By Allan Robinson, updated Aug 30, 2022
Oxalic acid (H₂C₂O₄) is a moderate organic acid that frequently serves as a reducing agent in organic synthesis. It can be synthesized in a laboratory using only sucrose and concentrated nitric acid, with a small quantity of vanadium pentoxide acting as a catalyst to accelerate the reaction.
Place the sugar in a flat-bottomed flask and add the nitric acid. Heat the flask in a boiling water bath. The sugar dissolves in a vigorous reaction that generates a large volume of nitric acid fumes.
Remove the flask from the water bath as soon as the reaction begins to produce fumes, and set it on a non-conductive surface. After the reaction subsides—typically about 15 minutes—pour the still-hot solution into an evaporating basin.
Evaporate the solution with gentle heat from a Bunsen burner for roughly 15 minutes until it reduces to about 20 mL. Add approximately 40 mL of water, then evaporate again to about 20 mL. Cool the solution thoroughly in an ice-water bath.
Allow the rapidly forming crystals of oxalic acid to complete crystallization in roughly 10 minutes. Filter the remaining solution through filter paper, then add the crystals to a small amount of hot water. Recrystallize the oxalic acid, which should take about 20 minutes.
Dry the crystals by pressing them between pads of drying paper or placing them in a desiccator. Avoid using a conventional oven, as this may cause the oxalic acid to decrystallize. The procedure typically yields about 7 g of oxalic acid.
This method generates a substantial amount of nitric acid fumes and must be performed under a fume hood with adequate ventilation. Personal protective equipment, including goggles and lab coat, is mandatory.
