DraconicAcid
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Destroying Oxalate
I regularly do gravimetric analysis on copper by precipitating copper(II) anthranilate. Oxalate and some other organic anions interfere, so I thought
I could oxidize the crap out of them by heating the sample with concentrated nitric acid before doing the analysis (because I have vague memories of
analyzing an iron oxalate complex decades ago by destroying the oxalate with concentrated acid).
Boiling nitric acid does not destroy oxalate ion.
Any suggestions?
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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j_sum1
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Potassium permanganate destroys oxalate.
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Boffis
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Would calcium interfere with your procedure? If not add calcium chloride/ acetate/ nitrate solution in small quantities at a time until no further ppt
appears, heat and cool to make the ppt filterable and filter or centrifuge off the calcium oxalate. You may need to check and adjust the pH to ensure
complete precipitation of the oxalate. Prolonged boiling with gypsum serves the same function. This will only remove a small number of organic acids
though.
Alternatively there is the oxidation route suggested by J_sum1. This requires >60 C in 5N sulphuric with K permanganate, I am not sure about other
oxidisers. The easiest oxidation method would be to dry and calcine the residue in air and then re-dissolve in a little acid +/- H2O2 depending on
which acid you use.
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RU_KLO
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I used calcium hydroxide to remove oxalate from chromium oxalate. (to get Chromium hydroxide)
This was the best method I found.
Tried oxidation, stronger acids, but the oxalate is difficult to remove (probably because its strong reducing properties or complexation with metals.)
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unionised
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Hot conc sulphuric acid will decompose it.
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chloric1
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What if want to reclaim oxalic acid as the dihydrate from calcium oxalate? Could concentrated hydrochloric dissolve the calcium and deposit oxalic
acid on dilution and cooling?
Fellow molecular manipulator
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woelen
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Peroxodisulfate with a tiny amount of silver ions as catalyst also oxidizes oxalate. The silver, however, may interfere with your analysis.
Maybe the oxalate is destroyed without the catalyst. Peroxodisulfate is a strong, but sluggish, oxidizer without catalyst. You could give it a try.
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Bedlasky
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Another option is MnO2. You can easily filter excess.
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RU_KLO
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Quote: Originally posted by chloric1 ![](images/xpblue/lastpost.gif) | What if want to reclaim oxalic acid as the dihydrate from calcium oxalate? Could concentrated hydrochloric dissolve the calcium and deposit oxalic
acid on dilution and cooling? |
I think, no. because calcium oxalate is less soluble that calcium chloride,
Maybe is the other way around, add some oxalic acid or sodium oxalate to calcium chloride, -> a precipitate of calcium oxalate.
I dont know if stronger acid takes precedence over the solubility. (I think not, because with decreasing solubility you remove the element from the
equation)
Maybe if calcium oxalate is soluble in concentrated HCl and it has more solubility than calcium chloride in concentrated HCl - because of common ion,
you can get oxalic acid, but you need to remove all calcium.
I think that the only way, is to calcinate (or destroy by heat) the calcium oxalate, to recover a carbonate or oxide (if you want to recover some
usefull reagent) . but oxalic acid, no.
edit: found new information:
https://bioresources.cnr.ncsu.edu/wp-content/uploads/2016/06...
"one explanation could be that calcium oxalate dissolves easily in HCl".
there is a study of HCl concentration in dissolution of oxalate.
Recovery of oxalic acid from hydrochloric acid and cobalt containing hydrometallurgical side-stream by solvent extraction
https://www.sciencedirect.com/science/article/pii/S089268752...
So, after this information, maybe you could extract oxalic acid from calcium oxalate with HCl.....
Also in the last reference, are methods of recovering oxalic acid, that could be used:
""In the literature available regarding the oxalic acid recovery from the solutions, various methods such as oxidation (Andreozzi et al., 1996), lime
neutralization (Haselhuhn and Kind, 2003), crystallization (Liu et al., 2019), ion exchange (Krisilova et al., 2014, Patil and Ganorkar, 2007, Verma
et al., 2022), and solvent extraction (Barnes et al., 1999, Hasret et al., 2019, Kirsch et al., 1997, Kostikova and Mal’tseva, 2020, Kyuchoukov et
al., 2008, Lebedev, 2008, Qiu et al., 2010, Tian et al., 2010, Qin et al., 2001) has been studied. Lime neutralization is a classical technique for
addressing acid wastewater and recovering carboxylic acid from aqueous solutions (Zhou et al., 2023).
[Edited on 13-1-2025 by RU_KLO]
[Edited on 13-1-2025 by RU_KLO]
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Precipitates
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Dissolve the calcium oxalate (CaOx) in excess HCl - it dissolves well at a low pH
Filter to remove unreacted CaOx, and dry the filtrate to a powder*
Dissolve the powder in ethyl acetate - oxalic acid will dissolve, calcium chloride will not (1) - filter to remove.
Remove ethyl acetate to recover oxalic acid.
1. Chemister solubility database
*A simple recrystallisation in water may be effective, given oxalic acid is less soluble than calcium chloride, although there is likely to be some
calcium contamination.
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clearly_not_atara
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Copper ions catalyze the reaction of oxalic acid with peroxydisulfate. Since copper sulfate is highly soluble, excess persulfate probably won't
interfere with your gravimetry.
https://pubs.acs.org/doi/pdf/10.1021/ja01152a012
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DraconicAcid
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That's an excellent suggestion, as long as xs peroxydisulphate doesn't mess up the anthranilate (hydrogen peroxide turns it black).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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RU_KLO
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I tried to remove oxalate from chromium oxalate with persulfate.
So based in the last post:
1) in 25 ml 1 Mol H2SO4, dissolved 1,36gr of chromium oxalate (impure - violet needle cristals, with white salt - probably oxalic acid, from a
cristallization). added 3 drops CuSO4 solution (unknown concentration)
2) then added first the same amount by weight of potassium persulfate (it was not anhydrous), because to start somewhere, but then when I thought
about the reaction added 2,6 grams more (a total of 4 grs persulfate).
This is what I thought:
3 K2S2O8 + 2 CrC2O4 -> Cr2(SO4)3 + 4 CO2 + 3 K2SO4
then I started heating.
The solution was dark violet, and when it reached 70C some bubbles where produced (not much).
at aprox 50 min - the solution color was shifted to a dark green.
at aprox 90 min, the solution became somewhat golden. (or coca cola color) with a green tint. (picture attached)
As I dont have proper temperature control, when the solution reached 85° some mild SO2 or SO3 odour was smelled. (it was more like a mild bleach
odour than pungent SO2) there was no chlorine used in the reaction, so....
Reactions starts at 70C, and with chromium, its bery difficult to stop at Cr(III). I think the golden color is a mixture of Cr(III) and Cr(VI).
![cr.jpg - 62kB](http://www.sciencemadness.org/talk/files.php?pid=698668&aid=104758)
[Edited on 26-1-2025 by RU_KLO]
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