Basic Properties of Humic Acid
Humic acid is an amorphous mixture of various macro-molecules; and its real molecular structure is still unknown. Usually in black or brown colloidal state, its shape, when dry, is gelatinous, like what we usually see on shell fractures. With a density between 1.330-1.448g/cm3, it has a large specific surface area; and it is as non-viscous as water in dilute solution. The basic properties of humic acid run as follows.
Solubility:Humic acid can be more or less soluble in acid, alkali, salt, water or some organic solvents, so these substances can be used as extracting agent of humic acid. These extractants are generally divided into five kinds: basic substances (such as KOH, NH4OH, NaCO3, Na4P207, etc.), neutral salts (NaF, Na2C204), weakly acidic substances (such as oxalic acid, citric acid, benzoic acid, etc.), organic solvents (like ethanol, ketones, pyridine, etc.) and mixed solutions (NaOH and Na4P2O7) .
Colloidal properties:As a kind of hydrophilic colloid, humic acid is a true solution with no viscosityat low concentration; while at high concentration, it is a colloidal solution (dispersion system), showing colloidal properties. When mixed with acid or high-concentration salt solution, coagulation can occur in it. Generally speaking, when mixed with dilute hydrochloric acid or dilute sulfuric acid, and when the pH of the solution is kept between 3 and 4, flocculent precipitation can be quickly precipitated in the solutionafter standing. The colloidal nature of humic acid also determines that the solubility of humic acid solution will not be too high, generally between 20-40.And it will appear in a paste state when itsconcentration in the solution is high.
Acidity:There are carboxyl and phenolic hydroxyl groups in the molecular structure of humic acid, making it weakly acidic. Therefore, humic acid can be quantitatively reacted with carbonate, acetate, etc. The buffer solution composed of humic acid and its salts can adjust the pH value of the soil and enable crops to grow under suitable pH conditions.
Ion exchangeability:Some functional groups on humic acid molecules, such as H+ on carboxyl-COOH, can be replaced by metal ions such as Na+, K+or NH4+ to form weak acid salts, thus endowing humic acid with a high ion exchange capacity. The ion exchange capacity of humic acid was related to pH. When the pH increased from 4.5 to 8.1, the ion exchange capacity of humic acid increased from 1.7 mmol/g to 5.9 mmol/g.
Complexing capability:Since humic acid contains a large number of functional groups, it can form complexes or chelates with some metal ions (such as Al3+, Fe2+, Ca2+, Cu2+, Cr3+, etc.). The functional groups involved in metal complexation or chelation in the humic acid are generally carboxyl and phenolic hydroxyl groups, and possibly carbonyl and amine groups.
Physiological activity:The physiological activity of humic acid refers to the ability of humic acid to promote the physiological activities of organisms. The physiological activity of humic acid is shown in plants via its stimulation upon plant growth and metabolism, its enhancing effect on fruit quality and plants’ stress resistance.The physiological activity of humic acid is related to its concentration and molecular weight.