Chemical properties
Tannic
Acid (Acidum Tannicum) is, as the name suggests, a
Tannin. It is an
ester of
gallic acid (3,4,5-
hydroxybenzoic acid) and
glucose, that is,
galloyl tannin (thank you
vuo) and has the chemical formula
C14H10O9 and is derived from
Oak. Tannic Acid is somewhat of a
misnomer, as this chemical is actually not an acid as such, but a
polyphenol compound. It is soluble in water (2 in 1), alcohol (10 in 6) and slowly, in
glycerin (1 in 3, or 1 in 1 on heating). It is, however, almost insoluble in pure
ether,
chloroform, or
benzene.
Use in tanning
It is one of many ways of
tanning animal hide, thus producing
leather. It is classified as a
Vegetable tannin, which works by transforming the gelatin in the animal hide into a
water insoluble form and thereby making the skin somewhat
water-resistant. It was for this reason that tannins first became
tools for humans, but they do have other uses, notable in the staining of woods.
Medical uses
When taken
orally, tannic acid has an
astringent property, that is it binds tissues. It will penetrate (although not deeply) into cells in the mouth, and cause a general feeling of astringency, in the stomach it forms coagulated proteins and in the gut tends to constipate. For this reason, it is sometimes
prescribed to stop
diarrhoea. It is also used externally on wounds, to act as a
coagulant. It is rarely taken internally, due to these coagulating properties. A typical dose is 1-3
decigrammes.
Warning: Tannic acid is
toxic. Do not ingest it unless a doctor has prescribed it to you. Regular exposure can lead to
liver or
kidney damage, any exposure can result in
irritation of the area exposed, or a binding and
constipated intestinal tract for ingestion.
Inhalation can induce
sneezing and a general bound feeling. If it is exposed to the eyes, or a rash develops from skin contact, seek medical attention immediately. If it is ingested
vomiting should be induced, but it would be best to
consult somebody with medical training for this.
Use in wood staining
It is also used in the staining of woods, as many
wood stains react with tannic acid woods with a low tannin content need to be first treated with tannic acid, in order to ensure they can be stained. The mixture required to stain the wood is 1 part tannin to 19 parts water. The staining itself is often done with
Potassium Dichromate or
Ammonia to produce
wood tones.
Solubility and medical applications from: http://www.ibiblio.org/herbmed/eclectic/bpc1911/acidum-tann.html
Wood staining information from:
http://www.woodfinishsupply.com/Tannin.html
Tannin
Tanning
Thanks to vuo for a correction about the compound nature of this chemical and excellent description of the chemical.
vuo used: http://medplant.nmsu.edu/Frhus.html