Structure & Acidity

• With a molecular formula of C2O4H2, oxalic acid consists of two carboxylic acid groups directly joined to each other, where a carboxylic acid is a -COOH group; the overall structure could be depicted as HOOC-COOH. Since the two carboxylic acids are directly joined, oxalic acid is one of the strongest organic acids --- much stronger than other carboxylic acids like citric acid that lack this feature. The strength of an acid is often measured in terms of its pKa, where a lower number indicates a stronger acid. Since oxalic acid can donate two hydrogen ions to water, it has two pKa values, pKa1 and pKa2. These are 1.187 and 4.215, respectively.
  
  

Physical Properties

• In pure form, oxalic acid is a white, crystalline substance. For every molecule of oxalic acid, two more molecules of water are present in the crystal lattice. Solid oxalic acid melts at 101-102 degrees Celsius and boils at 149-160 degrees Celsius. It is 1.6 - 1.7 times as dense as water at room temperature and highly water-soluble; roughly 1 gram can dissolve in 7 mL of water.
  
  

Chemical Properties

• Oxalic acid in solution is highly corrosive. The oxalate ions, when divested of their hydrogen ions, can act as what are called bidentate ligands, meaning they bind to a metal ion in two places, so they can form complexes with metal ions. Many of the salts they form with metals are insoluble. Kidney stones, for example, are often composed of calcium oxalate. Oxalic acid can react violently with strong oxidizing agents and silver compounds, so it should not be used in conjunction with these chemicals.
  
  

Biological Effects

• Some plants and vegetables contain small quantities of oxalic acid, typically in the form of an oxalate salt; parsley and spinach are two examples. Generally this is not harmful, although a solution of oxalic acid can be highly corrosive to your skin and thus very dangerous. According to the MSDS, the estimated fatal dose of concentrated oxalic acid is from 5 to 15 grams if ingested. Oxalic acid can also be absorbed through the skin, so it should never be handled without gloves in the lab.