AgCl

Research Note pubs.acs.org/IECR

Development and Evaluation of a Pseudoreference Pt//Ag/AgCl Electrode for Electrochemical Systems André A. G. F. Beati, Rafael M. Reis, Robson S. Rocha, and Marcos R. V. Lanza* Instituto de Química de São Carlos, Universidade de São Paulo, CP 780, 13560-970 São Carlos, SP, Brazil ABSTRACT: The use of standard reference electrodes, such as Ag/AgCl or saturated calomel electrodes, in potentiometric and amperometric studies involving miniaturized electrochemical systems, or those operating under positive hydraulic pressure, is often impractical. Placement of the reference electrode in the direct vicinity of the working electrode is often prohibited by the dimensions or layout of the electrochemical cell, while the alternative strategy of locating the reference electrode in a separate compartment often leads to electrolyte leakage and contamination of the system. In the present study, we have investigated the functionality of a pseudoreference electrode comprising a platinum wire, one end of which was maintained in intimate contact with the internal solution of an Ag/AgCl reference electrode while the other was connected, via a BNC connector, to a platinum probe located within the electrochemical cell. Linear and cyclic voltammetric studies, involving both aqueous and nonaqueous electrolytes, were carried out using the pseudoreference electrode and an electrochemical cup-type cell with three electrodes or an electrochemical flow reactor. In all cases, the functionality of the Pt//Ag/AgCl system was similar to that of a conventional Ag/AgCl reference electrode. Variations in the electrolyte did not alter the potential or voltammetric profile recorded when using the pseudoreference system, although peak currents were generally improved and potential values shifted by approximately +350 mV in comparison with the Ag/AgCl electrode, therefore, the system pseudoreference can be applied in any electrochemical system due to the constant potential difference. It is concluded that the pseudoreference electrode can be used with advantage to obtain potentiometric and amperometric measurements in both simple and complex electrochemical systems.

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INTRODUCTION The selectivity of an electrochemical transformation in an anodic or cathodic reaction is dependent on the application of an electric potential that is specific to the process of interest. Such potentials are characteristic of defined redox couples and are documented with reference to the standard hydrogen electrode (SHE, E0 = 0.000 V) or to a standard reference electrode such as the Ag/AgCl electrode (E0 = +0.222 V vs SHE) or the saturated calomel electrode (SCE; E0 = +0.241 V vs SHE) among others.1−5 A typical reference electrode comprises stable half-cell components that provide a known and constant potential while exhibiting a rapid response to changes in external conditions. For use in electrochemical studies and potentiometric titrations, it is imperative that the reference electrode is completely insensitive to the composition of the solution under study, irrespective of the conditions under which it is operating. In typical electrochemical cells or reactors, reference electrodes are generally located within the same compartment as the working electrode since this serves to minimize ohmic drop in the system. However, the major disadvantage of this type of arrangement is that it allows exchange of electrolyte solution between the internal reference electrode and the electrolyte used for the study (which are not necessarily the same) through the permeable diaphragm of the reference electrode.6−10 It is possible to locate the reference electrode in a compartment that is physically separated from that of the working electrode by means of a permeable barrier composed of nonselective sintered glass, polymeric microporous membrane, or ion-selective membrane.2,3 Unfortunately, these methods also permit contamination of the electrolyte system © 2012 American Chemical Society

with solution from the reference electrode compartment. An alternative strategy is to employ a Luggin capillary, although it is difficult to design and build reactors that allow a constant and stable reflux of electrolyte through such a capillary without introducing sources of leaks into the system.2,3,8−12 Additional problems arise when it is necessary to employ reference electrodes with miniaturized systems, such as the electrochemical microcells used in flow-injection analysis, or with those operating under positive hydraulic pressure, as is the case with filter-press electrochemical reactors. Ideally, a reference electrode should be located in the space between the working electrodes and the counter electrode in order to monitor the potential, but this is practically impossible to achieve in such systems. A number of attempts have been made to find a solution to these problems. In a study aimed at improving the efficiency of electrochemical processes, the monitoring of electrochemical potential of the system is extremely important. Since it makes use of an equilibrium approach, where the current through the cell is infinitesimal, and the reaction occurs through small passages of charge through the electrodes in regions close to the electrodes, the kinetics takes other features, since it then depends on surface phenomena, which necessarily involves the notion of surface energy. The case of an electrochemical cell, depending on the complexity of these interactions, does not necessarily follow Ohm’s law, that is, the current is proportional to the voltage Received: Revised: Accepted: Published: 5367

November 15, 2011 March 25, 2012 March 27, 2012 March 27, 2012 dx.doi.org/10.1021/ie2026025 | Ind. Eng. Chem. Res. 2012, 51, 5367−5371

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Research Note

approximately 3 mm apart from, a gas diffusion electrode (GDE; 60 mm diameter) as working electrode (Figure 2a). A constant flow of electrolyte was provided by means of a reservoir, hydraulic pump, and flow meter, and aliquots of the test solution could be removed by means of a sampling system (Figure 2b). The pseudoreference electrode was located as

applied to the cell, so that the readings of the potential parameters in the electrochemical systems are what enable you to guide the involvement of the redox reactions.8−16 The aim of the present study was to verify the functionality of a Pt//Ag/AgCl pseudoreference electrode for use in electrochemical systems that offer no or limited access to conventional reference electrodes. Since the Pt//Ag/AgCl system can be applied as a reference electrode in the majority of electrochemical techniques, including potentiometry and amperometry, it could be used to monitor redox reactions in order to establish conditions for electrochemical reactions or to describe such reactions in a less empirical manner.

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MATERIALS AND METHODS The Pt//Ag/AgCl pseudoreference electrode consisted of a standard reference electrode, type nonpolarizable with negligible current passing through them. The Ag/AgCl-coated wire of which was encircled by a spiral of platinum wire equipped with a BNC connector at the external end. The functionality of the Pt//Ag/AgCl system was investigated using a 250 mL capacity electrochemical cell and a 1.5 L capacity electrochemical reactor. All voltammetric experiments were conducted using an Autolab PGSTAT-302 potentiostat/ galvanostat. Cyclic voltammetric analyses were performed with an electrochemical cup-type cell fitted with a platinum or glassy carbon rotating disk working electrode (3 mm diameter; Metrohm, part numbers 6.1204.300 and 6.1204.310), a platinum screen counter electrode, and either an Ag/AgCl reference electrode (Figure 1a) or a Pt//Ag/AgCl pseudor-

Figure 1. Experimental setup employing an electrochemical cup-type cell (capacity 250 mL) fitted with (a) an Ag/AgCl reference electrode and (b) a Pt//Ag/AgCl pseudoreference electrode, showing [A] support for platinum or glassy carbon working electrode, [B] reference electrode, [C] platinum screen counter electrode, and [D] the pseudoreference electrode system connected via BNC connector.

eference electrode (Figure 1b). The test solutions were (i) K4[Fe(CN)6] 5 mmol L−1 and K3[Fe(CN)6] 5 mmol L−1 in phosphate buffer at pH 5, (ii) K2SO4 0.1 mol L−1, and (iii) 2ethyl anthraquinone 0.05 mol L−1 and NaClO4 0.1 mol L−1 in dimethylformamide (DMF). Aqueous test solutions were deoxygenated with a nitrogen stream for 20 min prior to analysis. Linear voltammetric analyses were conducted in a single compartment electrochemical reactor encompassing a dimensionally stable anode (DSA Cl2; 60 mm diameter; De Nora Do Brasil) as counter electrode arranged in parallel with, and

Figure 2. (a) Experimental setup employing a single compartment electrochemical reactor (capacity 1.5 L) showing the electrochemical reactor [1] equipped with flow meter [2], reservoir [3], sampling system [4], and hydraulic pump [5]. (b)Schematic representation of the electrochemical reactor compartment of a pseudoreference system. (c) Schematic of an electrochemical reactor compartment open showing the inside of the plate GDE faceted by wire platinum system pseudoreference. 5368

dx.doi.org/10.1021/ie2026025 | Ind. Eng. Chem. Res. 2012, 51, 5367−5371


Research Note

close as possible to the working electrode, without making contact with the surface (Figure 2c), in order to reduce the effect of ohmic drop across the platinum wire and, thus, to improve charge transfer.

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RESULTS AND DISCUSSION The functionality of the pseudoreference electrode was initially determined by conducting cyclic voltammetry of the Fe2+/Fe3+ redox couple in buffered medium using a three-electrode cuptype cell fitted with a glassy carbon rotating disk working electrode, a platinum counter electrode, and either an Ag/AgCl reference electrode or the Pt//Ag/AgCl system. Figure 3 shows Figure 4. Cyclic voltammetric analyses of K4[Fe(CN)6] 5 mmol L−1 and K3[Fe(CN)6] 5 mmol L−1 in phosphate buffer at pH 5, performed in an electrochemical cup-type cell with a glassy carbon working electrode, a platinum screen counter electrode and at scan rates in the range 5 mV s−1 ≤ v ≤ 150 mV s−1, showing cycles obtained with (1) Ag/AgCl reference electrode (RE), and (2) Pt//Ag/AgCl pseudoreference electrode (PR).

of electrolyte and to the nature of the working electrode. Figure 5 shows the cyclic voltammograms obtained using reference

Figure 3. Cyclic voltammetric analyses of K4[Fe(CN)6] 5 mmol L−1 and K3[Fe(CN)6] 5 mmol L−1 in phosphate buffer at pH 5, performed in an electrochemical cup-type cell with a glassy carbon working electrode, a platinum screen counter electrode, and at a scan rate of 20 mV s−1, showing the 1st and 50th cycles obtained with (1) Ag/AgCl reference electrode (RE), and (2) Pt//Ag/AgCl pseudoreference electrode (PR).

cyclic voltammograms obtained with the two electrode systems at a scan rate of 20 mV s−1 for the first and the 50th cycles. With the reference electrode, each scan commenced at an initial potential of −0.05 V vs Ag/AgCl and was reversed at 0.5 V vs Ag/AgCl, while for the pseudoreference system the initial potential was 0.35 V vs Pt//Ag/AgCl and was reversed at 0.91 V vs Pt//Ag/AgCl. The voltammograms show that the dynamic equilibrium of the redox pair Fe2+/Fe3 shifted approximately 350 mV toward more positive potential when measured using the pseudoreference system in comparison with the standard reference electrode. However, the Pt//Ag/AgCl electrode responded perfectly in terms of stability and reproducibility over 50 complete cycles. To verify the functionality of the pseudoreference system under conditions of hydrodynamic disturbance, cyclic voltammograms were recorded for the Fe2+/Fe3+ redox couple in buffered medium using scan rates in the range 5 mV s−1 ≤ v ≤ 150 mV s−1. The expected amplification of intensity of the current peaks with increased scan rate was observed with the standard reference as described in the literature2,4 and also observed in the pseudoreference electrode (Figure 4). Moreover, the Pt//Ag/AgCl system achieved voltammetric responses that were similar to those of the reference standard, but with a shift of approximately 350 mV toward more positive potential in relation to the Ag/AgCl electrode. In further experiments designed to verify the functionality and behavior of the pseudoreference electrode system with the electrochemical cell (Figure 1), changes were made to the type

Figure 5. Cyclic voltammetric analyses of 2-ethyl anthraquinone 0.05 mol L−1 and NaClO4 0.1 mol L−1 in dimethylformamide (DMF), performed in an electrochemical cup-type cell with a glassy carbon working electrode, a platinum screen counter electrode, and at a scan rate of 20 mV s−1, showing the 1st, 50th, and 100th cycles obtained with (1) Ag/AgCl reference electrode (RE) and (2) Pt//Ag/AgCl pseudoreference electrode (PR).

and pseudoreference electrodes with an organic electrolyte (2ethyl anthraquinone and NaClO4 in DMF) and a glassy carbon working electrode. Appropriate windows for the investigation of the known reversible redox reactions of the anthraquinone were determined for the reference and pseudoreference electrodes by cyclic voltammetry over 100 cycles at a scan rate 20 mV s−1. The Pt//Ag/AgCl electrode gave reproducible results in which the current peaks representing the redox couples in 2-ethyl anthraquinone were highly stable and similar to those obtained using the standard electrode, with ΔEp of the peaks (I) and (I′) with the peaks (III) and (III′) at 350 mV and the peaks (II) and (II′) with the peaks (IV) and (IV′) at 400 mV, showing that the use of commercial electrode reference or pseudoreference system did not change the ΔEp between peaks of the redox species of 2-ethyl anthraquinone. Figure 6 shows the 1st, 25th, and 50th cycles of voltammograms obtained at a scan rate of 20 mV s−1 using reference and pseudoreference 5369



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near to zero at about −0.78 V vs Pt//Ag/AgCl and decreased to more negative values at lower potentials. Replacing N2 for O2 gave rise to significant deviations in cathode current at all potentials, and these attained ∼600 mA at −2.0 V vs Pt//Ag/ AgCl. Figure 7 also shows that, at less negative potentials, there was an oxidation current that would not only be inappropriate for the synthesis of H2O2 but could also damage the electrode. Plotting the difference between the linear voltammograms recorded with the GDE pressurized with O2 and N2 (Figure 8)

Figure 6. Cyclic voltammetric analyses of an aqueous solution of K2SO4 (0.1 mol L−1) performed in an electrochemical cup-type cell with a platinum working electrode, a platinum screen counter electrode, and at a scan rate of 20 mV s−1, showing the 1st, 25th, and 50th cycles obtained with (1) Ag/AgCl reference electrode (RE), and (2) Pt//Ag/AgCl pseudoreference electrode (PR).

electrodes with an aqueous solution of K2SO4 (0.1 mol L−1) and a platinum working electrode, as shown in Figure 1. Once again the profiles produced by the standard Ag/AgCl and the pseudoreference electrodes were similar and reproducible. The Pt//Ag/AgCl pseudoreference electrode was finally tested for functionality in a complex electrochemical system comprising a single compartment electrochemical reactor encompassing a GDE working electrode powered by O2 and a DSA as counter electrode, as shown in Figure 2c. Figure 7

Figure 8. Difference plot of the linear voltammograms recorded with the GDE of the electrochemical reactor pressurized with O2 and N2 and monitored using a Pt//Ag/AgCl pseudoreference electrode. Experimental conditions were as shown in Figure 9.

allows the effect on cathode current associated with the reduction of O2 to H2O2 to be observed in the absence of competing effects occasioned by the reduction of O2 to water or H2. The linear voltammogram relating to the synthesis of H2O2 obtained using the pseudoreference electrode confirms that the Pt//Ag/AgCl system can be employed to study complex electrochemical processes that require potentiometric monitoring in situ.

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CONCLUSION We have demonstrated that the Pt//Ag/AgCl pseudoreference electrode system can be used to obtain potentiometric measurements in both simple and complex electrochemical systems with a functionality that is similar to that of a conventional Ag/AgCl reference electrode. Variations in the electrolyte in the electrochemical cell did not alter the potential or voltammetric profile recorded using the Pt//Ag/AgCl system, although peak currents were generally improved in comparison with the Ag/AgCl electrode. Potential values provided by the pseudoreference electrode were shifted by approximately +350 mV compared with the standard reference electrode, thus setting a new dynamic equilibrium for this system. The pseudoreference system will be particularly advantageous for use with miniaturized cells or electrochemical reactors in which it is impracticable to employ a reference electrode in the immediate vicinity of the working electrode. Moreover, the ability to employ the Pt//Ag/AgCl electrode in a compartment different from, and not connected to, that of the working electrode will eliminate the possibility of system contamination through leakage from the reference electrode. It is also worth noting that the pseudoreference system described herein can be adopted for application to other conventional reference electrodes such as the saturated calomel electrode.

Figure 7. Linear voltammetric analyses performed in a single compartment electrochemical reactor with a GDE cathode, a DSA anode, and a Pt//Ag/AgCl pseudoreference electrode. The electrolyte comprised an aqueous solution of K2SO4 (0.1 mol L−1) supplied at a flow rate of 150 L h−1, and the GDE was insufflated with N2 () or O2 (---) at a pressure of 0.2 kgf cm−2. The scan rate was 20 mV s−1 and the voltammograms presented were obtained following 30 cycles.

presents linear voltammograms obtained at a scan speed of 20 mV s−1 using this system with an aqueous solution of K2SO4 (0.1 mol L−1) as electrolyte supplied at a flow rate of 150 L h−1, and a constant pressure (0.2 kgf cm−2) with O2 and N2 purge applied to the GDE. The profiles recorded with the two gases differ since O2 was reduced to H2O2 when the GDE was powered with O2, whereas reduction of O2 to water or H2 occurred when the electrode was purged with N2. These differences were manifest in a variation in cathode current during linear voltammetry which, in the presence of N2, was 5370



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(15) González-García, J.; Montiel, V.; Aldaz, A.; Conesa, J. A.; Pérez, J. R.; Codina, G. Hydrodynamic behavior of a filter-press electrochemical reactor with carbon felt as a three-dimensional electrode. Ind. Eng. Chem. Res. 1998, 37, 4501. (16) González-García, J.; Frias, A.; Expósito, E.; Montiel, V.; Aldaz, A.; Conesa, J. A. Characterization of an electrochemical pilot-plant filter-press reactor by hydrodynamic and mass transport studies. Ind. Eng. Chem. Res. 2000, 39, 1132.

Due to the characteristics of the pseudoreference system, the application of this new system of reference has the advantage of not contaminating the electrolyte, be easy implementation and have a known behavior with a fixed shift of +350 mV. With these characteristics, the pseudoreference system allows the use of different designs of electrochemical flow reactors or electrochemical cells with reduced size, in experiments with potential control versus reference system, applications not permitted with conventional reference electrodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the following Brazilian Funding Institutions for financial support: Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP), ́ Superior Coordenaçaõ de Aperfeiçoamento de Pessoal de Nivel (CAPES), and Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq).

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REFERENCES

(1) Bard, L. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2001. (2) Pletcher, D. A First Course in Electrode Processes; The Electrochemical Consultancy: Romsey, UK, 1991. (3) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robison, J. Instrumental Methods in Electrochemistry; Ellis Horwood: Chichester, UK, 1990. (4) Pletcher, D.; Walsh, F. C. Industrial Electrochemistry; Blackie A&P: Glasgow, UK, 1993. (5) Harris, D. C. Quantitative Chemical Analysis; W.H. Freeman & Company: WA, 2010. (6) Hine, F. Electrode Processes and Electrochemical Engineering; Plenum Press: New York, 1983. (7) Trassati, S. Electrocatalysis: understanding the success of DSA. Electrochim. Acta 2000, 45, 2377. (8) Cañizares, P.; García-Gómez, J.; Sáez, C.; Rodrigo, M. A. Electrochemical oxidation of several chlorophenols on diamond electrodes. Part I. Reaction mechanism. J. Appl. Electrochem. 2003, 33, 917. (9) Polcaro, A. M.; Vacca, A.; Palmas, S.; Mascia, M. Investigation of transport phenomena in a hybrid ion exchange-electrodialysis system for the removal of copper ions. J. Appl. Electrochem. 2003, 33, 885. (10) Da Silva, A. I.; Araújo Filho, H. C.; Silva, R. C. Testes de desempenho de eletrodos: Eletrodos de referência. Quim. Nova 2000, 23, 512. (11) Forti, J. C.; Rocha, R. S.; Lanza, M. R. V.; Bertazzoli, R. Electrochemical synthesis of hydrogen peroxide on oxygen-fed grafite/ PTFE electrodes modified by 2-ethylanthraquinone. J. Electroanal. Chem. 2007, 601, 63. (12) Reis, R. M.; Beati, A. A. G. F.; Rocha, R. S.; Assumpçaõ , M. H. M. T.; Santos, M. S.; Bertazzoli, R.; Lanza, M. R. V. Use of gas diffusion electrode for the in situ generation of hydrogen peroxide in an electrochemical flow-by rector. Ind. Eng. Chem. Res. 2012, 51, 649. (13) Rocha, R. S.; Beati, A. A. G. F.; Oliveira, J. G.; Lanza, M. R. V. Avaliaçaõ da degradaçaõ do diclofenaco sódico utilizando H2O2/ ́ Fenton em reator electroquimico. Quim. Nova. 2009, 32, 254. (14) Beati, A. A. G. F.; Rocha, R. S.; Oliveira, J. G.; Lanza, M. R. V. Estudo da degradaçaõ de ranitidina via H2O2 eletrogerado/Fenton em ́ um reator eletroquimico com eletrodos de difusão gasosa. Quim. Nova. 2009, 32, 125. 5371


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