The oxidation of indigo carmine by potassium hexacyanoferrate(III) is catalyzed by trace of silver nanoparticle (AgNP). The reaction is followed UV-Vis absorption spectrophotometrically by measuring the change in the absorption spectra (max 612 nm). The catalytic oxidation reaction is used for the quantification of indigo carmine. The calibration graph was linear in the concentration range 50 nM–1.8 μM of indigo carmine. The variables affecting themethod have been optimized. Themethod is applied to the determination of indigo carmine in groundwater samples with the satisfactory results
