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Amperometric sensor for thiosulphate based on cobalt hexacyanoferrate modified electrode
Sensors and Actuators B 86 (2002) 180–184
Amperometric sensor for thiosulphate based on cobalt hexacyanoferrate modified electrode D. Ravi Shankaran*, S. Sriman Narayanan Department of Analytical Chemistry, University of Madras, Guindy Campus, Chennai 600025, India Received 23 January 2002; accepted 12 April 2002
Abstract A chemically modified amperometric sensor was constructed by mechanically immobilizing cobalt hexacyanoferrate onto the surface of a paraffin impregnated graphite electrode (PIGE). The cyclic voltammogram of the electrode exhibited two reversible redox peaks corresponding to Co2þ/Co3þ and Fe(CN)64/Fe(CN)63 reactions. Thiosulphate was found to undergo electrocatalytic oxidation at a reduced overpotential with good sensitivity in a wider pH range at the modified electrode. The sensor showed good stability and reproducibility. Under optimal condition, the sensor showed a linear response for thiosulphate in the concentration range from 4 105 to 4:8 104 M with a correlation coefficient of 0.9972 and the detection limit was 1:8 105 M (S/N ¼ 3). The sensor has been applied for the determination of thiosulphate in the photographic effluents. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Modified electrode; Mechanical immobilization; Electrocatalysis; Thiosulphate; Cobalt hexacyanoferrate
1. Introduction Over the past two decades, there has been a great deal of interest in the construction of potential chemical sensors for the analytical determination of compounds of biological, medicinal and environmental interest. Chemical sensors are analytical devices that are reliable, sensitive, robust, portable, easy to use and economical. Furthermore, they require only minimum sample treatment and conditioning. A variety of techniques have been exploited to prepare chemically modified electrodes such as adsorption, covalent bonding, polymer coating, sol–gel, screen-printing, and composites [1– 6]. In most cases, the surface modification involves tedious process resulting in poor stability and perturbed functions. Thus, there is a continuing interest in the fabrication of sensors with simple and reliable immobilization procedures. Recently, the technique of mechanical immobilization has opened a new area for the easy and effective construction of electrochemical sensors [7–9]. The mechanical immobilization of the mediator onto the electrode surface is a * Corresponding author. Present address: Satellite Venture Business Laboratory, Department of Applied Chemistry, Utsunomiya University, Yoto-7-1-2, Utsunomiya 321-8585, Japan. Tel.: þ81-28-689-6172(O), 663-4144(R); fax: þ81-28-689-6179. E-mail addresses: [email protected], [email protected] (D. Ravi Shankaran).
promising alternative to other modification methods owing to its simplicity, low cost and rapid. Despite the number of studies based on this approach, the analytical application for the determination of compounds of interest is rather limited [10,11]. In our laboratory, we have developed modified electrodes by mechanical immobilization using copper and nickel hexacyanoferrates for the determination of ascorbic acid [12], sulphur dioxide [13] and thiosulphate [14]. Thiosulphate is a prime pollutant resulting from photographic industry. It is, therefore, necessary to develop a sensitive, fast, inexpensive and user-friendly analytical method for the determination of thiosulphate. Despite different analytical methods for thiosulphate determination, the electrochemical sensors received increasing attention in recent years since they can offer a viable, promising and low cost solution to thiosulphate monitoring [14–16]. In this work, we describe a chemically modified electrode constructed by mechanical immobilization using cobalt hexacyanoferrate (CoHCF) as the mediator. CoHCF is an important member of the mixed valance metal hexacyanoferrates having similar characteristics as that of Prussian blue (PB). It has interesting chemical and electrochemical properties [9,17,18]. It does not undergo dissolution upon reduction or oxidation as the electrolyte ion diffuse in and out of the zeolitic structure of the compound to maintain charge neutrality. Thus, this material is an ideal candidate for electrode modification by mechanical immobilization. The
0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 1 7 1 - 5
D. Ravi Shankaran, S. Sriman Narayanan / Sensors and Actuators B 86 (2002) 180–184
modified electrode was characterized by cyclic voltammetry and used for the determination of thiosulphate. The analytical utility of the electrode for the determination of thiosulphate in photographic effluents was also studied.
2. Experimental 2.1. Reagents and apparatus Cobalt nitrate, potassium ferrocyanide and thiosulphate were received from Sigma (USA). All other chemicals and reagents used were of analytical grade. Double distilled water was used to prepare all the solutions. The pH of the solutions were adjusted using HCl, NaOH, Na2HPO4, NaH2PO4. The electrochemical experiments were carried out using EG & G PAR Electrochemical System (Model 263A) equipped with GPIP (IEEE-488) interface port and IBM personal computer. The CoHCF modified electrode was used as working electrode, while Ag/AgCl (saturated KCl) and a platinum wire as reference and counter electrode, respectively. All measurements were carried out under an atmosphere of high purity nitrogen. A magnetic stirrer and Teflon coated magnetic bar were used in hydrodynamic experiments. Hydrodynamic volatmmograms were registered by measuring the current response at a fixed interval of applied potential under the stirring condition of the electrolyte solution.
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2.4. Procedure The CoHCF modified electrode was scanned in 0.1 M NaNO3 between 0 and 1 V. The behavior of electrode at different pH was studied by varying the electrolyte pH using NaOH and HCl. The effect of scan rate and electrolyte variation has also been examined. For electrocatalytic studies fresh solution of thiosulphate was prepared in deareated double distilled water. A 0.05 M phosphate buffer was used for maintaining the pH during the electrocatalytic oxidation studies. Aliquots of analyte were added to the supporting electrolyte for quantitative analysis and calibration. Thiosulphate samples were received from photographic studio, filtered and diluted with double distilled water and used for the determination.
3. Results and discussion The cyclic voltammograms of the cobalt hexacyanoferrate modified electrode obtained in 0.1 M NaNO3 (A) and KNO3 (B) at a potential scan rate of 10 mV/s are shown in Fig. 1. In NaNO3 medium, the cyclic voltammogram exhibits two sets of reversible redox peaks with formal potentials (Em ¼ ðEp;a þ Ep;c Þ/2) of about 0.34 and 0.83 V. The first
2.2. Preparation of the complex The cobalt hexacyanoferrate complex was prepared by precipitation via dropwise addition of 100 ml of 0.1 M Co(NO3)2 to a well-stirred solution of 100 ml of 0.1 M K4Fe(CN)6 in a beaker. After the addition, the mixture was stirred for 15 min and kept undisturbed for 1 h. The precipitate formed was filtered, washed first with 0.1 M KNO3 solution and then with portions of double distilled water. The olive-brown CoHCF precipitate formed was dried at room temperature under a vacuum and then crushed and milled to a fine crystalline powder. 2.3. Construction of modified electrode Paraffin impregnated graphite electrodes (PIGEs) with a circular surface diameter of 3mm were used for electrode preparation. The PIGEs were prepared by immersing soft graphite rods into molten paraffin wax under vacuum until air bubble cease to evolve from the rods. After re-establishing atmospheric pressure, the rods are removed before paraffin solidifies. The lower end of the electrode is carefully polished on smooth paper and is used for immobilization of the complex. The modified electrode was prepared [7] by uniformly rubbing PIGE onto the CoHCF complex placed on a smooth glass plate. The surface of the electrode was then rinsed well with double distilled water.
Fig. 1. Cyclic voltammograms of CoHCF modified electrode in (A) 0.1 M NaNO3 and (B) 0.1 M KNO3: scan rate, 10 mV/s.
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peak at lower potential was assigned to Co2þ/Co3þ transition by analogy with the case of the PB modified electrode [19]. The assumption is also supported by the reports of cobalt oxidation peak observed in cobalt phthalocyanine [20]. The redox peak at higher potential was assigned to Fe(CN)64/Fe(CN)63 reaction by comparing the redox potentials of low spin iron transition in PB. In KNO3 medium, the CoHCF modified electrode exhibits two anodic peaks at potentials of 0.58 and 0.69 V and two cathodic peaks at 0.65 and 0.32 V. The different CV behavior in NaNO3 and KNO3 solution is due to the role of cation, which diffuses into and out of the film during redox reactions. Continuous cycling of the electrode in both 0.1 M NaNO3 and 0.1 M KNO3 exhibited constant response for nearly 100 cycles suggesting the stability of the modified electrode. The modified electrode has a wide working pH range from 2 to 8 (figure not shown). A decrease in response was observed at higher pHs, which may be due to the possible hydroxylation of the CoHCF film on the electrode surface in alkaline conditions. The peak currents of the both the redox peaks are linearly depends on the square root of the potential scan rate in the range from 10 to 200 mV/s, suggesting that the redox reaction was diffusion controlled. Fig. 2 compares the electrocatalytic activity of the CoHCF modified electrode and the bare graphite electrode towards the oxidation of thiosulphate in 0.1 M NaNO3 (A) and 0.1 M KNO3 (B). In the figure, the curves (a) and (c) corresponds to bare and modified electrode, respectively without thiosulphate. When 4:4 104 M thiosulphate was added to the electrolyte solution, a very small increase in anodic current at higher potential was observed (curve b) with the bare electrode. However, a remarkable increase in the anodic current at 0.70 V was observed at the modified electrode (curve d) indicating the electrocatalytic oxidation of thiosulphate. An increase in the anodic peak current with a decrease in the cathodic current was observed for each addition of thiosulphate and the catalytic peak current was linear with the square root of the scan rate indicating that the electrocatalytic process was diffusion controlled. As the electrocatalytic oxidation of thiosulphate showed an increase in current response at a potential before the start of the Fe(CN)64/Fe(CN)63 reaction and within the proximity of the Co2þ/Co3þ reaction, we believe that the electrocatalytic activity of the thiosulphate at the CoHCF modified electrode depends on the Co2þ/Co3þ reaction and not on the Fe(CN)64/Fe(CN)63 system [18]. This observation is rather interesting because the Fe(CN)64/ Fe(CN)63 reaction was involved in the electrocatalysis in most of the hexacyanoferrate based modified electrodes [3,11–15,17]. However, in the present system, the involvement of the Fe(CN)64/Fe(CN)63 reaction in electrocatalysis is uncertain at present. One possible reason may be because of the Co2þ/Co3þ reaction is more favored than the Fe(CN)64/Fe(CN)63 reaction, and, hence, the thiosulphate may get oxidized faster by Co3þ when it reaches the electrode surface.
Fig. 2. Cyclic voltammograms of (a) bare electrode, (b) 4:4 104 M thiosulphate at bare electrode, (c) CoHCF modified electrode and (d) 4:4 104 M thiosulphate at modified electrode. Electrolyte: (A) 0.1 M NaNO3 and (B) 0.1 M KNO3; pH 6 (0.05 M phosphate buffer); scan rate, 20 mV/s.
The pH dependence of the electrocatalytic oxidation was evaluated and it was found that there was no appreciable change in the electrocatalytic behavior in the pH range of 2–8. However, the catalytic current decreased with the increase of pH after 8, as depicted in Fig. 3. The possible
Fig. 3. Effect of pH on the anodic current for the catalytic oxidation of 2 104 M thiosulphate: scan rate, 20 mV/s; electrolyte, 0.1 M NaNO3.
D. Ravi Shankaran, S. Sriman Narayanan / Sensors and Actuators B 86 (2002) 180–184
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Fig. 6. Calibration plot for the determination of thiosulphate. Fig. 4. Hydrodynamic voltammograms of 4:4 104 M thiosulphate at (a) CoHCF modified and (b) bare electrode: electrolyte, 0.1 M NaNO3; pH 6; stirring rate, 300 rpm.
reason may be as indicated due to the hydroxylation of the mediator in alkaline conditions. Thus, a pH of 6 was used in the electrocatalytic oxidation studies. To study the applicability of the modified electrode as an amperometric sensor in flow systems, hydrodynamic voltammetric experiments have been carried out for the electrocatalytic oxidation of thiosulphate. The hydrodynamic voltammograms obtained for the electrocatalytic oxidation of 4:4 104 M thiosulphate at modified (curve a) and at bare electrode (curve b) are shown in Fig. 4. As expected, the electrocatalytic activity of the CoHCF permits the convenient detection of thiosuphate at lower potential with higher sensitivity. It shows similar response as that of cyclic voltammetric curves with the anodic peak currents increasing with potential and reaching a limiting value at 0.7 V at the modified electrode. In contrast, the unmodified electrode offers detection of the thiosulphate only at higher potentials with lesser sensitivity. Thus, a potential of 0.7 V was applied during the amperometric determination of thiosulphate. The CoHCF modified electrode exhibit good stability. The response of the modified electrode showed no observable change for nearly 11 days and 92% response was retained up to 1 month, when stored in air or dipped in the electrolyte solution. The stability of the electrode under continuous stirring condition was also examined. Fig. 5 shows the response of the CoHCF modified electrode for
4 104 M thiosulphate measured at 30 min interval for 6 h under hydrodynamic conditions. It is seen from the figure that the modified electrode retains good response in hydrodynamic conditions for long time, suggesting its possible application in flow systems for on-line monitoring of thiosulphate. A linear calibration range from 4 105 to 4:8 104 M thiosulphate with a correlation coefficient of 0.9972 was observed with the modified electrode (Fig. 6). The detection limit was 1:8 105 M (S/N ¼ 3). The precision of the method was evaluated for 10 successive measurements of 2 104 M thiosulphate and the R.S.D. was found to be 2.44%. The electrode-to-electrode reproducibility of the modified electrode was examined by cyclic voltammetry, the R.S.D. of the peak current was 3.7%. The long time stability and the lower working potential are the substantial advantage for practical application.
4. Analysis of thiosulphate in photographic effluents In order to evaluate the application of the method for the real sample analysis, the modified electrode was used for the determination of thiosulphate in photographic effluents. The results obtained are given in Table 1. The result suggests that the present method is comparable with the standard method but has the advantage of being simple and fast without the need for much reagents for the determination. The influence of some excipients commonly present with thiosulphate was analyzed. The influence of the interfering substance was evaluated at an interferent/thiosulphate concentration ratio of 1:1. It was observed from the experiments that most of the species such as Ni2þ, Ca2þ, Mg2þ, Agþ, Br and Cl, etc. did not interfere with the determination. Table 1 Results of the analysis of photographic effluents Sample
4
Fig. 5. CoHCF modified electrode response for 4 10 M thiosulphate vs. time: electrolyte, 0.1 M NaNO3; pH 6; stirring rate, 300 rpm.
Color film fixing waste Black and white film fixing waste
Thiosulphate (mg/l) Present method mean (n ¼ 5)
Iodimetry
34.23 40.56
34.19 41.24
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5. Conclusion It has been shown that the CoHCF modified electrode constructed by mechanical immobilization exhibited good and favorable characteristics for the determination of thiosulphate. The modified electrode can overcome the kinetic limitation for thiosulphate oxidation at bare electrode by a catalytic process and can lower the overpotential for the oxidation reaction. The advantages of ease of construction, low cost, reliable sensitivity, remarkable stability and good reproducibility of the sensor suggests its promising application for on-line monitoring of thiosulphate.
Acknowledgements One of the authors (DR) acknowledges the Council of Scientific and Industrial Research, New Delhi for providing Senior Research Fellowship and the other (SSN) acknowledges University Grants Commission for providing financial assistance in the form of major research project. References [1] R.P. Baldwin, K.N. Thomsen, Chemically modified electrodes in liquid chromatography detection: a review, Talanta 38 (1991) 1–16. [2] S.A. Wring, J.P. Hart, Chemically modified carbon-based electrodes and their application as electrochemical sensors for the analysis of biologically important compounds, Analyst 117 (1992) 1215–1229. [3] J.A. Cox, M.E. Tess, T.E. Cummings, Electroanalytical methods based on modified electrodes: a review of recent advances, Rev. Anal. Chem. 15 (1996) 173–223. [4] J.P. Hart, S. A Wring, Recent developments in the design and application of screen-printed electrochemical sensors for biomedical, environmental and industrial analyses, Trends Anal. Chem. 16 (1997) 89–103. [5] J. Lin, C.W. Brown, Sol–gel glass as a matrix for chemical and biochemical sensing, Trends Anal. Chem. 16 (1997) 200–211. [6] J.J. Gooding, D.B. Hibbert, The application of alkanethiol selfassembled monolayers to enzyme electrodes, Trends Anal. Chem. 18 (1999) 525–533. [7] F. Scholz, B. Meyer, Electrochemical solid state analysis: state of the art, Chem. Soc. Rev. (1994) 341–347. [8] F. Scholz, B. Meyer, in: A.J. Bard, I. Rubinstein (Eds.), Electroanalytical Chemistry—A Series of Advances, Voltammetry of Solid Microparticles Immobilised on Electrode Surfaces, Vol. 20, Marcel Dekker, New York, NY, 1998, pp. 1–86. [9] P.J. Kulesza, M.A. Malik, M. Berrettoni, M. Giorgetti, S. Zamponi, R. Schmidt, R. Marassi, Electrochemical charging, counter cation accomodation, and spectrochemical identity of microcrystalline solid cobalt hexacyanoferrate, J. Phys. Chem. B 102 (1998) 1870–1876. [10] A. Dostal, B. Meyer, F. Scholz, U. Schroder, A.M. Bond, F. Marken, S.J. Shaw, Electrochemical study of microcrystalline solid Prussian blue particles mechanically attached to graphite and gold electrodes: electrochemically induced lattice reconstruction, J. Phys. Chem. 99 (1995) 2096–2103. [11] S. Sriman Narayanan, F. Scholz, A comparative study of the electrocatalytic activities of some metal hexacyanoferrates for the oxidation of hydrazine, Electroanalysis 11 (1999) 465–469.
[12] D. Ravi Shankaran, S. Sriman Narayanan, Characterisation and application of an electrode modified by mechanically immobilised copper hexacyanoferrate, Fresenius J. Anal. Chem. 364 (1999) 686– 689. [13] D. Ravi Shankaran, S. Sriman Narayanan, Chemically modified sensor for amperometric determination of sulphur dioxide, Sens. Actuators B 55 (1999) 191–194. [14] D. Ravi Shankaran, S. Sriman Narayanan, Evaluation of mechanically immobilised nickel hexacyanoferrate electrode as an amperometric sensor for thiosulphate determination, Fresenius J. Anal. Chem. 365 (1999) 663–665. [15] X. Zhou, S. Wang, Z. Wang, M. Jiang, Electrocatalytic oxidation of thiosulfate on a modified nickel hexacyanoferrate film electrode, Fresenius J. Anal. Chem. 345 (1993) 424–427. [16] A.H. Liu, L. Xu, T. Li, S.I. Dong, E.K. Wang, Electrocatalytic oxidation and ion-chromatographic detection of bromide, iodide, sulfite, thiosulfate and thiocyanate at a platinum particle-based vitreous carbon modified electrode, J. Chromatogr. A 699 (1995) 39–47. [17] G. Shi, J. Lu, F. Xu, W. Sun, L. Jin, K. Yamamoto, S. Tao, J. Jin, Determination of glutathione in vivo by microdialysis using liquid chromatography with a cobalt hexacyanoferrate chemically modified electrode, Anal. Chim. Acta 391 (1999) 307–313. [18] S.M. Golabi, F.N. Mohammadi, Electrocatalytic oxidation of hydrazine at cobalt hexacyanoferrate modified glassy carbon, Pt and Au electrodes, J. Solid State Electrochem. 2 (1998) 30–37. [19] K. Itaya, T. Ataka, S. Toshima, Spectroelectrochemistry and electrochemical preparation method of Prussian blue modified electrodes, J. Am. Chem. Soc. 104 (1982) 4767–4772. [20] M.K. Halbert, R.P. Baldwin, Electrocatalytic and analytical response of cobalt phthalocyanine containing carbon paste electrodes toward sulfhydryl compounds, Anal. Chem. 57 (1985) 591–595.
Biographies D. Ravi Shankaran was born in Katary village, The Nilgirs, India, in 1972. He received BSc, MSc, and BEd degrees form Sri Ramakrishna Mission Vidyalaya, Bharathiar University. In July 2000, he obtained PhD from the Department of Analytical Chemistry, University of Madras for his research on ‘Mechanically immobilized modified electrodes for amperometric determination of compounds of biological and environmental interest’. He worked as a post-doctoral researcher in the group of Prof. Yoon-Bo Shim at the Department of Chemistry, Pusan National University, Korea. Presently, he is a post-doctoral researcher in the group of Prof. Teiji Kato at the Satellite Venture Business Laboratory, Department of Applied Chemistry, Utsunomiya University, Japan. He is engaged in the fabrication of new metal-dispersed amperometric chemical and biosensors based on the self-assembly, sol–gel biocomposite and layer-by-layer assembly techniques. S. Sriman Narayanan was born in Tamilnadu, India, in 1955. He received MSc degree in Analytical Chemistry from the University of Madras, in 1979. In 1983, he received PhD degree from Indian Institute of Technology, Madras for his research on ‘The development of radiorelease techniques for the determination of important gaseous air pollutants’. He joined as a lecturer in the Department of Analytical Chemistry at the University of Madras, in 1985 and he is presently professor in Analytical Chemistry since 1998. He has worked with Prof. Fritz Scholz at the Institute of Chemistry, Humboldt University at Berlin, Germany and Prof. James Hickman at the Department of Bioengineering, Clemson University, USA. His current interest is in the development of chemical and biosensor for biological and environmental analysis and self-assembled monolayers for trace metal analysis.
Amperometric sensor for thiosulphate based on cobalt hexacyanoferrate modified electrode D. Ravi Shankaran*, S. Sriman Narayanan Department of Analytical Chemistry, University of Madras, Guindy Campus, Chennai 600025, India Received 23 January 2002; accepted 12 April 2002
Abstract A chemically modified amperometric sensor was constructed by mechanically immobilizing cobalt hexacyanoferrate onto the surface of a paraffin impregnated graphite electrode (PIGE). The cyclic voltammogram of the electrode exhibited two reversible redox peaks corresponding to Co2þ/Co3þ and Fe(CN)64/Fe(CN)63 reactions. Thiosulphate was found to undergo electrocatalytic oxidation at a reduced overpotential with good sensitivity in a wider pH range at the modified electrode. The sensor showed good stability and reproducibility. Under optimal condition, the sensor showed a linear response for thiosulphate in the concentration range from 4 105 to 4:8 104 M with a correlation coefficient of 0.9972 and the detection limit was 1:8 105 M (S/N ¼ 3). The sensor has been applied for the determination of thiosulphate in the photographic effluents. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Modified electrode; Mechanical immobilization; Electrocatalysis; Thiosulphate; Cobalt hexacyanoferrate
1. Introduction Over the past two decades, there has been a great deal of interest in the construction of potential chemical sensors for the analytical determination of compounds of biological, medicinal and environmental interest. Chemical sensors are analytical devices that are reliable, sensitive, robust, portable, easy to use and economical. Furthermore, they require only minimum sample treatment and conditioning. A variety of techniques have been exploited to prepare chemically modified electrodes such as adsorption, covalent bonding, polymer coating, sol–gel, screen-printing, and composites [1– 6]. In most cases, the surface modification involves tedious process resulting in poor stability and perturbed functions. Thus, there is a continuing interest in the fabrication of sensors with simple and reliable immobilization procedures. Recently, the technique of mechanical immobilization has opened a new area for the easy and effective construction of electrochemical sensors [7–9]. The mechanical immobilization of the mediator onto the electrode surface is a * Corresponding author. Present address: Satellite Venture Business Laboratory, Department of Applied Chemistry, Utsunomiya University, Yoto-7-1-2, Utsunomiya 321-8585, Japan. Tel.: þ81-28-689-6172(O), 663-4144(R); fax: þ81-28-689-6179. E-mail addresses: [email protected], [email protected] (D. Ravi Shankaran).
promising alternative to other modification methods owing to its simplicity, low cost and rapid. Despite the number of studies based on this approach, the analytical application for the determination of compounds of interest is rather limited [10,11]. In our laboratory, we have developed modified electrodes by mechanical immobilization using copper and nickel hexacyanoferrates for the determination of ascorbic acid [12], sulphur dioxide [13] and thiosulphate [14]. Thiosulphate is a prime pollutant resulting from photographic industry. It is, therefore, necessary to develop a sensitive, fast, inexpensive and user-friendly analytical method for the determination of thiosulphate. Despite different analytical methods for thiosulphate determination, the electrochemical sensors received increasing attention in recent years since they can offer a viable, promising and low cost solution to thiosulphate monitoring [14–16]. In this work, we describe a chemically modified electrode constructed by mechanical immobilization using cobalt hexacyanoferrate (CoHCF) as the mediator. CoHCF is an important member of the mixed valance metal hexacyanoferrates having similar characteristics as that of Prussian blue (PB). It has interesting chemical and electrochemical properties [9,17,18]. It does not undergo dissolution upon reduction or oxidation as the electrolyte ion diffuse in and out of the zeolitic structure of the compound to maintain charge neutrality. Thus, this material is an ideal candidate for electrode modification by mechanical immobilization. The
0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 1 7 1 - 5
D. Ravi Shankaran, S. Sriman Narayanan / Sensors and Actuators B 86 (2002) 180–184
modified electrode was characterized by cyclic voltammetry and used for the determination of thiosulphate. The analytical utility of the electrode for the determination of thiosulphate in photographic effluents was also studied.
2. Experimental 2.1. Reagents and apparatus Cobalt nitrate, potassium ferrocyanide and thiosulphate were received from Sigma (USA). All other chemicals and reagents used were of analytical grade. Double distilled water was used to prepare all the solutions. The pH of the solutions were adjusted using HCl, NaOH, Na2HPO4, NaH2PO4. The electrochemical experiments were carried out using EG & G PAR Electrochemical System (Model 263A) equipped with GPIP (IEEE-488) interface port and IBM personal computer. The CoHCF modified electrode was used as working electrode, while Ag/AgCl (saturated KCl) and a platinum wire as reference and counter electrode, respectively. All measurements were carried out under an atmosphere of high purity nitrogen. A magnetic stirrer and Teflon coated magnetic bar were used in hydrodynamic experiments. Hydrodynamic volatmmograms were registered by measuring the current response at a fixed interval of applied potential under the stirring condition of the electrolyte solution.
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2.4. Procedure The CoHCF modified electrode was scanned in 0.1 M NaNO3 between 0 and 1 V. The behavior of electrode at different pH was studied by varying the electrolyte pH using NaOH and HCl. The effect of scan rate and electrolyte variation has also been examined. For electrocatalytic studies fresh solution of thiosulphate was prepared in deareated double distilled water. A 0.05 M phosphate buffer was used for maintaining the pH during the electrocatalytic oxidation studies. Aliquots of analyte were added to the supporting electrolyte for quantitative analysis and calibration. Thiosulphate samples were received from photographic studio, filtered and diluted with double distilled water and used for the determination.
3. Results and discussion The cyclic voltammograms of the cobalt hexacyanoferrate modified electrode obtained in 0.1 M NaNO3 (A) and KNO3 (B) at a potential scan rate of 10 mV/s are shown in Fig. 1. In NaNO3 medium, the cyclic voltammogram exhibits two sets of reversible redox peaks with formal potentials (Em ¼ ðEp;a þ Ep;c Þ/2) of about 0.34 and 0.83 V. The first
2.2. Preparation of the complex The cobalt hexacyanoferrate complex was prepared by precipitation via dropwise addition of 100 ml of 0.1 M Co(NO3)2 to a well-stirred solution of 100 ml of 0.1 M K4Fe(CN)6 in a beaker. After the addition, the mixture was stirred for 15 min and kept undisturbed for 1 h. The precipitate formed was filtered, washed first with 0.1 M KNO3 solution and then with portions of double distilled water. The olive-brown CoHCF precipitate formed was dried at room temperature under a vacuum and then crushed and milled to a fine crystalline powder. 2.3. Construction of modified electrode Paraffin impregnated graphite electrodes (PIGEs) with a circular surface diameter of 3mm were used for electrode preparation. The PIGEs were prepared by immersing soft graphite rods into molten paraffin wax under vacuum until air bubble cease to evolve from the rods. After re-establishing atmospheric pressure, the rods are removed before paraffin solidifies. The lower end of the electrode is carefully polished on smooth paper and is used for immobilization of the complex. The modified electrode was prepared [7] by uniformly rubbing PIGE onto the CoHCF complex placed on a smooth glass plate. The surface of the electrode was then rinsed well with double distilled water.
Fig. 1. Cyclic voltammograms of CoHCF modified electrode in (A) 0.1 M NaNO3 and (B) 0.1 M KNO3: scan rate, 10 mV/s.
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peak at lower potential was assigned to Co2þ/Co3þ transition by analogy with the case of the PB modified electrode [19]. The assumption is also supported by the reports of cobalt oxidation peak observed in cobalt phthalocyanine [20]. The redox peak at higher potential was assigned to Fe(CN)64/Fe(CN)63 reaction by comparing the redox potentials of low spin iron transition in PB. In KNO3 medium, the CoHCF modified electrode exhibits two anodic peaks at potentials of 0.58 and 0.69 V and two cathodic peaks at 0.65 and 0.32 V. The different CV behavior in NaNO3 and KNO3 solution is due to the role of cation, which diffuses into and out of the film during redox reactions. Continuous cycling of the electrode in both 0.1 M NaNO3 and 0.1 M KNO3 exhibited constant response for nearly 100 cycles suggesting the stability of the modified electrode. The modified electrode has a wide working pH range from 2 to 8 (figure not shown). A decrease in response was observed at higher pHs, which may be due to the possible hydroxylation of the CoHCF film on the electrode surface in alkaline conditions. The peak currents of the both the redox peaks are linearly depends on the square root of the potential scan rate in the range from 10 to 200 mV/s, suggesting that the redox reaction was diffusion controlled. Fig. 2 compares the electrocatalytic activity of the CoHCF modified electrode and the bare graphite electrode towards the oxidation of thiosulphate in 0.1 M NaNO3 (A) and 0.1 M KNO3 (B). In the figure, the curves (a) and (c) corresponds to bare and modified electrode, respectively without thiosulphate. When 4:4 104 M thiosulphate was added to the electrolyte solution, a very small increase in anodic current at higher potential was observed (curve b) with the bare electrode. However, a remarkable increase in the anodic current at 0.70 V was observed at the modified electrode (curve d) indicating the electrocatalytic oxidation of thiosulphate. An increase in the anodic peak current with a decrease in the cathodic current was observed for each addition of thiosulphate and the catalytic peak current was linear with the square root of the scan rate indicating that the electrocatalytic process was diffusion controlled. As the electrocatalytic oxidation of thiosulphate showed an increase in current response at a potential before the start of the Fe(CN)64/Fe(CN)63 reaction and within the proximity of the Co2þ/Co3þ reaction, we believe that the electrocatalytic activity of the thiosulphate at the CoHCF modified electrode depends on the Co2þ/Co3þ reaction and not on the Fe(CN)64/Fe(CN)63 system [18]. This observation is rather interesting because the Fe(CN)64/ Fe(CN)63 reaction was involved in the electrocatalysis in most of the hexacyanoferrate based modified electrodes [3,11–15,17]. However, in the present system, the involvement of the Fe(CN)64/Fe(CN)63 reaction in electrocatalysis is uncertain at present. One possible reason may be because of the Co2þ/Co3þ reaction is more favored than the Fe(CN)64/Fe(CN)63 reaction, and, hence, the thiosulphate may get oxidized faster by Co3þ when it reaches the electrode surface.
Fig. 2. Cyclic voltammograms of (a) bare electrode, (b) 4:4 104 M thiosulphate at bare electrode, (c) CoHCF modified electrode and (d) 4:4 104 M thiosulphate at modified electrode. Electrolyte: (A) 0.1 M NaNO3 and (B) 0.1 M KNO3; pH 6 (0.05 M phosphate buffer); scan rate, 20 mV/s.
The pH dependence of the electrocatalytic oxidation was evaluated and it was found that there was no appreciable change in the electrocatalytic behavior in the pH range of 2–8. However, the catalytic current decreased with the increase of pH after 8, as depicted in Fig. 3. The possible
Fig. 3. Effect of pH on the anodic current for the catalytic oxidation of 2 104 M thiosulphate: scan rate, 20 mV/s; electrolyte, 0.1 M NaNO3.
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Fig. 6. Calibration plot for the determination of thiosulphate. Fig. 4. Hydrodynamic voltammograms of 4:4 104 M thiosulphate at (a) CoHCF modified and (b) bare electrode: electrolyte, 0.1 M NaNO3; pH 6; stirring rate, 300 rpm.
reason may be as indicated due to the hydroxylation of the mediator in alkaline conditions. Thus, a pH of 6 was used in the electrocatalytic oxidation studies. To study the applicability of the modified electrode as an amperometric sensor in flow systems, hydrodynamic voltammetric experiments have been carried out for the electrocatalytic oxidation of thiosulphate. The hydrodynamic voltammograms obtained for the electrocatalytic oxidation of 4:4 104 M thiosulphate at modified (curve a) and at bare electrode (curve b) are shown in Fig. 4. As expected, the electrocatalytic activity of the CoHCF permits the convenient detection of thiosuphate at lower potential with higher sensitivity. It shows similar response as that of cyclic voltammetric curves with the anodic peak currents increasing with potential and reaching a limiting value at 0.7 V at the modified electrode. In contrast, the unmodified electrode offers detection of the thiosulphate only at higher potentials with lesser sensitivity. Thus, a potential of 0.7 V was applied during the amperometric determination of thiosulphate. The CoHCF modified electrode exhibit good stability. The response of the modified electrode showed no observable change for nearly 11 days and 92% response was retained up to 1 month, when stored in air or dipped in the electrolyte solution. The stability of the electrode under continuous stirring condition was also examined. Fig. 5 shows the response of the CoHCF modified electrode for
4 104 M thiosulphate measured at 30 min interval for 6 h under hydrodynamic conditions. It is seen from the figure that the modified electrode retains good response in hydrodynamic conditions for long time, suggesting its possible application in flow systems for on-line monitoring of thiosulphate. A linear calibration range from 4 105 to 4:8 104 M thiosulphate with a correlation coefficient of 0.9972 was observed with the modified electrode (Fig. 6). The detection limit was 1:8 105 M (S/N ¼ 3). The precision of the method was evaluated for 10 successive measurements of 2 104 M thiosulphate and the R.S.D. was found to be 2.44%. The electrode-to-electrode reproducibility of the modified electrode was examined by cyclic voltammetry, the R.S.D. of the peak current was 3.7%. The long time stability and the lower working potential are the substantial advantage for practical application.
4. Analysis of thiosulphate in photographic effluents In order to evaluate the application of the method for the real sample analysis, the modified electrode was used for the determination of thiosulphate in photographic effluents. The results obtained are given in Table 1. The result suggests that the present method is comparable with the standard method but has the advantage of being simple and fast without the need for much reagents for the determination. The influence of some excipients commonly present with thiosulphate was analyzed. The influence of the interfering substance was evaluated at an interferent/thiosulphate concentration ratio of 1:1. It was observed from the experiments that most of the species such as Ni2þ, Ca2þ, Mg2þ, Agþ, Br and Cl, etc. did not interfere with the determination. Table 1 Results of the analysis of photographic effluents Sample
4
Fig. 5. CoHCF modified electrode response for 4 10 M thiosulphate vs. time: electrolyte, 0.1 M NaNO3; pH 6; stirring rate, 300 rpm.
Color film fixing waste Black and white film fixing waste
Thiosulphate (mg/l) Present method mean (n ¼ 5)
Iodimetry
34.23 40.56
34.19 41.24
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5. Conclusion It has been shown that the CoHCF modified electrode constructed by mechanical immobilization exhibited good and favorable characteristics for the determination of thiosulphate. The modified electrode can overcome the kinetic limitation for thiosulphate oxidation at bare electrode by a catalytic process and can lower the overpotential for the oxidation reaction. The advantages of ease of construction, low cost, reliable sensitivity, remarkable stability and good reproducibility of the sensor suggests its promising application for on-line monitoring of thiosulphate.
Acknowledgements One of the authors (DR) acknowledges the Council of Scientific and Industrial Research, New Delhi for providing Senior Research Fellowship and the other (SSN) acknowledges University Grants Commission for providing financial assistance in the form of major research project. References [1] R.P. Baldwin, K.N. Thomsen, Chemically modified electrodes in liquid chromatography detection: a review, Talanta 38 (1991) 1–16. [2] S.A. Wring, J.P. Hart, Chemically modified carbon-based electrodes and their application as electrochemical sensors for the analysis of biologically important compounds, Analyst 117 (1992) 1215–1229. [3] J.A. Cox, M.E. Tess, T.E. Cummings, Electroanalytical methods based on modified electrodes: a review of recent advances, Rev. Anal. Chem. 15 (1996) 173–223. [4] J.P. Hart, S. A Wring, Recent developments in the design and application of screen-printed electrochemical sensors for biomedical, environmental and industrial analyses, Trends Anal. Chem. 16 (1997) 89–103. [5] J. Lin, C.W. Brown, Sol–gel glass as a matrix for chemical and biochemical sensing, Trends Anal. Chem. 16 (1997) 200–211. [6] J.J. Gooding, D.B. Hibbert, The application of alkanethiol selfassembled monolayers to enzyme electrodes, Trends Anal. Chem. 18 (1999) 525–533. [7] F. Scholz, B. Meyer, Electrochemical solid state analysis: state of the art, Chem. Soc. Rev. (1994) 341–347. [8] F. Scholz, B. Meyer, in: A.J. Bard, I. Rubinstein (Eds.), Electroanalytical Chemistry—A Series of Advances, Voltammetry of Solid Microparticles Immobilised on Electrode Surfaces, Vol. 20, Marcel Dekker, New York, NY, 1998, pp. 1–86. [9] P.J. Kulesza, M.A. Malik, M. Berrettoni, M. Giorgetti, S. Zamponi, R. Schmidt, R. Marassi, Electrochemical charging, counter cation accomodation, and spectrochemical identity of microcrystalline solid cobalt hexacyanoferrate, J. Phys. Chem. B 102 (1998) 1870–1876. [10] A. Dostal, B. Meyer, F. Scholz, U. Schroder, A.M. Bond, F. Marken, S.J. Shaw, Electrochemical study of microcrystalline solid Prussian blue particles mechanically attached to graphite and gold electrodes: electrochemically induced lattice reconstruction, J. Phys. Chem. 99 (1995) 2096–2103. [11] S. Sriman Narayanan, F. Scholz, A comparative study of the electrocatalytic activities of some metal hexacyanoferrates for the oxidation of hydrazine, Electroanalysis 11 (1999) 465–469.
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Biographies D. Ravi Shankaran was born in Katary village, The Nilgirs, India, in 1972. He received BSc, MSc, and BEd degrees form Sri Ramakrishna Mission Vidyalaya, Bharathiar University. In July 2000, he obtained PhD from the Department of Analytical Chemistry, University of Madras for his research on ‘Mechanically immobilized modified electrodes for amperometric determination of compounds of biological and environmental interest’. He worked as a post-doctoral researcher in the group of Prof. Yoon-Bo Shim at the Department of Chemistry, Pusan National University, Korea. Presently, he is a post-doctoral researcher in the group of Prof. Teiji Kato at the Satellite Venture Business Laboratory, Department of Applied Chemistry, Utsunomiya University, Japan. He is engaged in the fabrication of new metal-dispersed amperometric chemical and biosensors based on the self-assembly, sol–gel biocomposite and layer-by-layer assembly techniques. S. Sriman Narayanan was born in Tamilnadu, India, in 1955. He received MSc degree in Analytical Chemistry from the University of Madras, in 1979. In 1983, he received PhD degree from Indian Institute of Technology, Madras for his research on ‘The development of radiorelease techniques for the determination of important gaseous air pollutants’. He joined as a lecturer in the Department of Analytical Chemistry at the University of Madras, in 1985 and he is presently professor in Analytical Chemistry since 1998. He has worked with Prof. Fritz Scholz at the Institute of Chemistry, Humboldt University at Berlin, Germany and Prof. James Hickman at the Department of Bioengineering, Clemson University, USA. His current interest is in the development of chemical and biosensor for biological and environmental analysis and self-assembled monolayers for trace metal analysis.