White Yeti
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On the acid catalysed hydrolysis of tannic acid.
Hello everyone,
I recently attempted to depolymerise some tannic acid to get hold of gallic acid. I got the tannic acid by letting a lot of plant matter sit in a
container of hot water. I read that the same mechanism used to catalyse the formation of esters can be used to catalyse their hydrolysis.
I don't have muriatic acid, and I don't have sulphuric acid either. So I decided to make some HCl myself. The acid turned out to be extremely weak and
essentially useless, even as a catalyst.
Out of frustration, I wanted to see if sodium bisulphate was a strong enough acid to catalyse the hydrolysis and... it was.
However I'm not quite sure I got what I was going for. Upon the addition of sodium bisulphate, most of the pigment precipitated out, leaving a dark
brown precipitate and a pale yellow aqueous solution above. I'm guessing that the precipitate consists mainly of flavonoids and the aqueous solution
contains the gallic acid I'm looking for. However, gallic acid has limited solubility in water, on the order of 10 grams per litre, so it may also
have precipitated out alongside the flavonoids.
My question is mainly analytic, how can I test the solution and the precipitate to make sure I have what I want?
I'll post a picture when I find batteries for my camera.
In the meantime, any suggestions?
"Ja, Kalzium, das ist alles!" -Otto Loewi
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bbartlog
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Different plants give different tannins, not all of which will give gallic acid on hydrolysis. You should distinguish between 'tannic acid' (a
specific commercial form of tannin) and the much broader class of plant tannins. So anyway, what plant matter did you use as a tannin source?
For qualitative analysis: first, add some ferric salt to a small portion of the liquid. Either tannin or gallic acid should form a blue or black
precipitate (I realize this doesn't tell you whether you have gallic acid *specifically* but would be a good step to see whether your first step, the
extraction, worked).
Second, to a slightly acid solution of the extract, add an aqueous solution of gelatin. Tannin will produce a curdy white precipitate. Gallic acid
will produce no precipitate.
Unfortunately this test will not allow you to determine whether you have achieved partial hydrolysis of the tannin, i.e. it would still show a
precipitate even if some gallic acid is now present alongside the tannins. I suppose that if you have a way of separating the precipitate from the
second test you could re-run the first test on the remaining liquid...
The less you bet, the more you lose when you win.
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White Yeti
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My first choice for obtaining tannins was sumac (the non-poisonous variety) but the flowers were not mature yet. My second choice, which I ended up
using, was to use the leaves of some kind of goldenrod, Solidago gigantea (I think), primarily because it grows all too well around where I
live and I don't have to go through too much trouble to obtain very large quantities.
I am pretty sure that my preliminary extraction worked. The very first test I ran was mixing the tannin solution with iron acetate. The solution
turned murky and an extremely fine precipitate formed. The precipitate was almost colloidal, so much so, that it took an entire day to settle out. The
precipitate was a dark green colour, perhaps because I was using iron(II) acetate instead of the iron(III).
I don't have gelatine on hand, but I did try mixing some of the tannin solution with egg whites, some white stringy proteins precipitated out as
expected.
I'm waiting for the precipitate from the alleged hydrolysis (which I think is composed of flavonoids) to settle so that I can test the aqueous
solution for gallic acid. The problem is that even if the hydrolysis worked, it's still an equilibrium, whereby both gallic acid and tannins should be
found in the aqueous layer.
"Ja, Kalzium, das ist alles!" -Otto Loewi
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Nicodem
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White Yeti, please provide the references for the processes you talk about. I doubt we have many mind readers on the forum, and even if we do, it is
unlikely these are the same people who have the right type of experience to give you proper advices.
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White Yeti
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Acid catalysed ester hydrolysis:
http://www.chemguide.co.uk/physical/catalysis/hydrolyse.html
"Ja, Kalzium, das ist alles!" -Otto Loewi
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Nicodem
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Thread Moved
25-6-2012 at 08:48 |
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