Here’s a explanation:
Primary Standards
- Very pure and stable substances.
- Can be used directly to make a solution of known concentration.
- Examples: Sodium carbonate (Na₂CO₃), Potassium hydrogen phthalate (KHP), Oxalic acid.
- Used to standardize other solutions.
Secondary Standards
- Solutions whose exact concentration is not known.
- Must be standardized using a primary standard before use.
- Examples: Hydrochloric acid (HCl), Sodium hydroxide (NaOH), Potassium permanganate (KMnO₄).
- Used for routine titrations after standardization.
Simple rule:
- Primary = very pure, direct use
- Secondary = needs standardization first