Ironn (II) and iron(III) determination in sea water at the nanomolar level with selective on-line preconcentration and spectrophotometric determination

A method is presented for the shipboard determination of iron(II) and iron(III) at the sub-nanomolar level. A preconcentration step using a C18 phase column is required to remove the major ions as well as to concentrate iron. This column is impregnated with ferrozine, a selective ligand for Fe(II). After passing the sample through the chelating resin, the complex is eluted with methanol and detected with a spectrophotometer. The determination of Fe(III) is realised in the same manner after reduction by an ascorbic acid solution. To reduce the risks of contamination, the manifold developed includes on-line filtration, acidification, reduction, preconcentration and detection. The main parameters influencing the different phases studied were for the preconcentration step: flow-rate, concentration of ferrozine and pH; the optimum values of these parameters were 2 ml min−1, 10−3 M and pH 4–5, respectively. For the detection step these parameters were reduction of the Schlieren effect and interfering ions; for the reduction step they were flow-rate and concentration of reducing agent (0.1 ml min−1 and 100 μM, respectively) at pH 4–5, temperature 60 °C and reaction time 2 min. The method is characterised by its precision. It varied from 3% for 2 min of preconcentration to 15% near the limit of detection. The analysis of the certified sea water NASS-4 demonstrated the accuracy of the method, the blank was below the limit of detection for Fe(II) and 0.3 nM for Fe(III). The limits of detection were 0.1 nM for Fe(II) and 0.3 nM for Fe(III)