A Fluorescence Technique to Determine Low Concentrations of Ferrate(VI)

A fluorescence technique to determine low concentrations of aqueous ferrate(VI), [FeVIO42-], in water was developed over a wide pH range using the reaction of ferrate(VI) with scopoletin reagent. The rates of the reaction of ferrate(VI) with scopoletin as a function of pH at 25°C were determined using the stopped-flow technique to demonstrate the reaction is rapid (< 1 min). Spectral measurements on scopoletin at different pH showed that the maximum in absorption varies with the pH while the emission maximum is independent of pH. The absorbance measurements were used to determine the acid dissociation constant, Ka = 1.55 ± 0.01 × 10-9 (pKa = 8.81 ± 0.05) for scopoletin. The intensity of fluorescence for scopoletin decreases linearly with increase in the concentration of ferrate(VI), which suggests the suitability of the method. Moreover, a relatively large decrease in intensity per micromolar ferrate(VI) concentration was observed, especially at low pH, which makes fluorescence a sensitive technique to determine low ferrate(VI) concentrations.