Discuss, how to purify the given crude oxalic acid and recrystallize it.
Answers
I am set out to purify a batch of commercial oxalic acid with the aim in mind to end up with a highly pure product (starting from 4,000 ppm total impurities and doing so with basic lab equipment and no advanced analytical capacity).
Looking at the distribution of solubility vs. temperature in aqueous medium (which - despite the widely discrepancy in data from different sources - seems to generally adopt a somewhat exponential shape), I was thinking that the best may be to carry out dissolution/filtration at high temperature, followed by fractional recrystallization.
A few questions remain though:
I have no idea how I could discriminate against Vanadium impurities with the basic equipment I have at my disposal.
According to the data I've found V2O5 has a fairly low solubility at ~10−1 g/100 ml at 25 ∘C, a level which makes it much too soluble for proper filtration, and too little for the recystalization to efficiently remove it.
I found data for the solubility of Vanadyl Oxalate to be of ~22 g/100 ml at 20 ∘C, and that of vanadium oxalate complexes of ~14.5 g/100 ml at 25 ∘C, which makes it very close to the solubility of oxalic acid itself (though, unlike for oxalic acid, I don't have data for how the solubility of Vanadium evolves with temperature).
I had the notion that slowly recrystalizing the oxalic acid would yield better results, but I found an old paper* on its purification which found/states exactly the opposite, for a reason I don't very well understand. If someone had enough understanding to explain to me why a faster recrystalization would yield better results and/or to confirm/refute the paper's statement on the subject, I would greatly appreciate.
I read upon a preliminary search here on Chemistry Stack Exchange that upon excessive heating, oxalic acid decomposes into CO2+CO+H2O (in the ~103−185 ∘C range), thus I was wondering if it would be worth it to attempt to further exploit the exponential shape of its water solubility vs temperature by carrying out the dissolution/filtration in a CO2 atmosphere, maybe under a slightly increased pressure. Would that significantly decrease the rate of its decomposition or simply complicate things without significant advantage?
Oxalic acid dihydrate recrystallization
If I dissolve oxalic acid dihydrate in water until supersaturation, will it recrystallize to dihydrate or anhydrous form? I'm quite confused of their differences other than the presence of water in the solid structure. are they mutually exclusive? Will a different process be needed to dehydrate the dihydrate?
Recrystallizing any water-soluble substance from water (or a solvent mixture containing water) will usually give you the hydrated crystal form. If the crude oxalic acid you have is already anhydrous, you could recrystallize it from ethanol instead, provided that your source of ethanol doesn't have any water in it.
If your crude oxalic acid is the hydrated form, there are several ways to dehydrate it, depending on whether you want to recrystallize it at the same time. One way would be to dissolve it in ethanol and stir the solution with some activated molecular sieves, which would selectively absorb the water. Then you just decant or filter the solution away from the sieves, concentrate it by boiling off (or distilling off) most of the ethanol, then crystallizing.
Another way of removing the water would be to heat the crude oxalic acid to melting point. This alone can tell you whether you have the dihydrate or the anhydrous form. Hydrated oxalic acid melts at about 100 o C, whereas anhydrous oxalic acid melts closer to 200 o C. So if it melts easily at first, then resolidifies (as the heat drives the water off), then melts again when you raise the temperature further, then you had the hydrate to begin with.
Be careful not to overheat oxalic acid, since it can decompose into water vapour, carbon dioxide and carbon monoxide. So don't go too far above its melting point. Heating it under dynamic vacuum can greatly accelerate the drying process. Anhydrous oxalic acid is hygroscopic, so you should keep it in an airtight container to stop it absorbing water vapour from the air. If you need to recrystallize it once it has been dried, then again use anhydrous ethanol (or methanol) as the solvent.
Answer:
The correct answer to the question -"Discuss, how to purify the given crude oxalic acid and recrystallize it." is-
Explanation:
Purifying crude oxalic acid and recrystallize it
- It can be done by first dissolving it in ethanol
- then stiring the solution with some activated molecular sieves,
- the molecular sieves which selectively absorb water.
- Then the solution should be decanted or filtered from the sieves,
- after that it is to be concentrated by boiling off
- the ethanol will then crystallize.
The other method can be-
- First the oxalic acid solution is heated in adsorption tanks,
- then barium carbonate and polyacrylamide precipitates are to be added
- ,after which the coacervation reaction process,
- inorder to recrystallize it, first oxalate salts are to be converted to calcium oxalate
- as we know that, calcium oxalate is insoluble in water,,
- thenit is to be dissolved in dilute HCl to get oxalic acid back and it recrystallize.
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