Introductory Chapter: Advanced Electrochemical Technique for Arsenic Determination in Complex Samples

1. Introduction

Arsenic contamination is a serious environmental and health threat worldwide. Pollution is due to various natural and artificial processes as degradation of industrial waste, pesticide leaching, rock and mineral erosion with the contribution of acid rains, magmatic processes, industrial practices such as mining, metal smelting, coal-fired power plants emissions, etc. Arsenic determination in environmental samples is carried out applying numerous methods: atomic absorption spectrometry, inductively coupled plasma mass spectrometry, neutron activation analysis, inductively coupled plasma-atomic emission spectrometry, X-ray fluorescence, molecule adsorption spectrophotometry, etc. [1, 2]. Unfortunately, these methods are complicated and require sophisticated and expensive equipment. In addition, most of them are used for total arsenic content determination, i.e. they do not allow the quantification of the individual arsenic species, whose distribution in the environmental and health effects are different. This work describes some of the properties of arsenic and the electrochemical methods applied to its determination, as an alternative to the aforementioned techniques, with special emphasis on the differential alternative pulses voltammetry (DAPV) as an advanced electrochemical method for arsenic determination in complex samples.

2. Arsenic and its properties

Arsenic is a chemical element, which exists in various inorganic and organic forms.

Inorganic arsenic species mainly include As(V): H₃AsO₄, H₂AsO₄⁻, HAsO₄²⁻, and AsO₄³⁻. H₃AsO₄ is the most commonly found in the environment. It is the predominant arsenic species in natural waters, soils, and drinking water. It is also present in plants, algae, and aquatic animals. Arsenates have a high ionization capacity. The molecule, by losing the hydrogen ion by dissociation, remains negatively charged, forming several anions. The As(III) species: H₃AsO₃, H₂AsO₃⁻, HAsO₃²⁻, and AsO₃³⁻ are considered as the most toxic. The oxidation state of arsenic depends on the conditions, such as the redox potential and pH. Under oxidizing conditions As(V) predominates over As(III) in the form of H₂AsO₄⁻ (pH < 6.9) and in the form of HAsO₄²⁻ (at higher pH). At pH < 2 (extremely acidic) the dominant species is H₃AsO₄. At pH > 12 (extreme basicity) the dominant species is AsO₄³⁻. Under reducing conditions and pH < 9.2 the neutral specie H₃AsO₃ prevails. In surface waters As(V) predominates over As(III), while in groundwater both may be present.

Organic arsenic species are widely distributed in the atmosphere, aquatic systems, soils, sediments, and biological tissues. They are involved in biologically mediated methylation, which occurs in terrestrial and marine organisms and which converts inorganic arsenic into nontoxic methylated compounds or in methylated compounds of moderate toxicity. Organic forms of arsenic usually occur in lower concentrations than inorganic species, although their ratio may increase as a result of methylation reactions catalyzed by microbial activity (bacteria, algae). The predominant organic forms are: dimethylarsinic acid and monomethylarsonic acid, where arsenic is present in both cases as As(V).

The toxicity of arsenic species decreases in the following order: AsH₃ > As(III) species > As(V) species > organic arsenic species. The lethal dose of As(III) for adults is 1–4 mg/kg body weight. For the other arsenic compounds, the lethal dose varies between 1.5 and 500 mg/kg. The human body is particularly protected by the methylation of arsenic, producing less toxic and more extractable metabolites. The World Health Organization advises that the maximum allowed arsenic concentration in drinking water is 10 μg L⁻¹.

3. Electrochemical methods for arsenic determination

Electrochemical methods provide accurate measurements of As concentration in the ppb range, using simple and cheap equipment. The most commonly used techniques are square wave voltammetry (SWV) and differential pulse voltammetry (DPV) including their stripping mode [3, 4, 5]. Arsenic detection is performed applying various bare (Pt, Au, carbon) and chemically modified electrodes [6, 7]. Unfortunately, in contrast to the mercury electrode, the application of all types of solid electrodes is limited by the so-called “memory effect”. As an alternative, the hanging mercury drop electrode (HMDE) with a renewed mercury surface or mercury film electrodes (MFE) can be used. However, their application is limited by the toxicity of mercury.

The measurement of As(III) concentration applying voltammetry can be complicated due to the interference of other metals present in the sample such as: Pb(II), Cu(II), Sb(III), Ag(I), Se(IV), Bi(III), Hg(II), Fe(II), Tl(I). As their peak potentials are close to the peak potential of As(III), peaks overlapping is observed, which impedes the precise As(III) peak height evaluation and hence – the precise As(III) concentration determination.

Recently, Zlatev et al. [8] developed a high-resolution voltammetric technique: differential alternative pulses voltammetry (DAPV) allowing the simultaneous quantification of species having very close peak potentials. DAPV potential-time waveform and the corresponding current responses are shown in Figure 1. This technique provides the simplicity and the high sensitivity of the DPV combined with the high resolution of the second-order voltammetric techniques [9, 10, 11, 12, 13]. The high resolution of the DAPV is due to the registered curve shaped as the first derivative of a peak consisting of a cathodic and an anodic peak with a small half-width for any of the species. In the case of very close peak potentials, one of the species is determined by its cathodic peak, while the other by its anodic peak remained on the voltammogram after the overlapping [8, 9].

Figure 1.

DAPV potential-time waveform (left) and the corresponding current response (right).

The anodic dI_p+ and the cathodic dI_p- current responses can be expressed by the following equations for reversible electrochemical reactions:

$$\mathrm{d}{I}_{p_{:}}=\frac{n^{\mathbf{2}}{F}^{\mathbf{2}}}{RT}\mathit{AC}\left(-\mathrm{d}E\right)\sqrt{\frac{D}{\pi t}}\frac{P_{:}}{{\left(1+P_{:}\right)}^2}{P}_{:}=exp\left[\left(E-E_{\frac{\mathbf{1}}{\mathbf{2}}}-\frac{\mathrm{d}E}{2}\right)\frac{\mathit{nF}}{RT}\right]$$

$$\mathrm{d}{I}_{p_{+}}=\frac{n^{\mathbf{2}}{F}^{\mathbf{2}}}{RT}\mathit{AC}\mathrm{d}E\sqrt{\frac{D}{\pi t}}\frac{P_{+}}{{\left(1+P_{+}\right)}^2}{P}_{+}=exp\left[\left(E-E_{\frac{\mathbf{1}}{\mathbf{2}}}+\frac{\mathrm{d}E}{2}\right)\frac{\mathit{nF}}{RT}\right]$$

where R is the gas constant, T is the absolute temperature, n is the number of transferred electrons, F is the Faraday constant, A is the electrode area, C is the concentration, dE is the pulse amplitude, D is the diffusion coefficient, t is the current measurement delay time, E is the electrode potential and E_1/2 is the half-wave potential.

DPV curve of a water sample containing 1 μmol L⁻¹ As(III) and 1 μmol L⁻¹ Pb(II) in 1 mol L⁻¹ HCl supporting electrolyte is shown in Figure 2 left. The DPV peaks of the two species are completely overlapped forming a common peak even in concentration ratio 1:1, which makes its distinguishing and determination impossible. Separate peaks however are registered on the DAPV curve at the same conditions as seen in Figure 2 right. The two species can be distinguished even in concentration ratio As(III)/Pb(II) as high as 4 to 1 as previously reported by Zlatev et al. [14].

Figure 2.

DPV (left) and DAPV (right) voltammograms of 1 μmol L⁻¹ As(III) and 1 μmol L⁻¹ Pb(II) at HMDE. Pulse amplitude = 10 mV, scan rate = 10 mV s⁻¹, scan step = 5 mV.

4. Conclusion

The importance of measuring the arsenic concentration is due to the great environmental pollution with this toxic and carcinogenic element. The methods currently used for this purpose (atomic absorption spectrometry, inductively coupled plasma, chromatography, etc.) require complicated and expensive equipment. In this context, electrochemical methods represent an attractive alternative. Special attention has to be paid to the advanced technique differential alternative pulses voltammetry allowing the precise arsenic determination in complex samples despite the interference of the species with close peak potentials.