Q1(c). Non-absorbing analyte by UV-vis spectrophotometry

Answer in brief

Convert the analyte into an absorbing species with a chromogenic reagent and apply Beer’s law, A = εbc; or measure it indirectly through the change in absorbance of a coloured reagent.

Key points

• Derivatisation: analyte + excess chromogenic reagent → absorbing complex; measure A at λmax against a reagent blank and read c from a calibration curve.
• Examples: Fe²⁺ + 1,10-phenanthroline (orange-red, 510 nm); phosphate → molybdenum blue; NH₃ → indophenol blue; Ti(IV) + H₂O₂ (yellow, 410 nm).
• Indirect methods: analyte bleaches a coloured reagent (F⁻ with Zr–SPADNS lake); photometric titration; indirect UV detection; coupled enzyme reaction giving NADH (340 nm).
• Conditions: excess reagent, fixed pH, fixed colour-development time, measurement at λmax, concentration within the Beer’s law range.