Mercury-coated platinum microelectrodes for steady-state voltammetry in aqueous solutions at high temperature

www.elsevier.nl/locate/elecom Electrochemistry Communications 2 (2000) 399–403

Mercury-coated platinum microelectrodes for steady-state voltammetry in aqueous solutions at high temperature Salvatore Daniele a,*, Carlo Bragato a, Emanuele Argese b b

a Department of Physical Chemistry, University of Venice, Calle Larga, S. Marta, 2137, 30123 Venice, Italy Department of Environmental Science, University of Venice, Calle Larga, S. Marta, 2137, 30123 Venice, Italy

Received 17 February 2000; received in revised form 10 March 2000; accepted 10 March 2000

Abstract Sphere-cap mercury microelectrodes fabricated on a platinum disk substrate were tested in aqueous solutions over the temperature range 295–353 K. The performance of these electrodes was assessed in solutions containing Ru(NH3)6Cl3 and LiClO4 as supporting electrolyte. From the steady-state limiting current obtained at three different mercury microelectrodes, the diffusion coefficient of the electroactive species at different temperatures was determined. It was found that the diffusion coefficient values were consistent with the Stokes–Einstein and activation energy models. The results obtained also allowed us to conclude that sphere-cap mercury microelectrodes can be useful for elevated temperature electroanalysis in aqueous solutions. q2000 Elsevier Science S.A. All rights reserved. Keywords: Sphere-cap microelectrodes; Mercury; Voltammetry; Steady state; Temperature

1. Introduction Steady-state voltammetry has been widely employed for the investigation of thermodynamics, mass transport properties, electrode kinetics and chemical stability of electroactive species in both aqueous and organic media [1–3]. Quite often the measurements are carried out at room temperature. However, there are instances that require measurements to be performed at elevated temperature. These include assessing the thermal stability of the investigated species [4], electrochemistry in near-critical fluids [5,6], and determination of activation energy [1,7–9]. A rise of the solution temperature may also provide several advantages in electroanalysis. In fact, the mass transport is enhanced because of decreased solution viscosity; this brings higher sensitivity and improved signal-to-noise ratio in voltammetry [10]. Moreover, the heterogeneous electron transfer rate is generally enhanced, while the electrode surface can be cleaned more easily from adsorbed interfering compounds [11]. Microelectrodes allow steady-state conditions to be achieved rapidly [3,12,13], and mercury, because of its high hydrogen overvoltage, may be the material of choice for the investigation of processes occurring over the negative potential region in aqueous solutions [14]. The development of * Corresponding author. Tel.: q39-041-2578-630; fax: q39-041-2578594; e-mail: [email protected]

mercury microelectrodes has been the subject of numerous papers over the past 15 years [15–30]. Mercury microelectrodes are usually prepared by electrodeposition of mercury on metal substrates [15–30]. When mercury wets the surface of an inlaid microdisk, the mercury deposit is usually in the shape of a sphere cap with the basal plane coincident with the original inlaid disk [21,24,26,29,31–33]. This is a consequence of the large surface energy of the solutionNmercury interface [24,31,32]. The sphere-cap height (h) can be determined from the total charge spent during the electrodeposition of mercury [21,24,26,28,29]. Sphere-cap microelectrodes with h over a wide range have been prepared on platinum microdisks [17,24,28–30]. These microelectrodes have shown good stability at room temperature and have found numerous analytical applications [34–38]. The rise of temperature may affect the surface energy of the solutionNmercury interface and in turn the stability of the mercury deposit. In a recent paper, by exploiting hot-wire technology [10,11,14], it has been shown that iridium-based mercury film electrodes heated directly are stable and can be used for anodic stripping voltammetry [14]. However, the critical dimension of the iridium substrate was larger than that typical for microelectrode behaviour. Also, no detail has been given for the dimension and shape of the mercury deposit. To the best of our knowledge, no report exists on the performance of mercury microelectrodes in a sphere-cap shape above room temperature.

1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S 1 3 8 8 - 2 4 8 1 ( 0 0 ) 0 0 0 4 7 - 3

Thursday May 04 10:18 AM

StyleTag -- Journal: ELECOM (Electrochemistry Communications)

Article: 224

400

S. Daniele et al. / Electrochemistry Communications 2 (2000) 399–403

The aim of this paper has then the objective to assess the performance of mercury-coated platinum microelectrodes for measurements in aqueous solutions above room temperature. The measurements were carried out on ruthenium(III) hexaammine trichloride solutions over the temperature range 295–353 K. 2. Experimental Voltammetric experiments were carried out in a two-electrode cell located in a Faraday cage made of aluminium sheets. The platinum microdisk, which served as the substrate for mercury deposition, was prepared by sealing a wire of 10 mm radius into a glass capillary. Prior to mercury deposition, the microelectrode was polished with graded alumina powder of different sizes (1, 0.3 and 0.05 mm) on a polishing microcloth. The effective electrode radius of the exposed microdisk was calculated using Eq. (1) [39] after recording the steadystate limiting currents in a 1 mM ferrocene solution in acetonitrile (Ds2.4=10y5 cm2 sy1 [40]): b

I ds4nFDc a

(1)

where Id is the diffusion-controlled limiting current, a is the radius of the disk electrode and the other symbols have their usual meaning. Sphere-cap mercury microelectrodes of different sizes were prepared ex situ at room temperature by cathodic deposition of mercury onto the platinum microdisk, as reported elsewhere [28,29]. In the following, the dimension of the mercury deposit will be referred to as h/a, that is, the ratio of the sphere-cap height to the electrode’s basal radius. Mercury microelectrodes with h/a in the range 0.6– 1 were investigated. The reference electrode was an AgNAgCl saturated with KCl. Both working and reference electrodes were housed in the same cell containing the test solution. Cyclic voltammetric experiments were controlled by a PAR 175 function generator. A Keithley 428 picoammeter was used to measure the current, and data were plotted with a Hewlett-Packard 7045 B X–Y recorder. All chemicals employed were of analytical-reagent grade. Ruthenium(III) hexaammine trichloride was purchased from Aldrich and purified as reported in the literature [41]. All aqueous solutions were prepared with Milli-Q water. All measurements were carried out in solutions that had been deaerated with pure nitrogen (99.99%, from SIAD, Italy). In order to avoid a large amount of vaporisation of the working solution, the highest temperature investigated was 353 K, that is, below the solution’s boiling point. To minimise thermal gradients and convection, the system was thermostatcontrolled and allowed to equilibrate for about 30 min after heating the solution to the desired temperature. 3. Results and discussion Fig. 1 shows typical cyclic voltammograms recorded using a mercury microelectrode with h/as0.96 at 5 mV sy1 for

Thursday May 04 10:18 AM

Fig. 1. Cyclic voltammograms recorded at 5 mV sy1 on a mercury-coated platinum microelectrode (h/as0.96) in 0.75 mM (room temperature) Ru(NH3)6Cl3 and 0.1 M LiClO4 solutions, at 295.3 (1), 303.2 (2), 312.6 (3), 321.8 (4), 329.2 (5), 343.3 K (6).

the reduction of 0.75 mM Ru(NH3)6Cl3 solution containing 0.1 M LiClO4 as supporting electrolyte. All the voltammograms are well shaped and display the expected sigmoidal behaviour in both forward and backward scans, typical of microelectrodes under steady-state conditions [3,12,13]. Voltammetric waves very similar to those of Fig. 1 were also obtained with the other mercury-coated platinum electrodes investigated here. The only difference was in the current plateau that, at a given temperature, was proportional to the surface area of each mercury microelectrode employed, as shown in Fig. 2. Table 1 summarises potential parameters of the voltammetric waves obtained at various temperatures. It is evident that the half-wave potential (E1/2) is less negative the higher the temperature. The wave shift probably reflects the dependence of the formal potential of both the Ru(NH3)63q/ Ru(NH3)62q couple and the AgNAgCl reference electrode on temperature [42,43]. No further attempt was made to

Fig. 2. Plots of the steady-state limiting current against temperature for different mercury-coated platinum microelectrodes having h/as0.96 (d), 0.89 (h) and 0.61 (m). Solution containing 0.75 mM (room temperature) Ru(NH3)6Cl3 and 0.1 M LiClO4. Scan rate 5 mV sy1.

StyleTag -- Journal: ELECOM (Electrochemistry Communications)

Article: 224

S. Daniele et al. / Electrochemistry Communications 2 (2000) 399–403 Table 1 Voltammetric parameters for the reduction of Ru(NH3)6Cl3 in aqueous solutions at different temperatures a T ("0.5)/K

yE1/2 ("0.002)/V

(E1/4yE3/4)exp ("1)/mV

(E1/4yE3/4)th/ mV

295.3 303.2 312.6 321.8 329.2 337.0 343.3 346.5 353.3

0.126 0.120 0.116 0.110 0.106 0.100 0.096 0.094 0.092

55 57 59 60 63 64 66 66 67

55.9 57.4 59.2 60.9 62.3 63.8 65.0 65.6 66.9

a

expsexperimental; thstheoretical.

evaluate the contribution of each redox couple to the overall wave shift. The Tomes potential difference (E1/4yE3/4) [44] was used to assess electrochemical reversibility. For a microelectrode working under steady-state conditions, a reversible process should produce a Tomes difference given by [3]: (E 1/4yE 3/4)sRT/nF ln(9)

(2)

where R is the gas constant, T the temperature and the other symbols have their usual meaning. Experimental and theoretical Tomes difference values calculated by Eq. (2) are included in Table 1. The two sets of data are in very good agreement within experimental error. This indicates that, at all temperatures investigated, the electrode process is reversible. To assess whether the mercury microelectrodes worked properly, the dependence of the steady-state limiting current with temperature was examined in detail. The steady-state limiting current Il for a sphere-cap electrode can be written as [21,24,26,28,29,32,33,45]: Ilsa nFDc ba

(3)

where a is a geometrical coefficient whose value depends on h/a [21,24,32,33,45], and the other symbols have their usual meanings. The three mercury microelectrodes examined here had h/a values equal to 0.61, 0.89 and 0.95; the corresponding a values, calculated by Eq. (11) in Ref. [45], are 5.17, 5.99 and 6.18, respectively. It should be considered that the increase of temperature affects parameter a as mercury expands to some extent. However, on the basis of changes in density of mercury with temperature, it was estimated that, between 295 and 353 K, the volume variation was less than 2%. Correspondingly, the variation of a was lower than 0.7%, which is well within the experimental error made in the steady-state current measurements. Therefore, the room-temperature a value was employed as the coefficient in the steady-state limiting current equation, regardless of the temperature of the medium. At a given microelectrode and concentration of the electroactive species, the variation of the current plateau should

Thursday May 04 10:18 AM

401

mainly reflect the dependence of diffusion coefficient of the electroactive species with temperature. The diffusion coefficient of Ru(NH3)6Cl3 was calculated from experimental steady-state limiting currents and by Eq. (3). At high temperature, expansion of the solution results in a drop in the concentration of the diffusing species. To account for this in calculating the diffusion coefficients, a correction was made considering the known initial concentration at 293 K and the changes in density of the solution with temperature.Table 2 shows the diffusion coefficient values thus calculated. Each diffusion coefficient value was averaged from at least nine data points, which were obtained from steady-state limiting currents measured at a given temperature with the three mercury microelectrodes. The low-temperature diffusion coefficient value is very close to 6.30=10y10 m2 sy1 found from the steady-state limiting current at the bare 10 mm platinum microdisk and using Eq. (1). It also compares well with 6.29=10y10 m2 sy1 [46] and 6.0=10y10 m2 sy1 [26] reported in the literature for the same redox system in sodium perchlorate and in phosphate media, respectively. This result confirms that the sphere-cap model is adequate for calculating the coefficient a values. The dependence of the diffusion coefficient with temperature was examined according to the Stokes–Einstein and activation diffusion models. The Stokes–Einstein equation is given by [47]: DskB T/6phrh

(4)

where kB is the Boltzmann constant, h the absolute viscosity, and rh the hydrodynamic radius of the diffusing species. The viscosity of the medium in turn depends on temperature. On the basis of Eq. (4), the parameter Dh/T should be constant provided that rh is constant over the temperature range investigated. Table 2 includes the Dh/T values along with viscosity data for water, which were available in the literature [48]. From this table, it is evident that Dh/T is constant at 2.16 ("0.10)=10y15 Pa m2 Ky1 which suggests that the change in D with temperature is solely caused by a decrease in solution viscosity. Table 2 Variation of diffusion coefficient of Ru(NH3)6Cl3 in 0.1 M LiClO4 aqueous solution with temperature T ("0.5)/ K

D=1010/ m2 sy1

h a=103/ Pa s

(Dh/T)=1015/ Pa m2 Ky1

295.3 303.2 312.6 321.8 329.2 337.0 343.3 346.5 353.3

6.29"0.19 7.66"0.21 10.1"0.3 12.1"0.3 15.1"0.4 16.9"0.5 18.8"0.4 20.1"0.6 20.6"0.9

0.961 0.798 0.668 0.568 0.499 0.442 0.404 0.389 0.354

2.05 2.02 2.16 2.13 2.29 2.21 2.21 2.26 2.07

a

Ref. [46].

StyleTag -- Journal: ELECOM (Electrochemistry Communications)

Article: 224

402

S. Daniele et al. / Electrochemistry Communications 2 (2000) 399–403

The dependence of D with temperature should also follow an Arrhenius-type equation [1,49]: DsDU exp(yE A/RT)

(5)

U

where D is the pre-exponential factor, and EA the energy of activation for diffusion. EA can be obtained from a plot of ln D against 1/T. Fig. 3 shows the experimental ln D versus 1/T plot and that a linear dependence applies (correlation coefficient of 0.998). A deviation from linearity of this plot would imply a marked structural change in the solvated species. No evidence of such behaviour is present in Fig. 3. This result is congruent with that found from the Stokes–Einstein equation. In fact, the constancy of Dh/T applies only if rh does not change with temperature. From the slope of the above straight line the activation energy for diffusion was calculated as 18.1"0.7 kJ moly1. Similar activation energy values were found earlier for Cu2q [5], ferrocyanide and ferricyanide ions [1] in aqueous solutions. As the temperature increases, the onset of convection may affect the height of the voltammetric wave. In the above discussion, the latter effect has been neglected. It has been generally assumed that natural convection does not influence the steady-state voltammetric response of a microelectrode [13]. However, because of uneven heating of the electrochemical cell and density gradients between the bulk solution and the depletion layer, natural convection may contribute to mass transport [50]. This would reflect on the calculated diffusion coefficient values. The good linearity of the Arrhenius plot (see Fig. 3) and the constancy of the Dh/T parameter (see Table 1) suggest that the effect of natural convection on the above voltammetric measurements is either negligible or constant over the temperature range investigated. On the other hand, previous studies have shown that the relative importance of natural convection decreases rapidly with decreasing electroactive species concentration [50] and when the electrochemical cell is well thermostatted [10]. In particular, negligible effects due to natural convection were observed when measurements were performed with microdisks having radii less than 25 mm, in solutions with electroactive species concentrations lower than 10 mM [50]. Hence, under our experimental conditions (i.e., electrode radii about 10 mm and electroactive species concentration F1 mM) a small effect due to natural convection is expected, in agreement with the results reported above.

4. Conclusions The present study has shown that sphere-cap mercury microelectrodes prepared on platinum disk substrates behave well in aqueous solutions at elevated temperature up to 353 K. No erratic behaviour in the steady-state limiting current trend has been observed on increasing the temperature of the solution. This has been assessed by both the constancy of the parameter Dh/T and the linearity of the plot shown in Fig. 3. Hence, for the system examined here, thermodynamic sta-

Thursday May 04 10:18 AM

Fig. 3. Plot of ln D vs. Ty1 for data in Table 2.

bility [31] corresponding to sphere-cap geometry is attained even at elevated temperature. Since an exact equation for steady-state limiting current is available for sphere-cap mercury microelectrodes, the diffusion coefficient of an electroactive species in aqueous solutions at higher temperature can be evaluated straightforwardly. Mercury microelectrodes may also find practical applications in electroanalysis. For instance, anodic stripping voltammetric measurements performed at high temperature may benefit from both enhanced mass transport and improved signal-to-noise ratio. Hence, the mercury microelectrode can be a valuable tool for trace metal analysis in high-temperature wastewater.

Acknowledgements Financial support from the Italian Research Council (CNR) and from the Ministry of University and Technological Research (MURST) is acknowledged.

References [1] F. Opekar, P. Beran, J. Electroanal. Chem. 69 (1976) 1. [2] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980. [3] A.M. Bond, K.B. Oldham, C.G. Zoski, Anal. Chim. Acta 216 (1989) 177. [4] E. Argese, B. De Carli, E. Orsega, A. Rigo, G. Rotilio, Anal. Biochem. 132 (1983) 110. [5] A.C. McDonald, F. Fan, A.J. Bard, J. Phys. Chem. 90 (1986) 196. [6] W.M. Flarsheim, Y. Tsou, I. Trachtenberg, K.P. Johnston, A.J. Bard, J. Phys. Chem. 90 (1986) 3857. [7] H. Luo, W. Zhang, L. Gan, C. Huang, N. Li, Electroanalysis 11 (1999) 238. ¨ [8] A. Beckmann, A. Schneider, P. Grundler, Electrochem. Commun. 1 (1999) 46. [9] B. Marczewska, J. Wloszek, U. Kozlowska, Electroanalysis 11 (1999) 243. ¨ [10] P. Grundler, G.U. Flechsig, Electrochim. Acta 43 (1998) 3451. ¨ [11] P. Grundler, A. Kirbs, T. Zerihum, Analyst 121 (1996) 1805.

StyleTag -- Journal: ELECOM (Electrochemistry Communications)

Article: 224

S. Daniele et al. / Electrochemistry Communications 2 (2000) 399–403 [12] R.M Wightmann, D.O. Wipf, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 15, Marcel Dekker, New York, 1989, pp. 267–353. [13] M.I. Montenegro, M.A. Queiros, J.L Daschbach, Microelectrodes: Theory and Applications, vol. E197, NATO ASI Series, p. 199. ¨ [14] M. Jasinski, A. Kirbs, M. Schmehl, P. Grundler, Electrochem. Commun. 1 (1999) 26. [15] M. Ciszkowska, Z. Stojek, J. Electroanal. Chem. 191 (1985) 101. [16] A.S. Baranski, H. Quon, Anal. Chem. 58 (1986) 407. [17] K.R. Wehmeyer, R.M. Wightman, Anal. Chem. 57 (1985) 1989. [18] S. Baranski, Anal. Chem. 59 (1987) 662. [19] J. Golas, Z. Galus, J. Osteryoung, Anal. Chem. 59 (1987) 389. [20] S.P. Kounaves, J. Buffle, J. Electroanal. Chem. 216 (1987) 53. [21] Z. Stojek, J. Osteryoung, Anal. Chem. 61 (1989) 1305. [22] S.P. Kounaves, W. Deng, J. Electroanal. Chem. 301 (1991) 77. [23] R.R. De Vitre, M.L. Tercier, M. Tsacopoulos, J. Buffle, Anal. Chim. Acta 249 (1991) 419. [24] C.L. Colyer, D. Luscombe, K.B. Oldham, J. Electroanal. Chem. 283 (1990) 397. [25] T.J. O’Shea, S.M. Lunte, Anal. Chem. 65 (1993) 247. [26] R.L. Birke, Z. Huang, Anal. Chem. 64 (1992) 1513. [27] M. Ciszkowaska, M. Donten, Z. Stojek, Anal. Chem. 66 (1994) 4112. [28] M.A. Baldo, S. Daniele, M. Corbetta, G.A. Mazzocchin, Electroanalysis 7 (1995) 980. [29] M. Corbetta, M.A. Baldo, S. Daniele, G.A. Mazzocchin, Ann. Chim. (Rome) 86 (1996) 77. [30] M.A. Baldo, S. Daniele, G.A. Mazzocchin, Electrochim. Acta 41 (1996) 811. [31] C.L. Colyer, K.B. Oldham, S. Fletcher, J. Electroanal. Chem. 290 (1990) 33.

Thursday May 04 10:18 AM

403

[32] L.C.R. Alfred, K.B. Oldham, J. Phys. Chem. 100 (1996) 2170. [33] Y. Selzer, D. Mandler, Electrochem. Commun. 1 (1999) 569. [34] S. Daniele, M.A. Baldo, P. Ugo, G.A. Mazzocchin, Anal. Chim. Acta 219 (1989) 9. [35] S. Daniele, M.A. Baldo, P. Ugo, G.A. Mazzocchin, Anal. Chim. Acta 219 (1989) 19. [36] S. Daniele, G.A. Mazzocchin, Anal. Chim. Acta 273 (1993) 3. [37] M.A. Baldo, S. Danile, G.A. Mazzocchin, Anal. Chim. Acta 340 (1997) 77. [38] M.A. Baldo, C. Bragato, S. Daniele, Analyst 122 (1997) 1. [39] Y. Saito, Rev. Polarogr. 15 (1968) 177. [40] R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969, p. 221. [41] J.R. Pladziewicz, T.J. Meyer, J.A. Broomhead, H.Y. Taube, Inorg. Chem. 12 (1973) 639. [42] R.S. Greeley, W.T. Smith Jr., R.W. Stoughton, M.H. Lietzke, J. Phys. Chem. 64 (1960) 652. [43] D.D. Macdonald, A.C. Scott, P. Wentrek, J. Electrochem. Soc. 126 (1979) 909. [44] J. Tomes, Collect. Czech. Chem. Commun. 9 (1937) 50. [45] J.C. Myland, K.B. Oldham, J. Electroanal. Chem. 288 (1990) 1. [46] S. Daniele, P. Ugo, G.A. Mazzocchin, D. Rudello, in: R.A. Mackay, J. Texter (Eds.), Electrochemistry in Colloids and Dispersions, VCH, New York, 1992, p. 55. [47] J.O. Bokris, A.K.N. Reddy, Modern Electrochemistry and Physics, vol. 1, Plenum, New York, 1970. [48] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 61st ed., CRC Press, Boca Raton, FL, p. F-51. [49] H. Chen, R. Neeb, Fresenius’ J. Anal. Chem. 319 (1984) 240. [50] X. Gao, J. Lee, H. White, Anal. Chem. 67 (1995) 1541.

StyleTag -- Journal: ELECOM (Electrochemistry Communications)

Article: 224