**1. Introduction**

202 Electrochemical Cells – New Advances in Fundamental Researches and Applications

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Arsenic is one of the most feared contaminants because of its high toxicity at low concentrations. Exposure to high levels of arsenic can cause problems in humans ranging from gastrointestinal symptoms to arsenicosis. Once this element is dissolved in water and is ingested, it is accumulated in the body. Contamination of groundwater with arsenic is one of the major environmental and public health problems on a global scale (NRC, 1999).

The World Health Organization guideline has established a concentration of 10 µg l-1 as the maximum residue limit for arsenic in drinking water (WHO, 2004). Well-known arsenic contaminated regions include Bangladesh, India and other countries. In Mexico, sources of drinking water exceeding 10 µg l-1 have been found in Baja California Sur, Chihuahua, Coahuila, Durango, Zacatecas, Hidalgo, Morelos, Guanajuato, Sonora and San Luis Potosí (Camacho et al., 2011). In Spain, the presence of naturally occurring arsenic in groundwater has been reported in the sedimentary Duero and Tajo Cenozoic basins, located in central Spain (García-Sánchez et al., 2005; Gómez et al., 2006; Vega et al., 2008) .

Arsenic is usually distributed as water soluble species, colloids, suspended forms and sedimentary phases. Mobilized arsenic is most likely transported by water and accumulated in downstream river sediments as a result of the great affinity of arsenic to iron rich phases. The most common sources of non-naturally arsenic worldwide arise from the presence of alloys used in manufacture of transistors, laser, semi-conductors and mining industry (Smedley & Kinniburgh, 2002).

There are more than 20 arsenic compounds identified in environmental and biological systems (Gong et al., 2002). The dominant forms of arsenic present in the environment are As(III) (arsenite) and As(V) (arsenate) (Mondal, 2006). As(III) binds to sulfhydryl groups impairing the function of many proteins and affects respiration by binding to the vicinal thiols in pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. As(V) is a molecular

Sequential Injection Anodic Stripping Voltammetry

of ASV at gold film electrodes.

voltammogram.

higher degree of automation.

Edeposition

Ecleaning

E0 As

at Tubular Gold Electrodes for Inorganic Arsenic Speciation 205

Anodic stripping voltammetry (ASV) provides an alternative technique for measuring inorganic arsenic in water samples. ASV at gold film electrodes (Sun et al. 1997) or solid gold electrodes (Kopanica & Novotny, 1998) have been extensively used for inorganic arsenic speciation as they allow to determine separately As(III) and total As. The analysis by ASV involves three major steps (Figure 1). First, the electrode surface is conditioned for analysis (cleaning the surface of the solid electrode and/or plating a gold film). The As(III) is then deposited as elemental arsenic on the working gold electrode by electrochemical reduction. After the deposition step, the elemental arsenic is electrochemically oxidized (stripped) back to As(III). As(V), the most stable form of the element in oxidizing environments, is determined after chemical or electrochemical (Muñoz & Palmero, 2005) reduction to As(III), total As is then determined and As(V) is calculated by difference between total As and As(III). The Environmental Protection Agency (EPA) has approved an analytical method (EPA, 1999) for arsenic determination in water samples based on the use

> Accumulation step Mn+ + ne- → M0

> > Current

Fig. 1. Anodic stripping voltammetry: the potential-time waveform with the resulting

The remarkable sensitivity, broad scope and low cost of stripping analysis have led to its application in the determination of arsenic in water, soils and food samples. From early years of stripping analysis two main different research areas have been considered. The use of microelectrodes and disposable electrodes (Gibbon et al., 2010), and the development of hyphenated techniques using flow manifolds (Economou, 2010). On-line stripping analysis using flow analysis has demonstrated the viability and potentialities of this coupling such as: a) lower consumption of sample and reagents, b) higher precision and accuracy and c)

Time

Potential

Stripping step Mn+ → M0

analogue of phosphate and inhibits oxidative phosphorylation, the main energy generation system. As(V) is most frequently present in surface water while As(III) is commonly found in anaerobic groundwaters. Redox potential, pH and organic matter control the species present in water.

To determine the potential transformation and risk of arsenic in the environment, the analysis of arsenic should include identifying and quantifying both, the total quantity of arsenic present and the specific chemical forms, a procedure known as speciation (Bednar et al., 2004; Burguera & Burguera, 1997; Gong et al., 2002).
