Tannins from gallic acid

The two main types of hydrolysable tannins may be derived respectively from gallic acid and ellagic acid (the depside of gallic acid). These are known as gallo tannins and ellagitannins. 

Solid phase extraction (SPE), described in Technique 12, Section 12.14, avoids the use of mefhylene chloride, and fhe technique will be employed in Experiment 13B for exfracfing caffeine from fea or coffee. The material used in this SPE procedure is prepared by alkylating silica with 18-carbon chains. This process changes the silica from a polar subsfance to a relatively nonpolar one. The resulting material, called C-18 silica, is placed in a tube that resembles a syringe body. Because the silica is now nonpolar, water and other polar substances such as tannins and gallic acid will pass through the column, whereas the C-18 silica retains caffeine. To remove fhe caffeine from the column, ethyl acetate is added. 

Contradictory opinions have been referred to in the literature particularly on the nature of the iron-tarmate and its interaction with the rusted steel due to the diversity of the material used in different studies. Studies have included the use of tannic acid [7-10], gallic acid [11], oak tannin [12, 13], pine tannin [14] and mimosa tannin [15]. In order to establish the correlation between the ferric-taimate formation and the low inhibition efficiency observed at high pH from the electrochemical studies, phase transformations of pre-rusted steels in the presence of tannins were evaluated. In this work the quantum chemical calculations are conducted to analyse the relationship between the molecular stracture and properties of ferric-taimate complex and its inhibitory mechanism. 

Hydrolyzable tannins are comparatively restricted in the human diet and there are no human metabolic data. Studies in rats have indicated that some 63% of a dose of 1 g/kg commercial tannic acid is excreted unchanged in the feces accompanied by small amounts of gallic acid, pyrogallol, and resorcinol. Plasma after enzymic hydrolysis was found to contain 4-O-methylgallic acid, pyrogallol, and resorcinol. Urine also contained a small amount of gallic acid after enzymic hydrolysis. The most notable observation from this study is the failure of the gut microflora to metabolize the galloylglucoses efficiently, at least at this substantial dose. The viability or composition of the gut microflora was not reported. ... 

Again, one pound of galls is exhausted as first directed, and the tannin of the decoction changed, by exposure, into gallic acid, as in the last instance. The menstruum is then mixed with a saturated solution of logwood, freshly made by boiling a pound and a half of the chips in five quarts of water, till the quantity is reduced to seven pints eighteen ounces of sulphate of iron, and as many of gum, are dissolved in this mixed liquid the whole is allowed to stand for two or three days, and the ink is then, decanted from the sediment. 

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