Difference in preparation of oxalic acid and sodium hydroxide
Oxalic acid is regarded as the primary standard solution.A standard solution of such a substance can be prepared by dissolving a known weight of it in a known volume of water. Sodium hydroxide solutions pick up carbon dioxide from the air. This contamination can affect the strength of the base solution and can spoil the sharpness of the end point in the titration. That is why sodium hydroxide solution has to be standardized (for example with the oxalic acid).