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 $\pu{4,000 ppm}$ 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 $\ce{V2O5}$ has a fairly low solubility at ~$\pu{10^{-1} g}/\pu{100ml}$ at $\pu{25 ^\circ C}$, 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 ~$\pu{22 g}/\pu{100ml}$ at $\pu{20 ^\circ C}$, and that of vanadium oxalate complexes of ~$\pu{14.5 g}/\pu{100ml}$ at $\pu{25 ^\circ C}$, 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 $\ce{CO2 + CO + H2O}$ (in the ~$\pu{103-185 ^\circ C}$ 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 $\ce{CO2}$ atmosphere, maybe under a slightly increased pressure. Would that significantly decrease the rate of its decomposition or simply complicate things without significant advantage?