mlp
Harmless
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Pyrogallol and iron-gall ink
My husband is a huge fountain-pen enthusiast, and recently he's started to get interested in the history and chemistry of ink. Iron-gall ink is pretty
easy to make from tannin (obtained from a brewery supply), ferrous sulfate (obtained as the heptahydrate, from the garden store), HCl and phenol, but
it has a minor drawback: although the ink reacts with the cellulose in paper to produce a very dark black line, when it goes onto the paper it is
nearly clear and thus difficult to see.
This problem can be solved by adding a dye (prussian blue is apparently rather common, thus "blue-black" permanent fountain pen ink). I synthesized
several 100mL batches of iron-gall ink earlier this evening, and he tried saturating them with various water-soluble dyes, but we haven't been happy
with any of the ones we had; some precipitated out (probably due to the acidity of the ink), some didn't have much effect on the colour on paper, some
just smelled too nasty to use. We're going to look for some more ink-friendly dyes, but I want to try a couple of other possibilities first.
In particular, there's an excerpt from an 1856 book, "Cooley's Cyclopaedia of Practical Receipts" (http://www.clt.astate.edu/elind/pdf%20folder/CooleyInk.PDF), which describes two ways to produce ink that writes black initially: calcination of
the ferrous sulfate, or conversion of some of the gallic acid in the tannin to pyrogallic acid. I don't want to go the calcination route, because ink
produced in this way doesn't react as strongly with the cellulose in the paper and thus can be erased more easily. So, pyrogallic acid sounds like the
way to go.
Later this week I plan to hydrolyze some tannic acid (well, tannin really) to gallic acid with NaOH, then convert the gallic acid to pyrogallic acid.
However, just for grins I dropped some tannin in a test tube and heated it with a butane torch. The tan powder turned lighter (a sort of yellowish
beige, not quite white), then became darker and started to bubble. A white odorless vapor was produced. I didn't have a thermometer handy, but I'm
pretty confident that I got at least some of the tannin up past the melting point of gallic acid (523K). Lifting some of the molten mass with a glass
rod stretched the mass out like taffy, and it solidified quickly on cooling, with a glassy look after it had cooled.
I'm fairly certain that I've turned at least some of the tannin into plain old carbon, but I'm also curious what the likelihood is that I've also made
some pyrogallol. The brown glassy substance does dissolve in water, leaving some particulate matter which I assume is carbon, so I plan on trying to
heat up a larger sample and making ink with it (filtering first to remove particulates, since that's not the point).
More generally, when one is decarboxylating via heat, how do you compute the target temperature? It's been well over a decade since my last chemistry
class, and I'm not sure that subject ever came up.
Finally -- I have no interest in methylating the stuff, this is for ink.
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not_important
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The carboxylic group in tannin is fairly well tied up as esters, one structure is that in the attached image and you'll find other variants. I don't
think that it will decarboxylate very well in that form.
Below is a bit from another old book of formula or receipts, which might be useful. I also remember a process from tannin to gallic acid that
saturated a 5% solution of HCl with tannic acid, stoppered it up to keep out air, and left it in a dark warm place for several weeks. Gallic acid is
much less soluble in water than tannic and in part crystalises out, after chilling the solution gallic acid was collected on a filter.
Note that this is an old book from the USA, all temperatures are in Fahrenheit. Also, nowadays there are better ways to do the heating than a zinc
chloride bath.
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This section is from the "Encyclopedia Of Practical Receipts And Processes" book, by William B. Dick. Also available from Amazon: Dick's encyclopedia
of practical receipts and processes.
Acids. Part 15
3907. To Obtain Gallic Acid from Tannin
3907. To Obtain Gallic Acid from Tannin. Add a strong aqueous solution of tannic acid (tannin) to sulphuric acid, as long as a precipitate falls;
collect the powder, wash, and dissolve it by the aid of heat in diluted sulphuric acid; boil for a few minutes, cool, and collect the crystals of
gallic acid which will form in considerable quantity.
3908. To Distinguish Gallic Acid from Tannic Acid
3908. To Distinguish Gallic Acid from Tannic Acid. Gallic acid does not affect solutions of gelatine, the protosalts of iron, or the salts of the
alkaloids; but it produces a black precipitate with the sesquisalts of iron, which disappears when the liquid is heated.
3909. Pyrogallic Acid
3909. Pyrogallic Acid. This acid, is formed in white, shining scales, inodorous, very bitter; soluble in water, alcohol, and other; fusible at
239° Fahr., and subliming at 410°. When quite pure, it has no action on litmus paper. It is used in photography. A solution of the crude acid mixed
with a little alcohol imparts a fine brown color to the hair, but stains the skin also.
3910. To Obtain Pyrogallic Acid
3910. To Obtain Pyrogallic Acid. It may be prepared by heating gallic acid (previously dried at 212° Fahr.) in a glass retort, by means of a
chloride of zinc bath, to 410°, when the pure acid sublimes, and forms in crystals on the neck of the retort, and in the receiver, which should be
kept well cooled.
3911. Tannic Acid, also called Tannin
3911. Tannic Acid, also called Tannin. Pure tannic acid is solid, uncrystallizable, white, or slightly yellow; strongly astringent, but without
bitterness; very soluble in water, less so in alcohol and ether, and insoluble in fixed or volatile oils. Its solution reddens litmus. With the bases
tannic acid forms
Among the incompatibles of tannin are the alkaloids of opium, and it is altogether unavoidable that if solutions of them are brought together, a
precipitate will form of tannates; also, if the preparation of opium contain saffron, as in acetum opii and Sydenham's laudanum, this will cause a
further precipitation of the extractive of saffron. (See No. 3908 (To Distinguish Gallic Acid from Tannic Acid).)
3912. To Obtain Tannic Acid
3912. To Obtain Tannic Acid. Expose nut-gall in fine powder to a damp atmosphere for 24 hours, then mix it with sufficient ether, previously
washed with water, to form a soft paste. Set this aside, closely covered, for 6 hours; then envelope it quickly in a close canvas cloth, and obtain
the liquid portion by pressing powerfully between tinned plates. Reduce the resulting cake to powder, mix it with sufficient ether shaken with 1/16
its bulk of water, to form again a soft paste, and express as before. Mix the liquids, and evaporate spontaneously to a syrupy consistence; then
spread it on glass or tinned plates, and dry quickly in a drying closet. Put the dry residue in a well-stopped bottle.
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[Edited on 15-3-2010 by not_important]
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mlp
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Thanks! I had read on another thread here that base hydrolysis is preferable to acid hydrolysis for tannic acid because acids (not sure if this is any
Lewis acid or just Bronsted-Lowry acids) tend to strip off the -OH groups, but the test for gallic vs. tannic is extremely useful -- thanks!
210C should be more than doable in a sand bath; I will give that a try. I'll report back once I've tested it.
And yeah, I wouldn't expect the pentagalloyl ester to decarboxylate on its own; the original preparation calls for roasting the gallnuts before
soaking them to extract the useful acids, so I assume there's some free gallic acid in the galls. I have no idea what the actual composition of my
tannin is; it might be all pentagalloyl glucose, or there might be some ellagic acid esters in there, hard to say. 1g of tannin went immediately into
solution in 10mL of water; ellagic acid and its esters are apparently not very soluble in water (though I haven't found actual figures for solubility
yet), so I'm hoping I have more gallotannins than ellagitannins. I'll try saturating an aqueous solution of tannin as well.
[Edited on 15-3-2010 by mlp]
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