A good primary standard is a substance used to prepare a standard solution because its concentration can be known accurately. The main characteristics are:
- High purity:
- The substance should be very pure and stable.
- Stable:
- It should not decompose, absorb moisture, or react with air or carbon dioxide.
- Non-hygroscopic:
- It should not absorb water from the air.
- High molar mass:
- A higher molar mass reduces weighing errors.
- Soluble in the solvent:
- It should dissolve easily in the solvent (usually water).
- React completely and predictably:
- It should react fully in the intended reaction without side reactions.
In short:
A good primary standard is pure, stable, non-hygroscopic, soluble, and reacts completely, making it suitable to prepare a solution of known concentration.