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Author: Subject: Extraction of gallic acid
Felab
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[*] posted on 14-11-2018 at 01:53
Extraction of gallic acid


I want to make some calotype negatives and I have all the components I need except for gallic acid. Researching in the internet I found that chestnuts are the food with the highest content in gallic acid, with a content of aprox 0,5 g/100g of chesnuts. that means that with one or two kilos of chestnuts I can have all the galic acid I need. I want to extract it by the folowing procedure:
1. First, crush the chesnuts into a powder or a paste.
2. Add water with low calcium and iron content (I think 3l is enough) and boil in a galvanized steel bucket for a few minutes.
3. Decant of the gallic acid contanining water and repeat two or three times for maximum yield.
4. Add acetic acid to hydrolise the gallotanins to gallic acid and sugar.
5. Precipitate gallic acid by the adition of calcium carbonate, and then boil the solution for a few minutes, generating insoluble calcium gallate.
6. Decant the solution and add sulfuric acid to the precipitate until the ph is around 4.
7. Add water and remove calcium sulphate by filtration, geting gallic acid in solution.
8. Dry, and store properly to prevent oxygen from degrading my gallic acid.
Is there someone who has tried something similar? Is the gallic acid in free state in the chesnuts, or is it trapped in the form of gallotanins? Is there a more straight foward method of extracting gallic acid? Can tannic acid be hydrolised to gallic acid? How can I remove off the egallic acid contaminant that is going to be present in the final product? Should I develop my calotypes with tea instead of doing such a procedure?
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[*] posted on 14-11-2018 at 02:13


I've thought about doing that. My plan was to extract tannins from maple leaves.

I am not sure that boiling is a good idea. Gallic acid is easily oxidized or decarboxylated.

Is acetic acid strong enough to hydrolyze tannins? Chestnuts likely contain some free gallic acid, but a lot of it would be locked up in tannins. Acorns are high in tannins. The exact variety of chestnut matters quite a bit too... sweet chestnuts would not contain as much gallic acid as horse chestnuts.

Gallic acid and tannins are easy to purchase. There are several papers on extracting/purifying gallic acid. These papers contain clear and detailed procedures.




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Felab
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[*] posted on 14-11-2018 at 02:49


Quote:


I've thought about doing that. My plan was to extract tannins from maple leaves.

I am not sure that boiling is a good idea. Gallic acid is easily oxidized or decarboxylated.

Is acetic acid strong enough to hydrolyze tannins? Chestnuts likely contain some free gallic acid, but a lot of it would be locked up in tannins. Acorns are high in tannins. The exact variety of chestnut matters quite a bit too... sweet chestnuts would not contain as much gallic acid as horse chestnuts.

Gallic acid and tannins are easy to purchase. There are several papers on extracting/purifying gallic acid. These papers contain clear and detailed procedures.

From the information I found, the classic procedure of extracting tannins involves boiling it. I think acetic acid is the perfect acid since it is relatively strong, non oxidising and easy to separate. Maybe hydrochloric bould be better though.
I live in the most arid region of Europe so to find acorns I need to travel several hundred kilometers. For the same reason I coudn´t dind gallic acid at a reasonable price. I can buy tannic acid very cheaply though so that could be an option. All the papers I found where not free acess so I need to develop my own procedure.
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[*] posted on 14-11-2018 at 16:50


Quote: Originally posted by Felab  


All the papers I found where not free acess so I need to develop my own procedure.


Did you check Sci Hub?
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[*] posted on 14-11-2018 at 17:36


If there is a particular paper you are looking for, someone can probably find a copy for you.



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ahill
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[*] posted on 14-11-2018 at 19:12


I tried a few times to hydrolise tannic acid to gallic acid - I kept getting stuck with a tarry mass that would not filter. (looking for the procedure I tried to follow now - as far as I remember, it was a base hydrolysis using NaOH).

In the end I just used tannic acid. (I was tinting cyanotypes). It (tannic acid) didnt seem to do a better or worser job than green or black tea.

Ah - found the procedure here :-
(the one I tried to follow was garage chemists')
https://www.sciencemadness.org/whisper/viewthread.php?tid=20...

..and now I read thru it - I remember that I thought oxygen was my problem - I remember stuffing around with plastic wrap ! I've got a good source of argon now - perhaps I should revisit it.
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[*] posted on 14-11-2018 at 22:25


Acetic acid will not hydrolyse complex tannins, you need a mineral acid, dilute hydrochloric or sulphuric acid but acid hydrolysis also cause much formation of insoluble (presumably polymerised) tannins. My experience is that caustic alkalis give better, faster yields and I have reported on this procedure in several previous threads. Depending where you live in Europe, oak tree galls are amongst the best source of soluble tannins, particularly the large Mediterranean galls that grow on cork oaks. I collected a bag full in central Italy two years ago.

As you can buy tannic acid I would go down this route as its often the cheapest but be aware that there are several types of tannin and purchased tannic acids can be either catchetutannin or gallotannins, only the latter yield gallic acid. Check out the last post on this thread:

http://www.sciencemadness.org/talk/viewthread.php?tid=20487#...

I explain how to type test bought tannins.

I recently purchased some tannic acid from a Chinese ebay seller and the quality was excellent.
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Felab
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[*] posted on 15-11-2018 at 01:15


Quote: Originally posted by ahill  
I tried a few times to hydrolise tannic acid to gallic acid - I kept getting stuck with a tarry mass that would not filter. (looking for the procedure I tried to follow now - as far as I remember, it was a base hydrolysis using NaOH).

In the end I just used tannic acid. (I was tinting cyanotypes). It (tannic acid) didnt seem to do a better or worser job than green or black tea.

Ah - found the procedure here :-
(the one I tried to follow was garage chemists')
https://www.sciencemadness.org/whisper/viewthread.php?tid=20...

..and now I read thru it - I remember that I thought oxygen was my problem - I remember stuffing around with plastic wrap ! I've got a good source of argon now - perhaps I should revisit it.


I´ve looked up this procedure and I will try it out in the future.

For tonning cyanotypes, you can even use coffe. The type of tannin you have doesn´t matter very much.

But for making calotypes, gallic acid is desired since it doesn´t stain the negative like tannic acid would, and it doesn´t react immediatly with silver nitrate, wich is very important to get a good density.

The only argon I can easily get is the one present in lightbulbs so I will just asume that the reacction will give a lower yield.
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[*] posted on 15-11-2018 at 20:24


Quote: Originally posted by Boffis  
dilute hydrochloric or sulphuric acid but acid hydrolysis also cause much formation of insoluble (presumably polymerised) tannins.


If dark precipitate comes out of a clear tannic solution, it's not the gallotannin, unless maybe you poured in 96% acid...the acid solutions are much less sensitive to air than alkaline, so that's not it. Addition to strong tannic acid solution does precipitate the gallotannin but this is good...decant, wash...heat w/dil. acid for a bit and cool for...a few days maybe. Sulfuric acid in water or methanol (methyl gallate) is used analytically on gallotannins, so one might think it's not such a bad method.

PS Before this came up I already planned to extract sumac over the holiday, if the experiment goes well it will be posted in the relevant thread. Sumac is the only reliable local plant source of pure gallotannin here.

[Edited on 16-11-2018 by S.C. Wack]




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Felab
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[*] posted on 18-11-2018 at 08:51


I want to buy some tannic acid to do experiments, but the page that sells it offers two types: water and alcohol soluble and water soluble. Wich one sould I choose to get gallic acid? I coudn´t find if gallotanic acid is soluble in alcohol.
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[*] posted on 18-11-2018 at 11:36


It's impossible to say without knowing the source or how it was extracted. Transesterification is even a possibility. I note ebay is full of Majuphal whole, powdered, or extracted with ???. This traditional source does not have as much free or easily lost gallic acid compared to a sumac gall or leaf source. Stahl and Schild (Isolierung und Charakterisierung von Naturstoffen) extract it with methanol in a Soxhlet and end up with 35-45% yield of tannin. In the old days they used ether with a little ethanol in it, so strong polarity is not required for this source. Other sources can be more stubborn and of course tannin can be lost just by bad treatment of the material.

I've bought beige "USP tannic acid" that most definitely did not conform to USP spec.




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[*] posted on 18-11-2018 at 13:17


I´ve seen that the shiping is 4 times more expensive than the tannin itself so I have changed plans. I will try several possible sources in a small scale such as chesnuts, pomegranate, wood, oak bark and whatever I find that can contain tannins. I will see which plant gives the best result and yields the most gallic acid.

I have also found an old 19th century book wich gives another pathway on the syntesis of gallic acid:http://albumen.conservation-us.org/library/monographs/sunbeam/chap14.html#heading51

If you have some idea of what plant I could use please tell me.
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[*] posted on 18-11-2018 at 14:37


Chestnuts have a high enough starch content that step 2 will make porridge.
I doubt decantation will be an option.
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[*] posted on 20-11-2018 at 11:28


Quote: Originally posted by unionised  
Chestnuts have a high enough starch content that step 2 will make porridge.
I doubt decantation will be an option.

I have tried pomegranates and they seem very promising. I am yet purifying the tannic acid but it definitely makes some ferric sulphate turn blue and it tastes very astringent.
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[*] posted on 25-12-2018 at 19:43


70% acetone (or MIBK) has been recommended for a long time for leaf material, and crushed air-dried sumac leaves were 57% soluble in it. After removing the acetone, a single wash with DCM (or chloroform) removes all the green acetone-solubles. What was really surprising was powdered nutgalls were 80% soluble, and the wash is unnecessary. BTW there is lit showing great variation in extractable tannin depending on how the material was treated before extraction.



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[*] posted on 5-1-2019 at 17:54


One may question involving organic solvent at all, but concentration of the extraction solution is generally desirable and acetone is much easier to remove with a minimum of heat. My sumac extract is still in the desiccator but some experiments have been done on the ease of acid hydrolysis of extract of alleged Q. infectoria galls.

Sulfuric acid hydrolysis is in the old books as the method of Liebig. One recipe calls for boiling 10% tannin in 2.5M acid for 20 min. and setting aside for a week. At the other end is the HPLC method calling for boiling M acid for 24 hrs. At 80-85C a 10% solution still forms a precipitate with aq. gelatin. At 105-110C with no air there is no precipitate. The color is the same tea-coffee. On cooling, there is a light brown fine but filterable mud of gallic acid, and a smaller amount of black glassy thin flakes on the walls of the digester. The flakes may contain condensed gallic acid but my guess is no. Gallic acid that escapes purification can be extracted out with ethyl acetate (substantial neutralization may be wise with ester solvent)

NaOH hydrolysis may be preferable industrially but I wonder if that involves considerations such as acid resistant construction, since one wouldn't want dissolved iron.




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