombacz,E., Varga, B., Alsberg,T., and Persson, L. (2003). Estimation of the average molecular weight of humic-like substances isolated from fine atmospheric aerosol. Atmos. 

Figure 13.4. Apparent MWs of HS from different environments as determined by SEC [compiled from the data reported by Perminova et al. (2003)]. Bars represent a range of weight-averaged molecular weights determined for different groups of humic materials. Two black dots represent two IHSS standards Suwannee River FA and HA. SEC chromatograms were obtained with column-packing Toyopearl HW-50S using 0.028 M phosphate buffer at pH 6.8 as a mobile phase. Two different sets of standards were used to calibrate the column polystyrene sulfonates (PSS) and polydextrans the upper MW scale corresponds to dextran calibration and the bottom one—to PSS calibration.

GiUam A. H. and Riley J. P. (1981) Correction of osmometric number-average molecular weights of humic substances for dissociation. Chem. Geol. 33, 355—366. 

The techniques that have been used to determine the approximate molecular weights of humic materials are often those used to study biological macromolecules. Such techniques are often prone to artifacts arising from adsorption, precipitation, and degradation. Many of the methods suffer from one common problem, the lack of appropriate standards. Only colligative properties are free of this problem however, they yield number-averaged molecular weight, with no indication of the polydispersity of the humic material. 

In addition to methods that yield average molecular weights, there are a number of methods (gel filtration, ultrafiltration, small-angle X-ray scattering) that measure molecular size. In these methods, model compounds of known molecular weight and composition are used to estimate the molecular weight of humic substances. Problems can arise if the model compounds are not sufficiently similar to the humic substances of interest. Choice of appropriate model compounds is hampered by lack of detailed information about the chemical structures of humic substances. 

The molecular weight obtained by measurement of a colligative property is a number-average molecular weight. Two serious problems need to be addressed in determining number-average molecular weights of humic substances ... 

The reported spin concentrations correspond to one free radical per 1100 molecules (number average molecular weight of 951) for an untreated fulvic acid in the work of Schnitzer and Skinner (1969) and one free radical per 250 molecules of humic acid (1.4 x 10 spins/g, with a molecular weight of 20,000) in the work of Steelink (1964). Consequently, the ESR spectrum is providing data on only a small fraction of the total molecules in the humic mixture, as pointed out by Riffaldi and Schnitzer (1972). Hayes et al. (1975) found that the free radical contents of humic and fulvic acids vary with the solvent used in the extraction, suggesting that the free radical in a humic substance may be an artifact of the extractive procedure. With the severity of these limitations in mind, any generalization concerning the functionality or structural nature of humic substances based on ESR data must be conservative. 

By measuring the excess ratio R d) at different concentrations, the molecular weight can be determined. Because in a humic sample (as with most macromolecular materials), a distribution of molecnlar weights is present, and because light dispersion is a property essentially dependent on M, the measuranent yields the weight average molecular weight ... 

Humic acid is composed of aromatic, aliphatic and carbohydrate carbon compounds. An average humic acid s elemental composition is 55.1% C, 5.0% H, 3.5% N, 35.6% O, and 1.8% S (Rice and MacCarthy, 1991). Its molecular weight distribution is typically broad, and it is a relatively high-molecular-weight material relative to the fulvic acid isolated from the same soil or sediment. It s predominantly functionalized by carboxylic acid and phenolic groups. At least some components of humic acid are surface-active, and these components have been shown to form micelles in concentrated, alkaline aqueous solutions (Piret et al., 1960 Visser, 1964 Wershaw et al., 1969 Tschapek and Wasowski, 1976 Chen et al., 1978 Rochus and Sipos, 1978 Hayano et al., 1982 Hayase and Tsubota, 1984 Guetzloff and Rice, 1994). 

The results from gel chromatography indicate that the 2 mixtures contain humic substances, the majority of which have molecular weight (MW) lower than 50,000. Mixture HS2 contains about 60% of molecules with average MW of 5,000. 

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