Analytical Chemistry - Classical and Instrumental Quantitative Analysis
What the exam tests
Analytical Chemistry tests the full quantitative workflow: sampling, calibration, classical titrimetry and gravimetry, instrumental methods (UV-Vis, AAS, potentiometry, chromatography), and the statistics that decide whether a number is reportable.
Core rules
- Primary standards must be high purity, stable, non-hygroscopic, and have a high equivalent weight to reduce weighing error (KHP, Na₂CO₃, K₂Cr₂O₇).
- Equivalence vs endpoint: indicators bracket the equivalence point; a blank correction removes indicator and solvent error.
- Titration families: acid–base (phenolphthalein pH 8.2–10, methyl orange 3.1–4.4); complexometric (EDTA at pH 10 with ammonia buffer and Eriochrome Black T); redox (KMnO₄ is self-indicating in acid); precipitation (Mohr, Fajans).
- Potentiometry: the glass electrode obeys Nernst - about 59.2 mV per decade at 25 °C.
- Beer–Lambert: A = εbc, linear only in the dilute regime.
- Statistics: report mean ± confidence interval (t-table), reject outliers with the Q-test, and never drop data without a stated criterion.
Worked example
A 1.200 g sample containing Na₂CO₃ is titrated to endpoint with 38.50 mL of 0.2500 M HCl. n(HCl) = 0.2500 × 0.03850 = 9.625 mmol. From Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂, n(Na₂CO₃) = 9.625/2 = 4.8125 mmol. Mass = 4.8125 × 105.99 = 0.5101 g. Percent = 0.5101/1.200 × 100 = 42.51 %.
Traps
Forgetting the 1:2 stoichiometry; overlooking the blank; drying a primary standard at the wrong temperature; treating the endpoint as the equivalence point; quoting a burette volume as 38.5 mL when the burette reads to 0.01 mL.