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In the course of performing certain syntheses, it becomes necessary to acquire oxalic acid from OTC sources. While the easiest way to do this is no doubt buying it pure as wood bleach, on occasion extraction is required, either from the slurry of digested paper and mixed alkalis/acids resulting from a starch oxidation, or a cleaning product containing fine grit, soap, and other garbage. Described here is a streamlined method by which pure oxalic acid may be easily prepared from any of these sources, with a minimum of equipment and no organic solvents.
This route takes advantage of the very large change in solubility between oxalic acid's sodium and calcium salts to recover it from other garbage it may be mixed in with. The acid-containing raw material is first basified with soda or lye to produce sodium oxalate, stirred well and let to settle, then decanted through a coffee filter to remove any residual particles. A strong solution of calcium chloride is then added, precipitating calcium oxalate, and the supernatant discarded. The precipitate is then washed and dissolved with dilute sulfuric acid, and the precipitated calcium sulfate discarded. Finally, the mixture is boiled down until crystals start to form, and then chilled, whereupon the mother lode of pure oxalic acid crystals will fall out and can be collected.
- Starting with a can of Zud
Assuming the zud is all oxalic acid (which it surely is not), and that you have a 175 g can, there would be 1.3 moles of oxalic acid in there (it is a dihydrate), so add 3 moles (252 g) of baking soda or lye (120 g), or 1.5 moles (429 g) of washing soda to the zud in 1 liter of water. Shake thoroughly in a jug, and pour into a decanter. I used a dollar store glass decanter, works like magic. Let sit for an hour or so, at which point all the little particles of abrasive will more or less have settled. Then, very gently, decant off the liquid through a coffee filter into a jug (or coffee pot). Refill the decanter with 1 liter water and swirl it around some more, and when it settles, decant the new water through the filter into the jug also. 160 g anhydrous calcium chloride (Damp-rid) is mixed with sufficient water to dissolve it, and then poured into the jug. The decanter emptied into a waste container and washed out. The jug is then shaken well and poured into the decanter, and the mixture let to settle. The water is then decanted off, and 1 liter of fresh water poured into the decanter, stirred, let settle, and discarded. This wash cycle is repeated till the water looks nice and clear after the particles settle. Then, instead of discarding the water, 160 ml sulfuric acid is carefully added. The mixture is decanted through filter into a coated pot, and boiled until crystals start forming. It is then cooled to room temperature and poured through a coffee filter to collect the crystals. The crystals are washed with cold water and set them to dry, and the pot placed in the freezer to form a second load, which is processed in the same manner as the first. The water can then be discarded, and all equipment cleaned. After the oxalic acid has dried, it can be weighed, and the amount of base and acid used for extraction adjusted for minimum waste.
- Starting from alkali fusion mass
All of the oxalic acid is already in a salt form, so the basification step is unnecessary. The fusion mass may contain valuable formate salt, which is worth a try to recover also, so it shouldn't be disposed of immediately. Otherwise, use the above procedure.
- Starting from nitric acid oxidation mass
When basifying, make sure to take into account the amount of acid added to the mass, and adjust the alkali added accordingly. Be careful.
(COOH)2 + 2 NaHCO3 → Na2(COO)2 + 2 CO2 + 2 H2O (Soluble)
Na2(COO)2 + CaCl2 → Ca(COO)2 + 2 NaCl (Insoluble)
Ca(COO)2 + 2 H2SO4 → Ca(HSO4)2 + (COOH)2 (Soluble)
Ca(COO)2 + H2SO4 → CaSO4 + (COOH)2
Calcium chloride is the ideal calcium compound to use for this purpose, as most other calcium salts are either insoluble, or only sparingly soluble, thus being difficult to separate from the desired oxalic acid precipitate if added in any excess. CaCl2 is very soluble, so an excess may be added without fear of contamination of the final product.
Note: For all who are interested, "Zud" contains approximately 50% oxalic acid. Not bad.