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Author: Subject: Oxalic Acid from Rhubarb
syntheticvibes
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[*] posted on 11-7-2021 at 17:28
Oxalic Acid from Rhubarb


Hello,

Lots of questions here. I know this topic isn't necessarily new to this forum, but I need some specific guidance based on some concerns I have about my procedure. I had a ripoff of a lab experience in orgo because of COVID, so there seems to be a disjunct between my theoretical and my practical knowledge.

I have already dried about 8 leaves of the rhubarb plant and crushed them. I plan on doing a hot Soxhlet extraction. I initially planned on using water, but I am also considering ethanol (oxalic is nearly perfectly soluble in water and still very good–237 g/L–in ethanol).

My main concern at this stage is that there will also be present a number of organoacids, such as citric acid and malic acid (these are the three main contributors, including oxalic). They all express very similar solubility and could pose a problem to my understanding.

My plan was to try to start with some ideas given here and try to either use calcium chloride or sodium bicarbonate to neutralize my retrieved solution. This would leave me with the following:

If neutralized with sodium bicarb:
Sodium oxalate: sparingly soluble (3.7 g/100mL @ 20 ºC) in water, insoluble in ethanol
Sodium malate: freely soluble in water, no ethanol data
Sodium citrate: sparingly soluble in water (29.4 g/L), insoluble in alcohols

If neutralized with calcium chloride:
Calcium oxalate: nearly insoluble (.67mg/L) in water, conflicting information about ethanol solubility
Calcium malate: slightly soluble in water, this page says it's soluble in ethanol

My first question is this: is either solvent a viable choice for salting out oxalic acid (it's okay if there's another acid or two present), or leaving oxalic acid in solution and salting out the other two? Or is there a better way?

After salting out, I planned to convert back to the main carboxylic acids using hydrochloric acid, evaporate all the liquid, and then use oxalic acid's tendency to sublime to my advantage and collect using a RBF full of cold water over a beaker on a heating mantle. Could this be carried out before reacting with either sodium bicarb or calcium chloride? That would be significantly more simple if so.

Also, one other question, when reacting calcium chloride with an acid, is there a visual reaction indicator such as bubbling? I need to work with something with an indication of a reaction proceeding because I will be figuring out the molar amount of acid present by how much bicarb or cacl2 is used to neutralize. I'll need that information when I re-acidfy the solution, and I'm not set up for titrations at home.

And yes, I know oxalic acid is dirt cheap, but I'm more doing this to improve upon my, as previously mentioned, lackluster orgo lab skills due to COVID.




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[*] posted on 14-7-2021 at 00:19


I forgot to mention:

Calcium citrate solubility: soluble in water, insoluble in alcohol




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[*] posted on 15-7-2021 at 18:42


I like your sublimation idea, but you might have issues with the oxalic acid decomposing at 189C: see https://www.sigmaaldrich.com/US/en/product/mm/816144. I don't know at which temperature oxalic acid supposedly sublimates.

Seems like mixing with CaCl2 in water is the way to go. BTW, that is not an acid-base reaction, the ions just float around and drop out of solution when they meet (ionization equilibrium gets driven to one side as the product of calcium oxalate leaves the solution). Then you can filter the calcium oxalate precipitate, rinse the precipitate with cold water, and convert to oxalic acid. You can dry the calcium oxalate precipitate and weigh it to get an idea of how much HCl to add.

Please post some pics of your experiment and let us know how it goes!
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[*] posted on 16-7-2021 at 11:05


Thanks for your response bob800.

I'm curious to hear if you think the calcium citrate and calcium malate would behave the same way. If they won't great--that solves my problem. If they will, how will I separate them?




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[*] posted on 16-7-2021 at 15:23


I would recommend buying a copy of the CRC Handbook of Chemistry and Physics for detailed solubility data.

According to my copy, calcium DL-malate solubility in water is 0.321 g/100mL at 0 C, or 0.451 g/100mL at 37.5C. Calcium oxalate hydrate is totally insoluble in water.

So, a large quantity of warm water should be sufficient for dissolving any calcium malate while preserving the oxalate precipitate, which you can separate by filtering. Make sure to rinse the precipitate with water afterwards.
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[*] posted on 17-7-2021 at 16:02


That’s a very helpful resource, thank you for your information. This should solidify my procedure, and as far as I can tell, will provide a complete and successful procedure. I’ll probably share it here when I’m done because I don’t see many really valid procedures for this sort of extraction for a chemist in a lab.



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[*] posted on 17-7-2021 at 16:40


Quote: Originally posted by bob800  
CRC Handbook of Chemistry and Physics



https://tinyurl.com/d5mfbtw9


/CJ




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[*] posted on 17-7-2021 at 20:42


Even better! Thanks! I’ll probably get a hard copy of an 8x-9x-th edition, but until then the PDF is great help!



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[*] posted on 18-7-2021 at 01:54


Sodium citrate is very soluble in water.

You might want to consider precipitating oxalate as the magnesium salt.
(The citrates and malates of magnesium are soluble in water; the oxalate is not.)


I take it you know this is an interesting, but inefficient, way of obtaining the acid.

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[*] posted on 18-7-2021 at 07:42


Quote: Originally posted by unionised  


I take it you know this is an interesting, but inefficient, way of obtaining the acid.



Yes I do.

I’ll look into the magnesium option, but it sounds like the calcium salts behave the same way. Plus I don’t have the material right now to do it that way.




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[*] posted on 18-7-2021 at 08:40


Hydrated magnesium sulphate (Epsom salts) is easy to buy.
Calcium citrate is only sparingly soluble in water
Ditto the tartrate and malate.

Funny thing is, if they they were the same as the calcium salts, I wouldn't have suggested using them.
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[*] posted on 18-7-2021 at 21:39


Ah I forgot about Epsom salts. We have some at home. I’ll take it into advisement.



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