The Issue of Molecular Weight in Humic Acid Chemistry

Since the day humic substances have been isolated by Berzelius, they are commonly considered polymeric in nature. The biopolymer theory became so popular that the concept was embraced by most scientists for more than 1.5 centuries. It has provided a suitable hypothesis that is among other things not running afoul of with the elemental composition of the humic compounds, which shows relatively consistent ranges, especially in their carbon and nitrogen contents. Though the basics of polymer chemistry suggest that the molecular weight of polymers equals the sum of molecular weights of the monomer components minus the moles of water lost in the condensation reaction, many people claim the concept to fail in providing a commonly accepted molecular weight for humic substances. The range, reported in the literature, shows values to vary from a few hundreds to >1,000,000. A range of 500 Da to 1,360,000 Da is also cited by Piccolo and coworkers, a range too large for any of the values to be the characteristic mass of humic substances. This is one reason for Piccolo to introduce an alternative theory, the supramolecular assembly concept. Aside from the interchangeable use of dalton and dimensionless units, addressed presently as creating more confusion, the new hypothesis has created several reactions of wait-and-see awaiting future developments. This new concept tries to show humic substances to be composed of groups of small heterogeneous biological compounds, called supramolecular associations, capable of self assembly. The idea originates from results of low pressure (LPSEC) and especially high pressure size exclusion chromatography (HPSEC), methods falling in the category of gel chromatography. Considered a relatively effective method for the selective separation of humic substances according to molecular sizes, the size exclusion fractionation depends on the type of gel substance used and a large variety of gels are available. Therefore, Piccolo’s supramolecular humic assembly — though providing a fascinating model — is derived from the chemical behavior of artificial or operational humic fractions created by HPSEC. The capacity of self assembly of broken humic segments into new humic associations has to be established and confirmed properly by separate analyses. As admitted by Piccolo, no immediate solution is also in sight on the issue of molecular weight.