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Extracted humic acid not a single acid; rather, it is a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Commercial humic acid used to amend soil is manufactured using these same well established procedures.
The International Humic Substances Society maintains a collection of standard and reference samples of humic and fulvic acids extracted and fractionated from leonardite, river water, a mineral soil, and peat, plus natural organic matter isolated from river water by reverse osmosis, [4] without fractionation. [5]
These largely neutralized the decomposing properties of the humic acid of the fens in the former Geiseltal, but were, as can be seen from the finds, only locally effective. To a lesser extent, silicic acid also ensured good preservation of the fossils.
Due to their very complex molecular structure, humic substances, including humin, do not correspond to pure substances but consist of a mixture of many compounds that remain very difficult to characterize even using modern analytical techniques.
They are defined operationally. The humic acids precipitate from solution at pH < 2. Likewise, the fulvic acids remain in solution. Both are complex mixtures of many organic compounds. Most aquatic NOM is fulvic acid, while a greater fraction of soil NOM (SOM) is humic acid. Humic acids generally have higher molecular weights.
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Fulvic acid is the genus that contains the matter that has the lowest molecular weight, is soluble in acids and alkalis, and is susceptible to microbial action. Humic acid is the genus that contains the intermediate matter that has medial molecular weight, is soluble in alkalis and insoluble in acids, and has some resistance to microbial action.
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