Analytica Chimica Acta 455 (2002) 327–333 Spectrophotometric catalytic determination of Fe(III) in estuarine waters using a flow-batch system Josiane Meire Tolotti Carneiro a,∗ , Ana Cristi Basile Dias a , Elias Ayres Guidetti Zagatto a , Ricardo Saldanha Honorato b a Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P.O. Box 96, Piracicaba, SP 13400-970, Brazil b Departamento de Qu´ ımica Fundamental, CCEN, Universidade Federal de Pernambuco, Recife, Brazil Received 29 August 2001; received in revised form 27 November 2001; accepted 13 December 2001 Abstract A flow-batch system was developed for the determination of Fe(III) in estuarine waters with high variability in salinity. The method is based on the catalytic effect of iron(III) on the oxidation rate of N,N-dimethyl-p-phenylenediammonium dichloride (DmPD) by hydrogen peroxide and the formed product is spectrophotometrically monitored at 554 nm. A controlled addition of sodium chloride to every assayed sample is accomplished for in-line individual salinity matching. The proposed system processes about 30 samples h -1 and yields reproducible results. Relative standard deviations were esti- mated as <1.5% after 10 injections of typical samples (10.0–50.0 gl -1 Fe; ca. 0.5 mol l -1 Cl). Synthetic samples (15.0 gl -1 Fe; 0.25–1.0 mol l -1 NaCl) were efficiently processed, and no significant differences in results were found at a probability level of 99.7%. The method works for the full range of salinities. Only 120 g DmPD are consumed per determination. The analytical curve is linear up to about 60 gl -1 Fe (r> 0.999; n = 5) and the detection limit is 5 gl -1 Fe. Results are in agreement with graphite furnace atomic absorption spectrometry. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Flow-batch system; Matrix matching; Catalytic methods; Iron; Estuarine waters; Spectrophotometry 1. Introduction An increasing demand towards high sensitive meth- ods for the determination of metals in seawater, in- cluding estuarine waters, has been recently observed. In this context, catalytic methods are becoming more and more attractive for ultra-trace determinations [1] in view of their ruggedness, ease of operation, low-cost and suitability to in situ analysis. Regarding the spectrophotometric determination of iron, the catalytic method based on the influence of Fe(III) on the oxidation rate of N,N-dimethyl-p- ∗ Corresponding author. Fax: +55-19-429-4610. E-mail address: josiane@cena.usp.br (J.M.T. Carneiro). phenylenediamine by hydrogen peroxide in slight acidic medium should be highlighted. The method is highly sensitive and was successfully applied to the analysis of natural waters without a prior step of con- centration/separation [2]. It is however susceptible to pronounced variations in ionic strength, therefore its applicability could be limited when it was applied to the analysis of sample lots with high variability in salinity. The effect becomes more severe in relation to the analyses of estuarine waters that usually contain high and variable sodium chloride contents. The limitation was reported when a flow-injection spectrophotomet- ric procedure was designed for the determination of dissolved iron in sea water based on the oxidation of 0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0003-2670(01)01611-7