Electrochemical and spectroscopic insights of interactions between alizarin red S and arsenite ions

Abstract

Interferences of arsenite ions on electrocatalytic oxidation of alizarin red S (ARS) was studied using Pt and ITO electrodes. A Pt electrode can oxidize both arsenite ions and ARS molecules simultaneously. The oxidation wave of ARS exceeds that of arsenite until the [AsO₂⁻]/[ARS] ratio surpasses 0.07. Meanwhile, an ITO electrode can oxidize only ARS molecules. It was seen that the diffusion coefficient of ARS molecules decreased from 4.3 × 10⁻⁶ cm² s⁻¹ to 1.68 × 10⁻⁷ cm² s⁻¹ in the presence of arsenite ions. The electrokinetic investigation shows that ARS oxidation was a two-electron transfer consecutive process. The EIS studies showed that charge transfer resistance was increased in the presence of arsenite ions during ARS oxidation.

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Introduction

Concerning toxicity, inorganic arsenic (arsenite) is one of the most notorious carcinogens, which causes malformations, primarily neural tube defects in animals, and it may contribute to human birth defects.¹ Arsenic contaminants come into the human body through drinking water. They enter into the natural water systems from earth deposits or from industrial and agricultural wastes.² In particular, the existence of extremely high As(III) concentrations in Bangladesh ground waters could be due to the reductive dissolution of hydrous ferric oxide by iron-reducing bacteria.³ In environmental chemistry, the behaviour of arsenic in natural water has become of great interest nowadays. Over the past several decades, a majority of the studies have been performed on electrochemical and spectroscopic detection of arsenic from ground water.^4–13 Nevertheless, development of an easy and low-cost detection method of arsenite ions from water still receives great interest. Under this concern, we have investigated the alizarin red S (ARS)–arsenite interactions to check the feasibility of detection of arsenite ions using ARS molecules.

ARS is a kind of anthraquinone type dye chemically known as 1,2-dihydroxy-9,10-anthraquinone sulfonic acid sodium salt.^14,15 Generally it is widely used in textiles which is derived from roots of madder genus plant or sulfonation of alizarin.¹⁶ ARS is also used for biological staining, drug analysis, sensing of metal ions, pH senor and in electro-analysis.^17–26 An ARS molecule has quinoid oxygen with two phenolic groups at α & β position (Fig. 1). Thus, ARS molecules can undergo both oxidation and reduction reactions at an electrode surface which is supported from the literature. In an article, it has been reported that ARS molecules can undergo two-electron transfer reduction reactions at HMDE.¹³ In another article, it is stated that ARS molecules involve in oxidation reactions at a GC electrode again by transferring two electrons.⁸

Fig. 1 Structural formula of alizarin red S (1,2-dihydroxy-9,10-anthraquinonesulfonic acid sodium salt, ARSH₂).

It is to note that phenolic groups of an organic dye molecule, particularly under deprotonated state, often can get coordinated with the metal ions. Due to availability of such coordinating properties, ARS has been exploited in detection and determination of a number of transition metal ions.^27–34 However, coordination between ARS molecules and arsenite ions is not possible. But since ARS molecules has phenolic groups and since according to pourbaix diagram, arsenite ions has strong affinity for H⁺ ions in a wide pH range,³⁵ thus ARS molecules may be deprotonated in the presence of arsenite ions. This probable deprotonation reaction may become a matter of interest in detection of arsenite species in relevance with waste water management process. In this paper, we have reported how the presence of arsenite ions influences the oxidation kinetics of ARS molecules using Pt and ITO electrodes that is associated to protonation–deprotonation reactions. We have reported our data based on cyclic voltammetry, convolution potential sweep voltammetry, electrochemical impedance spectroscopy, and UV-visible spectral analysis. Finally, based on the experimental evidences, we have made a recommendation concerning viability of detection of arsenite ions using ARS molecules.

Experimental

All the experiments were carried out under thermostatic condition at room temperature. Milli-Q water was used to prepare all the solutions. All of the electrochemical experiments were performed in 0.1 M KCl supporting electrolyte. Sodium meta arsenite (NaAsO₂) and alizarin red S were supplied by Sigma Aldrich chemical company. The cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) were recorded using an Autolab potentiostat (PGSTAT 128N, The Netherlands) in a conventional three electrode cell. A Teflon jacket-coated Pt electrode (2 mm in diameter) and an ITO disk (4 mm in diameter) served as a working electrode. In the experiments, counter and reference electrodes were the Pt wire and Ag/AgCl (sat. KCl), respectively. The cleaning action was repeated after each experiment. Before the experiments, bare Pt electrode was mechanically polished with alumina (0.3 μm) slurry on a soft lapping pad until a mirror like shiny surface was obtained. Then the surface was rinsed with double-distilled water and sonicated in ethanol and double-distilled water for 10 minute, respectively. Finally, the Pt surface was cleaned by potential cycling in a N₂-saturated 0.3 M H₂SO₄ solution from −0.2 to 1.5 V at scan rate of 100 mV s⁻¹ until the characteristic reproducible cyclic voltammogram of cleaned Pt surface was obtained.

The ITO electrode was cleaned by sonication in 0.01 M H₂SO₄ for 15 minutes each time. Then it was heated at 300 °C in oven. Before each experiment, the test solution was purged by N₂ for 5 minutes to remove dissolve oxygen. Convoluted voltammograms (CPSV) were obtained from cyclic voltammograms by using Nova 1.11 software. All the data obtained by CV and CPSV were fitted using Sigma Plot v.10 data analysis software. An UV-visible spectrophotometer (UV-1800, Shimadzu, Japan) was used to record spectral changes of ARS molecules in the aqueous medium.

Results and discussion

Diffusion

Fig. 2 shows the cyclic voltammograms of ARS molecules (5 mM) and arsenite ions (2.5 mM) in presence of 0.1 M KCl at a scan rate of 5 mV s⁻¹ recorded using a Pt electrode. The ARS molecules and arsenite ions got oxidized at peak potential (E_p) of 0.64 V (peak a(i)) and 0.79 V (peak b(ii)), respectively. Note that when arsenite ions were mixed with ARS molecules, keeping their concentrations invariant, the peak potential (peak c(iii)) shifted to a positive value (0.82 V) with increasing peak current (I_p) with respect to ARS or arsenite alone (see Fig. 2A). These observations indicate that a Pt electrode not only can oxidize ARS molecules but also can oxidise arsenite ions. Under the mixture condition, the individual features of ARS and arsenite species could not be isolated until the [AsO₂⁻]/[ARS] ratio was higher than 0.07. As the [AsO₂⁻]/[ARS] ratio exceeded 0.07, during potential scanning from 0 V to 0.9 V at scan rate of 5 mV s⁻¹, two consecutive oxidation peaks were seen at 0.65 V and 0.77 V which are marked as f(v) and f(vi) in Fig. 2B. At the peak potential f(v), ARS molecules formed quinoid structure by transferring two electrons (as per eqn (1)),^21,24 and at peak f(vi), arsenite ions became oxidized as of eqn (2), which can easily be isolated by comparing the peak position (e(iv)) of arsenite ions alone.

AsO₂⁻ + H₂O → AsO₄³⁻ + 2H⁺ + 2e⁻ (2)

Fig. 2 Cyclic voltammograms of [A] 5 mM ARS (a), 2.5 mM AsO₂⁻ (b), 5 mM ARS + 2.5 mM AsO₂⁻ (c) in presence of 0.1 M KCl; [B] 5 mM ARS (e), 5 mM ARS + 0.35 mM AsO₂⁻ (f) in presence of 0.1 M KCl at scan rate of 5 mV s⁻¹ on Pt surface (ϕ = 2 mm) [dotted curve (d) represents CV of 0.1 M KCl].

However, since both of the ARS molecules and AsO₂⁻ ions can exhibit voltammetric responses in the similar potential region, it was difficult to determine the possible interferences of arsenite ions on the properties of ARS molecules using a Pt electrode. Under this circumstance, we recorded CVs of arsenite and ARS solution separately using an ITO electrode. We observed that the AsO₂⁻ ions did not show any response at ITO electrode (see Fig. 3). Therefore, investigation of ARS electro-oxidation in presence of AsO₂⁻ ion became convenient at an ITO electrode. In Fig. 3, it is seen that an ARS solution generated oxidation peak current of 19.21 μA at the E_p of 0.77 V on an ITO surface. Meanwhile, when arsenite ions were mixed with ARS solution (1 : 1 molar ratio), the E_p decreased to 0.60 V with decreasing oxidation current (9.24 μA).

Fig. 3 Cyclic voltammograms of (a) 3 mM ARS, (b) 3 mM ARS + 2.5 mM AsO₂⁻, (c) 2.5 mM AsO₂⁻, (d) blank (0.1 M KCl) at scan rate of 5 mV s⁻¹ in 0.1 M KCl at ITO electrode (ϕ = 4 mm).

For an electrode process, the peak current (I_p) depends on the electrode surface area, diffusibility of the analyte and number of electron transfer. In the present case, except diffusibility ARS molecules, all other properties were invariant under the reaction condition. Thus we suspected that diffusion coefficient of ARS molecules was probably influenced by the presence of arsenite species. The convolution potential sweep voltammetry is the one of the easiest technique to determine the diffusion coefficient of an electro-active species. In this technique, background subtracted current divided by the square root of scan rate provides the current which is independent of scan rate.^36–39 Convolution of the scan rate independent current generates a limiting current I_l (as shown in Fig. 4) which is related to diffusion coefficient as per following equation.

I_l = nFACD_o^1/2 (3)

where, I_l is the limiting current, n is the number of electron transfer which is one for ARS,⁴⁰ F is the Faraday's constant, C is the concentration of solution and A is the area of electrode in cm².

Fig. 4 Cyclic voltammograms of (A) 3 mM ARS, and (B) 3 mM ARS with sodium arsenite (3 mM) in 0.1 M KCl at scan rate of 100 mV s⁻¹. The dotted lines indicate convoluted current for forward scan. ITO electrode: (ϕ = 4 mm).

By solving eqn (3), D value of ARS molecules was calculated as 4.3 × 10⁻⁶ cm² s⁻¹ and 1.68 × 10⁻⁷ cm² s⁻¹ for ARS alone and ARS in presence of arsenite, respectively. Consequently, it can be supposed that since diffusion coefficient of ARS decreased in presence of arsenite ions, the ARS molecules experienced some structural changes in the presence of arsenite ions which inhibited the rate of oxidation process of ARS molecules at the electrode surface.

Electro-kinetics

Fig. 5 shows the dependency of CV profiles of ARS molecules (both in absence (Fig. 5A) and presence of arsenite ions (Fig. 5B)) on scan rate. With the increase of scan rate, peak potential shifted to the higher positive values, which is very common feature of the irreversible electron transfer reactions. Fig. 5C shows the dependency of ratio between peak current and square-root of scan rate (I_p/v^1/2) on scan rate. It is seen that I_p/v^1/2 ratio decreased with the increase of scan rate. This observation indicates the non-constancy of the transfer coefficient (β) with respect to potential.³⁷ The oxidation kinetics of ARS molecules, under such a situation, cannot be explained using Butler–Volmer (B.V.) equation, according to which the value of β must be constant.

Fig. 5 Dependency of CVs of ARS (3 mM) oxidation on scan rate. (A) in absence of arsenite, (B) in presence of arsenite (3 mM), and (C) dependency of I_p/v^1/2 ratio on scan rate. ITO: (ϕ = 4 mm).

It noteworthy to mention that peak width (ΔE_p/2) is an ideal indicator of sensitivity of probable dependency of β on potential as per eqn (4).

In the present case, it was seen that the peak width showed an increasing tendency with the increase of scan rate in both cases. As a consequence, the value β decreased in both cases. Under this circumstance, we took the advantage of CPSV in determining the kinetics of electron transfer processes. According to CPSV, the heterogeneous rate constant (k_het) of an irreversible ET process can be estimated by using following equation.^36–39

where, I_(l) is the convulated limiting current, i_(t) is the cyclic voltammetric current and I_(t) is the convoluted current. The calculated ET rate constant (k_het) as a function of applied potential are shown in Fig. 6A. Indeed, the plots in Fig. 6 represent the parabolic relationships between ln k_het and the driving force (E). In such a case the observed activation driving force (shown in Fig. 6), can be related to the apparent transfer coefficient β_app as per eqn (6).

Fig. 6 Dependency of heterogeneous rate constant (k_het) of ARS oxidation in absence (A) and in presence (B) of arsenite ions (3 mM) at ITO surface [3 mM ARS, 100 mV s⁻¹].

The equation is inscribed in terms of the apparent value of the transfer coefficient (shown in Fig. 7) because such a value has not been corrected for the double layer. By considering transfer coefficients at various scan rates, the linear regression of eqn (6) gives β_app = 1.26 − 1.30E and β_app = 1.62 − 2.26 E, in absence and in presence of arsenite ions, respectively, for ARS oxidation. Setting β_app = 0.5, the standard oxidation potential (E^o′) of ARS molecules vs. Ag/AgCl reference electrode was evaluated as 0.58 V and 0.49 V in absence and presence of arsenite ions, respectively. Using these data, standard rate constants (k^o) were then evaluated. Several kinetic parameters are reported in Table 1.

Fig. 7 Potential dependence of the transfer coefficient (β_app) for ARS (3 mM) oxidation at various scan rates on ITO. (A) in absence of arsenite ions, (B) in presence of arsenite ions (3 mM).

Table 1 Electrokinetic properties of ARS molecules in absence and presence of arsenite^a

Analyte	Ep/V	E^o′/V	Ip/μA	Do/×10^−7 cm^2 s^−1	ln(k^o/cm s^−1)
a ARS: 3 mM, AsO2^−: 3 mM, 5 mV s^−1.
ARS	0.77	0.58	19.21	43.0	−10.8
ARS + AsO2^−	0.60	0.49	9.24	1.68	−9.5

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To isolate the mechanistic aspects further, the potential-dependent free energy of activation (ΔG^‡(E)) was evaluated by solving the following equation.^37,38

where, Z ([=(RT/2πM)^1/2], M = 324.25 g mol⁻¹) for ARS) is 3400 cm s⁻¹. The evaluated ΔG^‡ values as a function of ΔG^o (=−nF(E − E^o′) are displayed in Fig. 8. Now by examining the profiles of the plots shown in Fig. 6, 7 or 8, one can predict the nature of reaction mechanism that took place on the ITO surface.

Fig. 8 ΔG^‡(E) vs. ΔG^o plots in absence and in presence of arsenite pertaining to ARS oxidation at ITO. [3 mM ARS, 3 mM arsenite, 100 mV s⁻¹].

The nonlinear variation of ΔG^‡ with ΔG^o pertaining to ARS oxidation process may be interpreted by Marcus parabolic relationship (8)⁴¹

Here, λ is the reorganization energy contributed by the molecular reorganization and surrounding solvent of the molecules. The Marcus theory was originally formulated to address outer sphere electron transfer reactions, in which the electron donor and acceptor species only change their charge with an electron jumping (e.g., the oxidation of an ion like Fe²⁺/Fe³⁺), but do not undergo large structural changes.^37–41

In the present case, it can be seen from Fig. 7 that there are two kinetic regimes with one break point (ca. 0.65 V for ARS alone and 0.48 V in presence of arsenite) irrespective of presence of arsenite ions. This indicates the involvement of stepwise reactions.

From Fig. 7, it is also seen that the value of β varied from 0.64 to 0.20 between 0.60 V and 0.88 V, Meanwhile, in the presence arsenite ions, transfer coefficient varied from 0.87 (0.39 V) to 0.27 (0.60 V). The transfer coefficient value closer to unity indicates the involvement of a reversible step.⁴² Conversely, while the value of transfer coefficient decreased to the lower values (<0.5), it indicates that a chemical step is coupled with an electron transfer step.^43–45 Thus, it can be said that at the beginning of the potential scan, a reversible one electron transfer process was involved where the intermediate state had the similar chemical configuration as that of ARS as per Marcus theory. As the potential increased transfer coefficient decreased and around the peak potential, second electron transfer occurred along with deprotonation reaction. Thus, the probable oxidation mechanism of ARS molecules may be presented as shown in Scheme 1.

Scheme 1 Oxidation mechanism of ARS molecules on ITO surface.

EIS studies

In order to perceive the interference of arsenite on the properties of ARS oxidation, EIS spectra of ARS molecules were recorded in absence and presence of AsO₂⁻. The EIS spectra are classically presented in the form of a complex plane plot, where Z′ is the real and Z′′ is the imaginary part of impedance, respectively (shown in Fig. 9). For an ideal non polarizable system, the circuit comprises a series connection of solution resistance (R_s) with a parallel combination of a charge transfer resistance (R_ct), a double layer capacitance (C_dl) etc., is known as simplified Randle's circuit.^40,46

Fig. 9 EIS spectra of 1 mM ARS molecules in 0.1 M KCl. (A) Nyquist plots of ARS molecules (1 mM) in presence and absence of 1 mM arsenite ions at ITO electrode at 0.6 V. Inset shows the equivalent circuit.

The complex plane plot shows that the impedance of faradic process appeared as a semicircle at high frequency edge and the diffusion process appears as a diagonal line at the low frequency edge at a working potential of 0.6 V. This observation ascertains the relevance of occurring of oxidation reaction where a diffusional process was coupled with a charge transfer process. At an exciting frequency ca. 100 kHz, the capacitive impedance short-circuited, which practically diminished the R_ct.

Therefore, only the R_s remained at the high frequency intercept. Meanwhile, the low frequency intercepts represent the sum of R_s and R_ct. So the diameter of the semicircles represents R_ct for the concerning pH of the medium. It can be noticed from Fig. 9 that the maximum of the Z′′ occurred at Z′ = R_s + R_ct/2, which denotes the specific frequency associated to charge transfer (ω_max). The Bode magnitude plots shown in Fig. 10(A and B), Y1 axes indicates that the system exhibited two breakpoints. In the order of decreasing frequency; the two breakpoints correspond time constants τ₁ and τ₂ respectively defining R_s, R_ct and C_dl as follows:⁴⁶

Fig. 10 Bode module and Bode phase of (A) 1 mM ARS and (B) 1 mM ARS in presence of 1 mM NaAsO₂ in 0.1 M KCl at ITO electrode.

At the lower frequency intercept, the third break point refers the time constant τ_d of the diffusional process. Note that the Bode phase plots in Fig. 10(A and B); Y2 axes also expressed the similar significance to those of Bode magnitude plots for the simplified Randle's circuit.

The peak and valley like shapes connote the involvement of a diffusional process. Impedance study of a species in solution refers to the measurement of resistance for charge transfer process. It is clearly seen, by the application of ITO electrode, that after addition of arsenite ions, the EIS properties of the ARS molecules altered significantly. The R_ct value increased from 26.74 kΩ (ARS) to 73.68 kΩ (ARS + AsO₂⁻) implying that probable deprotonation reaction made the ARS molecules more resistant to be oxidized.

For the same reason, charge accumulation by the ARS molecules also decreased at the ITO-solution interface as reflected by the decrease of C_dl value from 248.42 μF cm⁻² to 75.48 μF cm⁻².

UV-visible spectroscopy

Finally, reasons of interferences of arsenite ions on the electro-oxidation of ARS molecules was investigated by means of UV-visible spectroscopy. In this case, absorption spectra of ARS solution were recorded within the wavelength range between 300 nm and 800 nm in presence and in absence of arsenite ions as shown in Fig. 11. The ARS molecules present one broad band with λ_max at 425 nm (due to quinonoid π–π* transition), a strong narrow band at 334 nm due to π–π* transition in the benzoid system (characteristic band of anthraquinone).⁴⁷ It is seen from Fig. 11 that as the arsenite ions were added to the ARS solution, the band for quinonoid moiety shifted to 520 nm indicating the extension of conjugation system. Here, the colour change was so sharp that it was even visualized by bare eyes (see Fig. 12). This means that upon addition of arsenite ions, phenolic groups of the ARS molecules were deprotonated. As a consequence, the structure of ARS molecules was reformed due to extended resonance which weakened the C=O bonds (Scheme 2). Thus, electronically different ARS structures, in absence and presence of arsenite ions, exhibited different oxidation kinetics and EIS properties.

Fig. 11 UV-visible spectra of 0.13 mM ARS molecules in the aqueous medium in absence and in presence of 0.1 mM sodium arsenite ions.

Fig. 12 Natural view of ARS (A) and ARS in arsenite (B).

Scheme 2 Deprotonation reaction of ARS molecules in the presence of arsenite ions.

Consequently, it can be said that presence of arsenite ions changes the electrochemical and spectroscopic properties of ARS molecules in the aqueous medium. Accounting these assayed results, it might be possible to detect arsenite ions selectively using electrochemical and spectroscopic methods.

Conclusion

Interferences of arsenite ions on ARS oxidation was investigated by electrochemical and spectroscopic analysis. A Pt electrode can oxidize both of arsenite and ARS molecules, meanwhile, an ITO electrode can oxidize only ARS molecules, which involved a two electron transfer process. Diffusion co-efficient of ARS molecules decreased in presence of arsenite. The arsenite ions increased the charge transfer resistance of the interfacial processes. The investigation of electron transfer kinetics revealed that ARS oxidation occurred using a stepwise mechanism. This discussion may be helpful for simultaneous determination of arsenite and ARS or easy and passive determination of arsenite ions in presence of ARS molecules from natural water systems.

Acknowledgements

Ministry of education of Bangladesh is acknowledged for the financial support (2015–16). The World Academy of Sciences (TWAS) is also acknowledged greatly for the development of our laboratory facilities (Ref. 14-050 RG/CHE/AS_G; UNESCO FR 34028605).

References

1. U.S. Environmental Protection Agency, Integrated Risk Information System (IRIS) on Arsine, Washington, 1994.
2. Y. Takahashi, M. Sakamitsu and M. Tanaka, DyePigm., 2011, 91, 324–331.
3. M. L. Polizzotto, B. D. Kocar, S. G. Benner, M. Sampson and S. Fendorf, Nature, 2008, 7, 454–505.
4. C. Bauer, P. Jacques and A. Kalt, J. Photochem. Photobiol. Chem., 2001, 140, 87–92.
5. M. Korolczuk, M. Ochab and I. Rutyna, Talanta, 2015, 144, 517–521.
6. A. Dhillon, M. Nair and D. Kumar, Anal. Methods, 2015, 7, 10088–10108.
7. J. H. T. Luong, E. Lam and K. B. Male, Anal. Methods, 2014, 6, 6157–6169.
8. F. R. Zaggout, A. E. A. Qarramam and S. M. Zourab, J. Matter. Lett., 2007, 6, 4192.
9. J. Suna, H. Lua, L. Duc, H. Lina and H. Li, Appl. Surf. Sci., 2011, 257, 6667–6671.
10. S. Schumacher, T. Nagel, F. W. Scheller and N. Gajovic–Eichelmann, Electrochim. Acta, 2011, 56, 6607–6611.
11. K. A. Francesconi and D. Kuehnelt, Analyst, 2004, 129, 373–395.
12. K. Ito, C. D. Palmer, A. J. Steuerwald and P. J. Parsons, J. Anal. At. Spectrom., 2010, 25, 1334–1342.
13. D. F. Wood and R. H. McKenna, Analyst, 1962, 87, 880–883.
14. K. R. Mahanthesha, B. E. Kumara Swamy, U. Chandra, Y. D. Bodke, K. V. Kumar Pai and B. S. Sherigara, Int. J. Electrochem. Sci., 2009, 4, 1237–1247.
15. B. Zhang, J. Liu, X. Ma, P. Zuo, B. Ye and Y. Li, Biosens. Bioelectron., 2016, 80, 491–496.
16. W. Siangproh, O. Chailapakul and K. Songsrirote, Talanta, 2016, 153, 97–202.
17. M. Zaib, M. M. Athar, A. Saeed and U. Farooq, Biosens. Bioelectron., 2015, 74, 895–908.
18. N. Rahman and N. Afaq, Anal. Methods, 2010, 2, 513–518.
19. R. Sharma, A. Kamal and R. K. Mahajan, Soft Matter, 2016, 12, 1736–1749.
20. V. M. Ivanova, E. M. Adamova and V. N. Figurovskaya, J. Anal. Chem., 2010, 65, 912.
21. B. Dadpou and D. Nematollahi, J. Electrochem. Soc., 2016, 163, H559–H565.
22. D. F. Wood and R. H. McKenna, Analyst, 1962, 87, 880–883.
23. Y. J. Huang, Y. B. Jiang, J. S. Fossey, T. D. James and F. Marken, J. Mater. Chem., 2010, 20, 8305.
24. J. E. Halls, S. D. Ahn, D. Jiang, L. L. Keenan, A. D. Burrows and F. Marken, J. Electroanal. Chem., 2013, 689, 168–175.
25. H. E. Zittel and T. M. Florence, Anal. Chem., 1967, 39, 320.
26. R. A. Hamid, M. K. Rabia and H. M. El-Sagher, Bull. Chem.Soc. Jpn., 1997, 70, 2389–2397.
27. S. P. Sangal, Microchem, J., 1967, 12, 326.
28. A. K. Dey, Mikrochim. Acta, 2009, 4, 2–4.
29. F. Ding, Wei Liu, J. –X. Diao, Y. Sun and F. Ding, J. Hazard. Mater., 2011, 186, 352–359.
30. Q. G. Jiang, Y. J. Ji and Y. X. Chang, Handbook on Toxic Pollutant in Environment Analytical Chemistry, Chemical Industry Press, Beijing, 1st edn, 2004.
31. A. D. Arulraj, M. Vijayan and V. S. Vasantha, Spectrochim. Acta, Part A, 2015, 148, 355–361.
32. Y. Ni and Y. Wu, An. Chim. Acta, 1997, 354, 233–240.
33. N. B. Stanton, Analyst, 1968, 93, 802–804.
34. R. Villamil-Ramos and A. K. Yatsimirsky, Chem. Commun., 2011, 47, 2694–2696.
35. P. L. Smedley and D. G. Kinniburgh, Appl. Geochem., 2002, 17, 517–568.
36. M. A. Bhat, P. P. Ingole, V. R. Chaudhari and S. K. Haram, J. Phys. Chem. B, 2009, 113, 2848–2853.
37. S. Antonello, M. Musumeci, D. D. M. Wayner and F. Maran, J. Am. Chem. Soc., 1997, 119, 9541–9549.
38. R. L. Donkers, F. Maran, D. D. M. Wayner and M. S. Workentin, J. Am. Chem. Soc., 1999, 121, 7239–7248.
39. C. L. Dufaure, F. Najjar and C. A. Barres, J. Phys. Chem. B, 2010, 114, 9848–9853.
40. L. R. F. Allen and J. Bard, Electrochemical Methods: Fundamentals and Applications, Wiley, 2nd edn, 2000.
41. Y. Liu and S. Chen, J. Phys. Chem. C, 2012, 116, 13594–13602.
42. R. G. Compton, Understanding Voltammetry, Oxford, 2nd edn, 2011.
43. A. Muthukrishnan, V. Boyarskiy, M. V. Sangaranarayanan and I. Boyarskaya, J. Phys. Chem. C, 2012, 116, 655–664.
44. S. Antonello, R. Benassi, G. Gavioli, F. Taddei and F. Maran, J. Am. Chem. Soc., 2002, 124, 7529.
45. J. C. Imbeaux and J.-M. Saveant, J. Electroanal. Chem., 1973, 44, 169.
46. B. A. Lasia, “Electrochemical impedance spectroscopy and its applications”, in Modern Aspects of Electrochemistry, ed. B. E. Conway, J. O. M. Bockris and R. White, Springer, New York, NY, USA, 2002, pp. 143–248.
47. M. M. El Jamal, A. M. Mousaoui, D. M. Naoufal, M. A. Tabbara and A. A. El Zant, Port. Electrochim. Acta, 2014, 32, 233–242.

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