A primary standard and a secondary standard are both used to make or check solutions in titrations, but they are different:
- Primary standard:
- Very pure and stable.
- Can be weighed and used directly to make a solution of known concentration.
- Example: Sodium carbonate, potassium hydrogen phthalate.
- Secondary standard:
- Not pure or may change over time.
- Must be standardized using a primary standard before use.
- Example: Sodium hydroxide, hydrochloric acid.
In short:
Primary standard is pure and ready to use, while secondary standard needs standardization first.