The use of anion-exchange disks in an optrode coupled to a multi-syringe flow-injection system for the determination and speciation analysis of iron in natural water samples

Abstract

A combination of multi-syringe flow-injection analysis (MSFIA) technique with an optical fibre reflectance sensor for the determination of iron in water samples has been developed in this work. Anion-exchange solid phase extraction (SPE) disks have been used as solid phase. Ammonium thiocyanate has been chosen as chromogenic reagent for Fe(III). The complex Fe[SCN] 6 3− is retained onto the SPE disk and spectrophotometrically detected at 480 nm. The complex is eluted with 0.25 mol l −1 hydrochloric acid in 75% ethanol. Total iron can be determined by oxidising Fe(II) to Fe(III) with hydrogen peroxide. A mass calibration was run within the range of 0.4–37.5 ng. The detection limit (3 s b/ S) was 0.4 ng. The repeatability (RSD), calculated from 9 replicates using 0.5 ml injections of a 25 μg l −1 concentration, was 3.6%. The repeatability between five anion-exchange disks was 5.4%. An injection throughput of 7 injections per hour for a sampling volume of 1 ml has been achieved. The applicability of the proposed methodology in natural water samples has been proved. The properties of anion-exchange and chelating SPE disks have been studied and compared.