Full Paper Speciation of Inorganic Arsenic in Waters by Potentiometric Flow Analysis with On-Line Preconcentration J. A. Rodríguez, a E. Barrado,* a M. Vega, a J. L. F. C. Lima b a Departamento de Química Analítica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain *e-mail: ebarrado@qa.uva.es b REQUIMTE/Departamento de Quimica-Fisica, Faculdade de Farmacia, Universidade do Porto, Rua Aníbal Cunha, 164-4050 Porto, Portugal Received: July 21, 2004 Accepted: October 4, 2004 Abstract A potentiometric flow system has been developed for the direct determination of arsenic(V) in water samples after on-line preconcentration in a minicolumn packed with a new adsorbent material selective to As(V). Iron oxyhydroxide embedded in silica gel was synthesized and used simultaneously as adsorbent material in the minicolumn and as selective component in a tubular potentiometric electrode. In the first step, the sample solution is mixed on-line with phosphate buffer of pH 7 and pumped through the minicolumn to adsorb As(V). The insertion of the minicolumn in the mobile section of a homemade injector-commutator allows to relocate the minicolumn to desorb the retained As(V) in the second step. The desorption is maximum in the presence of 0.1 M phosphate of pH 12. Maximum response of the tubular selective membrane is achieved in phosphate buffer of pH 7. On-line oxidation of As(III) to As(V) by iodine (I 3  ) in the presence of phosphate buffer of pH 7 allows the determination of total As as As(V) and then the speciation of inorganic arsenic, since As(III) can be estimated as the difference between total As and As(V). In the optimized experimental conditions, a linear range of 40 – 500 mgL 1 for As(V), and detection limits of 30.6 and 38.6 mgL 1 for As(V) and total As, respectively, were obtained.The flow system has been successfully applied to the determination of inorganic arsenic species in groundwater samples collected in areas affected by arsenic contamination. Relative standard deviation of replicated determinations of As in groundwater samples was always below 5%. Keywords: Arsenic, Speciation, Groundwater, Flow analysis, Potentiometry, Ion-selective electrode, On-line preconcentration 1. Introduction The recent finding that groundwater from large areas around the world is heavily enriched with arsenic [1, 2] has prompted the need of developing sensitive and fast methods to determine arsenic species in water. Arsenic occurs in water mainly as inorganic arsenite and arsenate, the former being recognized as more toxic [3, 4]. Therefore, the knowledge of the chemical forms of arsenic in water is crucial to understand the toxicity of the element to living organisms. Spectroscopic methods have been mostly used to measure total arsenic, but can only be applied to arsenic speciation if combined with previous separation steps [5 – 9]. Electrochemical methods such as stripping voltammetry allow to differentiate between As(III) and As(V). As(III) is the electrochemically active form and can be directly measured, whereas As(V), the most stable form of the element in oxidizing environments, can be determined after chemical reduction to As(III) [10 – 15]. Potentiometry using ion selective electrodes (ISE) is an adequate technique for metal speciation because it responds to the activity of an individual ion, and the natural distribution of species in the sample is not disturbed during the measurement [16]. However, with these inherent advantages, the use of potentiometry at ISEs for trace element determination in unpolluted waters is limited due to the lack of sensitivity required for direct measurements (lower limits of linear response usually ranging from 10 6 to 10 7 M) [8], and to the interfering effects caused by other ions present in the analytical matrix. Another limitation in the use of selective membranes in potentiometry is the need of preconditioning the electrode surface, which is a time consuming process rather difficult to reproduce in batch determinations. These drawbacks, along with the scarce number of substances selective to arsenic described to date, are likely responsible for the absence of published methods using potentiometric selective sensors for arsenic. In order to improve the sensitivity and selectivity of most analytical techniques, and thus potentiometry, preconcen- tration and separation procedures such as ion exchange, solvent extraction, solid sorbent extraction and coprecipi- tation are often used before analysis [17 – 20]. Such proce- dures operated in the batch mode require large sample volumes and undergo great risks of contamination and analyte loss. Flow systems with on-line preconcentration and separa- tion coupled with potentiometry have proved to be a powerful tool for the determination of trace elements [21]. 504 Electroanalysis 2005, 17, No. 5 – 6  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200403188