Primary and secondary standards
Classical Quantitative Analysis · Chemistry
Study notes
Why is KMnO₄ a secondary standard but K₂Cr₂O₇ primary? Step 1: Primary standard: pure, stable, non-hygroscopic, high MW — K₂Cr₂O₇ qualifies. Step 2: KMnO₄ contains MnO₂ impurities, decomposes in light — concentration drifts. Step 3: So KMnO₄ must be standardized against oxalic acid (primary) before use. Step 4: Stability decides — pure and stable = primary!