What are primary standard substance and examples

A primary standard substance is a chemical that is highly pure, stable, has a high molar mass, is non-hygroscopic (does not absorb moisture from the air), and can be weighed accurately to prepare a solution of precisely known concentration. These solutions are then used to standardize other solutions.

Examples of primary standard substances include:

• Sodium carbonate ($\text{Na}_2\text{CO}_3$) for standardizing acids.
• Potassium hydrogen phthalate ($\text{KHC}_8\text{H}_4\text{O}_4$, KHP) for standardizing bases.
• Benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$) for standardizing non-aqueous bases.
• Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) for standardizing reducing agents.