Why is oxalic acid suitable as a primary standard?

Answer 1

• Relatively high molar mass of the di-hydrate: C2O4H2.2H2O 108.05 g/mol (= (90.03+18.02)
• Chemically and physically stable under normal (preconditioned dry) air and in the prepared standard solution.
• In acidimetric titrations oxalic acid is a relatively strong acid, despite being a di-carboxylic acid:

  C2O4H2 → C2O4H− + H+; pKa = 1.38 and C2O4H− → C2O42− + H+; pKa = 4.28

  Both pKa values are low enough to give a sharp pH-change (pHendpoint close to 7) when added as titrant to strong base

• It is also a good reducing standard titrant

Answer 2

Oxalic acid is suitable as a primary standard because it can be obtained in a high degree of purity, has a high molar mass allowing for accurate weighing, and can be easily dried to remove any water of hydration, ensuring precise measurements for standardization purposes.