Talanta 69 (2006) 615–620 Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent Demetrius G. Themelis ∗ , Fotini S. Kika, Anastasios Economou Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Received 1 July 2005; received in revised form 4 October 2005; accepted 25 October 2005 Available online 23 November 2005 Abstract A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λ max = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO 4 . The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3–4000 gl -1 and 30–1200 gl -1 for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s r = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 gl -1 level, respectively). The method developed proved to be adequately selective and sensitive (c L = 1 and 10 gl -1 for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93–106%). Analytical results of real sample analysis were in good agreement with certified values. © 2005 Elsevier B.V. All rights reserved. Keywords: Flow-injection; Chromium speciation; Spectrophotometry; Chromotropic acid; Water; Lyophilised solution 1. Introduction The identification and quantitation of ultra trace of specific metal species is currently an important challenge for the analyti- cal community. Speciation measurements are made for a variety of reasons, including characterization and evaluation of systems in environmental science, medicine, biological processes mon- itoring nutrition and industry. In this work, the focus was on the speciation of trace chromium. Chromium is a naturally occurring element, mainly found in minerals, rocks, plants, soil, water and in volcano dust and gases. Trivalent Cr(III) and hexavalent Cr(VI) enter the envi- ronment as a result of effluent discharge, electroplating, tanning industries and oxidative dyeing. Although chromium is known to exist in all oxidation states from Cr(0) to Cr(VI), the Cr(III) and Cr(VI) species are the most widespread in nature. The “para- dox” with chromium is that its toxicity varies according to its ∗ Corresponding author. Tel.: +30 2310 997 804; fax: +30 2310 997 719. E-mail address: themelis@chem.auth.gr (D.G. Themelis). oxidation state. While trivalent Cr(III) is considered to play an essential role in the proper function of living organisms, hexava- lent Cr(VI) is a potential carcinogenic agent and exerts serious toxic effects on biological systems. Its toxicological action is mainly based on its oxidizing properties and on the formation of free radicals during reduction of Cr(VI) to Cr(III) occuring inside the cells [1]. The speciation of chromium can be achieved by many dif- ferent methods. The predominant trend in recently proposed methods for the separation—speciation of chromium is the use of liquid chromatography (LC) and flow methods of analysis. Considering the great analytical interest in the speciation of chromium, the development of an inexpensive, on-line proce- dure for the trace determination of chromium is both desirable and very useful. Flow injection (FI) analysis offers great advan- tages in chromium speciation analysis, in terms of instrumental versatility, cost-effectiveness, sample throughput and simplicity. In addition, the improvement in accuracy and precision enables the effective incorporation of pre-concentration and separation steps in the flow system, prior to the final measurement in order to increase the sensitivity of the analytical assays. 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.10.031