Analytical and Instrumental Chemistry: Choosing and Interpreting a Method
Core concepts
Candidates should be able to select a technique for a stated analyte and interpret its output. Titrimetry: acid–base, redox (permanganometry, iodometry), complexometric (EDTA) and precipitation methods; know primary standards (oxalic acid, potassium dichromate, sodium carbonate) and indicators (phenolphthalein, methyl orange, starch, Eriochrome Black T). Gravimetry depends on complete precipitation and a weighable form.
Instrumental methods: UV–visible absorption follows the Beer–Lambert law, A = εbc; deviations occur at high concentration or with polychromatic light. Atomic absorption and flame emission suit trace metals. Infrared identifies functional groups through characteristic absorptions (O–H broad near 3300 cm⁻¹, C=O near 1700 cm⁻¹). NMR gives chemical shift and integration. Mass spectrometry gives molecular mass and fragmentation. Chromatography (paper, TLC, GC, HPLC) separates by differential partitioning; retention factor R_f = distance travelled by solute / distance travelled by solvent.
Traps to watch
- Using Beer's law outside its linear range.
- Confusing atomic absorption (absorption by ground-state atoms) with flame emission (emission from excited atoms).
- Forgetting that TLC R_f must be between 0 and 1 and depends on the solvent system.
- Treating a single titration result as reliable without repeats and concordant readings.
Worked example
A solution in a 1 cm cell has absorbance 0.30 and ε = 1500 L mol⁻¹ cm⁻¹. From A = εbc, c = 0.30/(1500 × 1) = 2.0 × 10⁻⁴ mol L⁻¹. Rule of thumb: keep absorbance between roughly 0.1 and 1.0 for best precision; dilute concentrated samples rather than extrapolating.