A potentiometric rhodamine-B based membrane sensor for the selective determination of chromium ions in wastewater

The construction and performance characteristics of a novel chromate ion-selective membrane sensor are described and used for determining chromium(III) and chromium(VI) ions. The sensor is based on the use of a rhodamine-B chromate ion-associate complex as an electroactive material in a poly(vinyl chloride) membrane plasticized with o-nitrophenyloctyl ether as a solvent mediator. In a phosphate buffer solution of pH 6 - 7, the sensor displays a stable, reproducible and linear potential response over the concentration range of 1 × 10^-1 - 5 × 10^-6 mol l^-1 with an anionic Nernstian slope of 30.8 ± 0.5 mV decade^-1 and a detection limit of 1 × 10^-6 mol l^-1 Cr(VI). High selectivity for Cr(VI) is offered over many common anions (e.g., I^-, Br^-, Cl^-, IO₄^-, CN^-, acetate, oxalate, citrate, sulfate, phosphate, thiosulfate, selenite, nitrate) and cations (e.g., Ag⁺, Ca²⁺, Sr²⁺, Co²⁺, Ni²⁺, Cu²⁺, Mn²⁺, Fe²⁺, Zn²⁺, Cd²⁺, Al³⁺, Cr³⁺). The sensor is used for determining Cr(VI) and/or Cr(III) ions in separate or mixed solutions after the oxidation of Cr(III) into Cr(VI) with H₂O₂. As low as 0.2 µg ml^-1 of chromium is determined with a precision of ±1.2%. The chromium contents of some wastewater samples were accurately assessed, and the results agreed fairly well with data obtained by atomic absorption spectrometry.