Sciencemadness Discussion Board - Object: make Gallic acid through iron gall ink

Object: make Gallic acid through iron gall ink

CHRIS25 - 4-11-2012 at 04:15

I have been doing quite a lot of reading about gallic acid since that is what I want to make next. My understanding so far is that I can not make gallic acid from tannic acid, unlike other things in combination with Iron salts. However I can make iron-gall ink from Tannic acid and Ferrous Sulphate. This produces gallic acid + FeSO4.7H2O (not adding gum arabic because it is not the ink I want to make). So far much of the information is here there and everywhere, I have found no specific sources where gallic acid has been made and therefore am a bit unsure how to proceed further. Can someone start me off please? Forgot to add that I have no distilation equipment or lab, so my means are restricted to home chemistry.

[Edited on 4-11-2012 by CHRIS25]

[Edited on 4-11-2012 by CHRIS25]

bbartlog - 4-11-2012 at 05:18

Pretty sure you can produce gallic acid from tannic. Check out 'Outlines of chemistry for the use of students' by William Gregory, p436:

'The precipitate formed by sulphuric acid in a hot solution [of tannic acid], dissolves in hot diluted sulphuric acid, and when this solution has been boiled for a short time, it contains no tannic acid, the whole having been converted into gallic acid.'

On the other hand Watts distinguishes between quercitannic acid (which supposedly can't be hydrolyzed to produce gallic acid) and gallotannic acid (which can), so maybe it depends on how you got your tannic acid. Tannic acid is not after all the name of a specific compound...

Vargouille - 4-11-2012 at 08:15

I made iron gall ink previously, with ferrous sulphate, gallic acid, gallotannic acid, glycerol, and hydrochloric acid. Apparently salicylic acid can be used as a preservative. I presume you already have a source of tannic acid, and it can be made into gallic acid by a simple hydrolysis reaction, boiling it with HCl or any other strong acid.

The mixture of gallotannic acid and the ferrous salt does not visibly react (either indicating an unstable ferrous salt, or a soluble ferrous salt). Upon oxidation by air, the ferrous salt is converted to the ferric, and the ferric gallotannate makes the solution go black. The hydrochloric acid slows the reaction by reducing the amount of gallotannate in solution by the common ion effect, but over time, the solution will go black, but still appear grayish when freshly applied to a page. Gallic acid is not produced by this reaction. Gallic acid can be used, however, to alter the coloring a bit.

CHRIS25 - 4-11-2012 at 10:47

Quote: Originally posted by bbartlog

Pretty sure you can produce gallic acid from tannic. Check out 'Outlines of chemistry for the use of students' by William Gregory, p436:

'The precipitate formed by sulphuric acid in a hot solution [of tannic acid], dissolves in hot diluted sulphuric acid, and when this solution has been boiled for a short time, it contains no tannic acid, the whole having been converted into gallic acid.'

On the other hand Watts distinguishes between quercitannic acid (which supposedly can't be hydrolyzed to produce gallic acid) and gallotannic acid (which can), so maybe it depends on how you got your tannic acid. Tannic acid is not after all the name of a specific compound...

Hi, yes i just downloaded this book, nice, my Tannic acid is from a chemical company and is labeled for laboratory use. It has a formula of C76H52O46; but I note that the book refers to C19H5O9. I am new to tannic acid having only used it for toning cyanotypes. But its properties seem immensely interesting, and are a subject of ongoing scrutiny to modern science from what I have read so far. Anyway, what are the forms that you refer to? I read another document yesterday (but did not keep it) that said gallic acid could not be synthesized from tannic. So maybe I should just try what you say, problem is, I am not clever enough to know whether what I have made is indeed gallic acid.

Vargouille: Thankyou. "I presume you already have a source of tannic acid, and it can be made into gallic acid by a simple hydrolysis reaction, boiling it with HCl or any other strong acid". Now that I suppose is another method, but I have always read sulphuric acid when in proximity to the word tannic acid, never read hydrochloric acid in this relationship.

[Edited on 4-11-2012 by CHRIS25]

Just in the process of writing balanced equations and noticed something odd: Gallic acid has a g/mol of 1701.2 and Tannic has 170.2. No coincidence I am sure.

But the equation is H2SO4+C76H52O46 = C7H6O5+H2SO4 I presume there will be a huge amount of carbon dioxide and hydrogen gas given off by looking at the stoichemetry?

[Edited on 4-11-2012 by CHRIS25]

Vargouille - 4-11-2012 at 11:50

Sulfuric acid is generally better for long hydrolysis reactions of complex organics because it is much less volatile. It is, however, generally more expensive than hydrochloric acid.

And the reason for the differing usages of "C76H52O46" and "C19H5O9" is because tannic acid as sold commercially has a somewhat uncertain composition. The manufacturer is assuming that it is only, or is mostly, that form, and it's somewhat difficult to distinguish either way. In any case, gallic acid is much less soluble than tannic acid, so if a white, or yellowish-white, precipitate forms, you'll know that you made the gallic acid.

As for the hydrolysis, your equation isn't balanced. And you've got your molar masses switched, and the value is probably just a coincidence. Also, hydrolysis doesn't work like that. It breaks the ester linkage to form a hydroxyl group and a carboxylate group. The products here are an unknown number of gallic acid molecules and glucose, and maybe some quinic acid.

CHRIS25 - 5-11-2012 at 05:08

Hi Vargouille, interesting you say that it is not balanced. Actually two things here: 1. I agree, that is why I can see that there must be plenty of carbon dioxide and water evapourating. But also 2. I always use this site to double check my equations just in case I am wrong of course - www.gregthatcher.com/Chemistry/. And it is balanced on this calculation. now maybe it is not always right? But I find that to be a little difficult to believe. Hence when I checked the stoichemetry and saw that it was not balanced and that this site wrote it the same way I did, a little observation at the number of hydrogen and carbon and oxygen atoms that were floating around I simply put this down to the carbon dioxide and water that would be a by-product liberated in gaseous form of course. So am I to assume that my thinking (as non-chemist don't forget) is wrong?
===================================
Ok, it's official, you can not make gallic acid from Tannic acid,

I placed 28.5g tannic in 35mLs water and dissolved. Then added 20mLs concentrated H2SO4 and immediately got a white.brown precipitate that had the same consistency and colour and even texture as Cow dung after three hours in the sun. This solidified and unified mass I then washed and placed in another beaker and added 50% strength H2SO4. Brought this to the boil for three minutes until fully dissolved; and then allowed to cool outside in the 5 degree c outside temperature for two hours. What I ended up with was what can only be described as a solidified deep red/brown hardened toffee consistency at the bottom of the beaker was easily broken up with a plastic spoon, is quite hard and insoluble in ordinary warm water. Mmmm.............

[Edited on 5-11-2012 by CHRIS25]

Vargouille - 5-11-2012 at 13:39

I doubt that it would take only three minutes to achieve complete conversion of tannic acid to gallic acid. Also, it is noted that quercitannic acid "is precipitated by sulfuric acid in red flocks." This may indicate that your tannic acid is not of the gallotannic form, but rather of the quercitannic form, which is consistent with an "inability" to form gallic acid. I would, however, attempt it again both with a lower sulfuric acid concentration (around 10%) and for longer (around a half-hour to an hour). It should hydrolyze then to gallic acid. I find it somewhat unlikely that quercitannic acid is sufficiently different from tannic acid that hydrolysis would be impossible.

To respond to the previous half of the above post, yes, your thinking is wrong. In an aqueous solution with an ester (tannic acid) and a non-oxidizing strong acid (H2SO4), the ester will not be oxidized to carbon dioxide. Rather, the ester bond is broken, forming a carboxylic acid and an "alcohol". In this case, the carboxyl group of gallic acid is the carboxylic acid, and the hydroxyl groups present in gallic acid, glucose, and quinic acid are the "alcohols". This site goes into further detail on the specific mechanism of an acid-catalyzed ester hydrolysis. In any case, regarding the equation, assuming that the majority of the "tannic acid" is decagalloyl glucose, the equation is:

10H2O + C76H52O46 --(H⁺)--> 10C7H6O5 + C6H12O6

Of course, it is unlikely that the tannic acid is completely decagalloyl glucose, but it serves to show that water is consumed in the reaction rather than produced, and that no carbon dioxide is formed in the reaction. What the site does is adds up the number of each element on either side of the equation and applies coefficients until the numbers match. I do find it curious that you say that the site gave you that equation, when the equation it gave me was "H2SO4 + 0C76H52O46 --> 0C7H6O5 + H2SO4", meaning that tannic acid is not consumed and gallic acid is not produced, at least, not as in that equation. When I put CO2 and H2O in either side, the only way I get an equation that doesn't have negative coefficients is when I state that the reaction consumes CO2 and H2O. In short, it's a website. It's only as good as the information it's given.

EDIT: As even more proof that it's not perfect, I fed in this unbalanced equation: "NaOH + Ag = Ag2O + Na + H2". The answer it gave me was "2NaOH + 4Ag = 2Ag2O + 2Na + H2", not "Error: Sodium doesn't work like that".

[Edited on 5-11-2012 by Vargouille]

CHRIS25 - 6-11-2012 at 02:12

Hi Vargouille, I do not want to waste any more tannic acid unless I am sure. I remember reading this: It exhibits with ferric salts the same reactions as gallotannic acid. It differs however from the latter in not being convertible into gallic acid, and not yielding pyrogallic acid by dry distillation. It is precipitated by sulfuric acid in red flocks. Wikpedia here is referring to quercitannic acid, which I remember reading elsewhere. The reason I did the experiment was because I was reading in an old book online, http://chestofbooks.com/ the method that I used above came from this book. Unfortunatley no measurements given. I did not think that it would work but tried anyway, but all the modern day sources agree that it can not be done, only fro,as you say, the gallotannic form.

Sorry, forgot to add that the equation you typed in was the same as mine, yes I saw those '0's as well but I thought erroneously that it was a quirk of the software design, after all I was not aware of this nomenclature and this is the first time I have ever seen a zero in an equation.

[Edited on 6-11-2012 by CHRIS25]

Vargouille - 6-11-2012 at 03:25

"Oak bark also contains a phlobaphene, oak red, which is produced from quercitannic acid by hydrolysis..."

"Pure quercitannic acid yields no glucose on hydrolysis..."

The appearance of the red material would suggest the formation of this phlobaphene, oak red. This tendency, however, is more in line with a condensed tannin. I am curious as to the actual structure of quercitannic acid, as there is no simple explanation of it that I have found. If the problem is due to an odd form of dehydration leading to the formation of phlobaphene, perhaps a series of experiments are in order. Heat small amounts (one gram) in a test tube with water, a weak acid, like vinegar, and a dilute strong acid, like sulfuric, at varying temperatures. A data table can be used to compile all of the results. Its classification as a tannic acid implies that it can be hydrolyzed, but it may take some work to find the ideal conditions.

And, for future reference, don't use more than five grams of a reagent if you aren't confident in a desirable outcome of a reaction. Even better, use only one or two grams, and scale-up as needed.

[Edited on 6-11-2012 by Vargouille]

CHRIS25 - 6-11-2012 at 04:57

Ok, after the toning that I am now doing, I will investigate this because it actually looks interesting. I will do as you suggest - will take a couple of days before I get round to it. But I like the idea. On a side note - I noticed that while a pain to dissolve the solubility of Tannic acid is 2850g per litre. So I began originally with 14.25g in 5mLs. It dissolved but left me with a really toffee gooey globular mass that was difficult to handle. So I added another 30mLs of water to make it managable. Just to be sure, do you think the dilution would be ok, or keep it at its max solubility before I start the tests?

Alternate source of Tannins

ElizabethGreene - 6-11-2012 at 07:46

This is kind of a left turn from the original thread, but...

If you want to save your lab grade tannic acid, you can put roughly crushed acorns in boiling water for ~10 minutes and filter out the solids. The dark brown solution contains the tannic acid. It can be boiled or evaporated to increase concentration. The acorns mash can be leached several more times, until the resulting liquid stops turning brown. When it stops turning brown, the acorn meat can be roasted and eaten. The shells can be dried and used as a tumbling medium.

Trivia: an oak gall is a knob-like protrusion that results from an injury.

What are you going to use instead of Gum Arabic?

-ellie

CHRIS25 - 6-11-2012 at 08:16

Hi Ellie, I read about the acorns and their tannins and so forth, but from what I gather the gallic acid that exists in them is complexed with other tannins and goodness knows what other chemicals that I have never heard of. So I give this a miss. On the subject of Gum Arabic, I am not making ink, apparantly one can easily make the ink this way, that is how it has been made since 800 AD or so - iron gall ink. And one product is Gallic acid if you continue to work on the ink somehow which I never bothered reading fully because it appears beyond my knowledge.

ElizabethGreene - 6-11-2012 at 08:22

Chris25: "I never bothered reading fully because it appears beyond my knowledge."

Lol, that's my approach to organic chemistry in general. Place fingers in ears and say LALALALALA until someone changes the subject.

Vargouille - 6-11-2012 at 11:19

Diluting the tannic acid solution is fine as long as you keep the dilution constant for all trials, because that might affect the outcome.

On the subject of the ink, gallic acid is not a product, it is a reactant. The product that causes the black color is iron (III) gallate when starting from gallic acid, iron (III) gallotannate when starting from gallotannic acid, and iron (III) quercitannate when starting from quercitannic acid.