Quantitative Analysis: Titrimetry, Gravimetry, and Spectrophotometry
Quantitative Analysis: Titrimetry, Gravimetry, and Spectrophotometry
This domain tests whether a number you report is traceable to a primary standard, statistically defensible, and free of matrix bias.
Core rules
- Standardise against a primary standard: KHP for NaOH, anhydrous Na2CO3 for HCl, K2Cr2O7 for thiosulfate, EDTA for metal ions, sodium oxalate for KMnO4. Dry and weigh by difference; correct for buoyancy only in high-precision work.
- Distinguish equivalence point from endpoint and choose an indicator whose transition is close to equivalence. Run a blank and subtract it; use the same batch of indicator.
- Complexometric EDTA titrations need pH control (pH 10, NH3/NH4Cl) and a metal indicator such as Eriochrome Black T; mask interfering ions with citrate or triethanolamine.
- Gravimetry depends on precipitate quality: digest hot to promote Ostwald ripening, wash to remove adsorbed ions, ignite to constant mass, and cool in a desiccator.
- Spectrophotometry: A = εbc; calibrate over the linear range, use a reagent blank, and verify with a certified reference material or standard addition when the matrix is complex.
Worked example
A 25.00 mL water sample is titrated with 0.01000 M EDTA, mean titre 12.50 mL. Moles EDTA = 0.01250 × 0.01000 = 1.250 × 10⁻⁴ mol. With 1:1 Ca:EDTA, this is 1.250 × 10⁻⁴ mol Ca, equivalent to 1.250 × 10⁻⁴ × 100.09 = 0.01251 g CaCO3 in 25.00 mL. Scaling to 1 L: 0.01251 × 40 = 0.500 g L⁻¹, i.e. 500 mg L⁻¹ as CaCO3.
Traps
Overshooting the endpoint; ignoring the indicator blank; failing to dry the primary standard; forgetting the dilution factor; using a calibration curve beyond its linear range; treating a matrix effect as random error.