In this section a question and answer summary is given about the structure of humic acid molecules. For a full explanation of these answers please refer to the section entitled “Structure and Origin of Humic Acids and their Relationship to Kerogen, Bitumen, Petroleum and Coal.”
Q Are there many humic acid molecules?
A Yes, probably thousands (complexity like components in petroleum).
Q When one sees complex published structures containing many aromatic groups and links, are they correct?
A No, they do not resemble humic acid molecules.
Q What are the molecular weights of humic acids?
A In the vicinity of 800 – 1200 amu (unaggregated).
Q What structures do humic acid molecules contain?
A These structures:
Q Is the exact polymeric structure of any humic acid molecule known?
A No, but these can be postulated as major structures, either by themselves or with other groups listed above present in the chain:
A Are there minor polymeric components?
Q Yes, these have been found:
C17-C30 alkanes, considered to be degradation (probably decarboxylation) products
R[CH2CH2]n-H where R = C18H37O4
C20H40N305[(CH2)mO]n-H where m = 2 or 3 and n = 1-3
Q Are there minor non-polymeric components?
A Yes, such as succinic, maleic, oxalic and ketoglutaric acids. Minor amounts of aromatic acids such as vanillic and syringic acids in terrestrially-derived humic acids are considered to be contaminants.
Q Does the humic acid molecule change structure with the environment?
A Yes, there are two major structural changes. The gem-diol group present in the solid can lose a molecule of water on dissolution in certain solvents or at different concentrations in a given solvent. Also, the conjugate chelate group can change its equilibrium between an enol-keto and di-keto group, depending on the solvent and concentration. A minor keto-enol change may also occur.
Q What effect does the dynamic nature of the humic acid molecule as described above have on the properties of humic acids?
A A single molecule changes its structure and chemistry eternally, and is therefore very difficult to analyze, and gives the impression that humic acids are composed of an infinite variety of molecules.
Q Can the structure of underivatized humic acids be determined?
A Probably not, because of the dynamic nature of humic acid molecules described above.
Q What are the best humic acid derivatives?
A Butylated derivatives; butylated plus lactonized derivatives; reduction with Raney Nickel or lithium aluminium hydride followed by acetylation or derivatization with pentafluoropropionic anhydride.
Q What are the best techniques for investigating the structure of derivatized humic acids?
A Fluorescence and solution FTIR. HPLC-MS, FT-MS and NMR are also useful, but only for the most hydrophobic derivatives.
11-11
Hello, it really interesting, thanks
Comment by ShealaGag — 2009 @ 10:49 pm