Potential Uses of Tannic Acid

Tannic acid is a cheap natural product available from Aldrich. Synonyms are Gallotannic acid, Gallotannin, Galloylglucose, Glycerite, Quebracho or simply Tannins. It has in the past been used to make slow release pharmaceutical products, but that is long gone because the natural variability of the mixture of components leads to a pharmaceutical salt having too much inconsistency in its certificate of analysis.

The molecular weight of tannic acid is usually in the range 1250-1700. Natural tannin is typically derived from Turkish or Chinese nutgall.

Tannin is a polyphenol. It can also be regarded as a natural dendrimer of gallic acid with branching groups built on a scaffold of glucose. It forms a highly concentrated solution in water. One gram can dissolve in 0.35 ml of water according to the Merck Index. It is also soluble in glycerol and lower alcohols such as isopropanol and acetone while being insoluble in non-polar solvents.

Tannin is known to precipitate alkaloid as salts. There are other useful things it is likely to do usefully but for which now that I am retired I do not have the search tools to find evidence. For example, the molecule, because of its polyphenolic nature, might be expected to:

·        Trap halogens and make products that either dissolve in water or precipitate: therefore, tannin added to a wash layer would be expected to remove the colour of residual halogen agents.

·        Precipitate some other polymers; for example, it is known to precipitate albumin, starch, gelatin which are water soluble proteins or carbohydrates.

·        Precipitate small molecular weight water soluble amines as tannic acid salts.

·        Precipitate polyethylene glycol. 

with regard to this last idea, polyethylene glycol (PEG) preferentially binds with tannins to form an insoluble precipitate  [Jones, W. T., and J. L. Mangan. 1977]. Complexes of the condensed tannins of sanfoin (Onobrychis viciifolia Scop.) with fraction 1 leaf protein and with submaxillary mucoprotein and their reversal by polyethylene glycol and pH. [J. Sci. Food Agric. 28:126–136.]

There are other questions:

·        Would it quench free radical oxidants and remove the metal into an aqueous solution or precipitate it?

·        Would tannic acid remove boric acid by forming borate esters with the 1,2-diphenolic unit?

·        Would it remove chromium by treating an organic layer with an aqueous solution?

·        Would it complex hypervalent iodine?

·        Would it remove formaldehyde?

·        Would it react with aldehydes?
Would it quench residual diazonium salts at the end of a reaction?

Siderophores are 1,2-dihydroxybenzene containing materials. Metals chelated by siderophores are Aluminum, Gallium, Copper, Zinc, Lead, Tin, Manganese, Cadmium, Vanadium, Indium, Plutonium and Uranium.

Sodium borohydride might be beneficially worked up using a solution of tannic acid in water or ethanol. In some applications it is very important not just to destroy the borohydride anion but the borane that results.

Tannic acid, I would expect, would potentially destroy cyanide salts because they would attack the phenolic esters. It is known that tannic acid reacts with formaldehyde. Similarly, I think, the catechol functionality would react with other aldehydes.

What would an aqueous solution of tannic acid do with a basic ion-exchange resin? Because of the tannic acid component's significant size it could not enter the smaller voids in the resin. It would however, because of its acidity, develop ion pair bonds that would hold the polymers together.

Tannic acid also forms insoluble complexes with polyvinylpyrrollidone, a behaviour that suggests binding to amide carbonyls where it can share hydrogen atoms in strong hydrogen bonds.

Would tannic acid adsorbed on PVP be useful for removing traces of dipolar aprotic solvents from organic solvent streams?  Because gallic acid units are very electron rich, tannic acid might readily extract electron deficient charge transfer agents into an aqueous solution of tannic acid.

Phenols form strong hydrogen bonds with p-dioxane. Would tannic acid be
soluble or insoluble in dioxane?

Along the same line of thought, might a solution of tannic acid form hydrogen bonds that could complex triphenylphosphine oxide and help remove it from a difficult work-up?

The special characteristics of being both a polymer, and a polyphenol while being at the same time a cheap material suggest that tannin may have many as yet untested uses.