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Using Pt microelectrodes in liquid ammonia for studying proton reduction
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 477 (1999) 140 – 145
Using Pt microelectrodes in liquid ammonia for studying proton reduction A-M. Gonc¸alves *, C. Mathieu, M. Herlem, A. Etcheberry IREM (Institut La6oisier), Uni6ersite´ de Versailles St Quentin, Baˆt. La6oisier, UMR CNRS CO173, 45 A6. des Etats Unis, 78035 Versailles Cedex, France Received 21 June 1999; received in revised form 24 September 1999; accepted 24 September 1999
Abstract The electrical response of Pt microelectrodes, with a radius of 10 mm, was studied in liquid ammonia ( − 50°C). The question of a suitable time scale for steady state measurements in liquid ammonia is important. The proton reduction currents were measured for different scan rates. A suitable experimental time scale of 20 mV s − 1 was determined to reach a stationary state. The diffusion coefficient of protons in liquid ammonia was deduced from these electrochemical results: D(NH+ 4 )NH3 =(3.8 9 0.4)× 10 − 9 m2 s − 1. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Microelectrodes; Stationary state; Liquid ammonia; Diffusion coefficient
1. Introduction Microelectrode techniques have been used widely in electrochemistry during the past decade in many scientific areas, in for example monitoring fundamental biological events or electrochemical processes [1 – 5]. In comparison to electrodes of millimetre size, microelectrodes have at least three unique advantages resulting from their reduced size [4,6]. The first is that mass transport to and from the electrode is enhanced because of spherical diffusion due to edge effects. A second advantage is that the double-layer capacitance is reduced due to the reduction in surface area. A third advantage is that microelectrodes are almost free from ohmic drop (IR) phenomena. In contrast to macroelectrodes, it is not necessary to correct the data for IR drop. This property allows one to detect weak Faradaic currents which can be also observed using a solvent of low dielectric constant, in the absence or with a low concentration of supporting electrolyte. All these properties make microelectrodes very useful for studying intermediate species in non-aqueous solvents [7 – 10].
* Corresponding author. Fax: +33-1-39-254381. E-mail address: [email protected] (A.-M. Gonc¸alves)
Protons are often implied in many fundamental electrochemical processes like the oxygen reduction [11– 13]. An obvious way to understand these complex electrochemical processes is to isolate the effects of protons. Thus, a specific study of protons becomes crucial. Among all non-aqueous solvents, liquid ammonia appears to be unique both for its similarities to water (both are protic solvents) and its own chemical properties [14–18]. The interesting point in using liquid ammonia is that a large range of pH can be obtained: 33 pH units. This large pH range allows the study of electrochemical processes with and without proton contributions. Whatever the electrochemical reaction, the knowledge of the number of electrons exchanged is crucial for understanding the process. Steady state diffusion at the electrode solution interface gives a limiting current, which is directly proportional to the number of electrons exchanged and also to the concentration of electroactive species in the bulk solution. If the number of electrons is known, the limiting current can be related to a specific concentration. Electrochemical processes which are controlled by mass transport are useful for chemical analysis. Two classical electrochemical techniques are used usually to achieve a stationary state in both aqueous
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 3 9 8 - 8
A.-M. Gonc¸al6es et al. / Journal of Electroanalytical Chemistry 477 (1999) 140–145
and common non-aqueous solvents [19]. The first is related to forced convection techniques in, for example, the rotating disk electrode. The second technique makes use of spherical diffusion obtained, for instance, at a dropping mercury electrode. These two classical electrochemical techniques provide controlled diffusion at the electrode solution interface. The number of electrons involved in the electrochemical process can be deduced from the limiting current. Since experiments in liquid ammonia must be carried out with great care, rotating disk and dropping mercury electrodes have been hardly used. Even when a steady state could be reached with the dropping mercury electrode, the limiting diffusion current was not well-defined and consequently the number of electrons exchanged was uncertain [20]. Microelectrodes should provide new opportunities for obtaining a well-defined limiting diffusion current in liquid ammonia without involving significant experimental difficulties. In contrast to the two other classical techniques, there is no doubt about the practicality of microelectrodes resulting from their reduction in size. In spite of the fact that microelectrodes are used usually in both aqueous and non aqueous solvents, nothing is known about their properties in liquid ammonia. In this work, before using a microelectrode in liquid ammonia, we checked each electrode in usual solvents. For this purpose two reactions were used; the oxidation of ferrocene in acetonitrile and the oxidation of potassium ferrocyanide (K4Fe(CN)6) in aqueous solvent. These two electrochemical couples have been chosen since they are well studied systems and are commonly used to characterize microelectrodes [4]. Studies of electrochemical reduction of protons in liquid ammonia are relevant for at least two reasons. The first is that the basic medium allows the exclusion of all contributions from protons during an electrochemical process. The second point is that providing a steady state at a microelectrode, the limiting diffusion current due to proton reduction is directly proportional to the concentration of protons in acidic medium. Protons, in liquid ammonia, (NH+ 4 ) are solvated by the ammonia molecule because of the strong basicity of liquid ammonia. In contrast to aqueous media, ammonium ion (NH+ 4 ) is the strongest acid in liquid ammonia [18]. Even if the electrical conductivities of many ammonium salts were measured since 1900 in liquid ammonia [18], no data were given concerning the electrical conductivity of either cations or anions of the salt. This study gives an opportunity to compare electrical conductivity of ammonium ion (NH+ 4 ) to NMR results. A relation between the limiting currents of proton reduction (NH+ 4 ) and a large range of proton concentration becomes really relevant for detecting any contribution of protons during an electrochemical process.
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Since nothing is known of the interface between a microelectrode and liquid ammonia, the question of a suitable time scale for steady state measurements in liquid ammonia is important. This time scale depends on the potential scan rate: the longer is the time scale, the slower must be the scan rate. A stationary state is reached if the scan rate is slow enough to ensure spherical diffusion due to edge effects at the microelectrode. What kind of time scale can we expect in liquid ammonia? If the time scale is too long, corresponding to scan rates of less than 1 mV s − 1, variations of temperature could occur during the electrochemical processes and lifetimes of species and kinetic processes could be modified. The aim of this work is then to define a suitable time scale for steady state experiments using a platinum microelectrode with a radius of 10 mm.
2. Experimental Ammonia (‘electronic grade’) was obtained from Air Liquid. The device for condensing ammonia and the cell have already been described [21]. The volume of liquid ammonia is about 150 ml and the cell was maintained at − 50°C in a cryostat. An acidic medium is obtained by addition of NH4Br (from Aldrich). Platinum microelectrodes have been made using a platinum wire with a radius of 10 mm. The microelectrode was prepared by sealing the Pt wire into soft glass tubing. The microelectrode was polished on successively finer grades of alumina and finally cleaned in an ultrasonic water bath. The radius of the microdisk electrode was determined by linear voltammetry from the plot of steady state current against concentration of Fe(CN)46 − . We get complementary information by studying the oxidation of ferrocene (from Aldrich) in a volume of 10 ml of acetonitrile and also in studying the oxidation of K4Fe(CN)6 (from Aldrich) in a volume of 100 ml in a buffered basic aqueous medium (pH 10). These two electrochemical couples were chosen because they are model systems in these solvents [4,22]. A Pt wire (diameter l mm) was used as an auxiliary electrode. All potentials were measured against a silver reference electrode (SRE) in non-aqueous solvent (acetonitrile and liquid ammonia). In aqueous medium, all potentials were measured against the mercury sulphate electrode (MSE). Since the radius of our microelectrode was not small enough to allow us to omit a supporting electrolyte, 10 − 3 M of (Bu)4NPF6 was added in acetonitrile and 10 − 3 M of KBr was added to liquid ammonia to ensure sufficient conductivity. All electrochemical measurements were carried out under an argon stream to avoid electrochemical reactions with oxygen.
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3. Results and discussion
3.1. Microelectrode calibration 3.1.1. Ferrocene oxidation in acetonitrile The oxidation of ferrocene was performed in acetonitrile. A large range of ferrocene concentrations (from 3.2 ×10 − 4 to 1.4 × 10 − 3 M) was explored. A potential scan rate of 20 mV s − 1 provides a steady state at a 10 mm radius Pt microelectrode [22]. For each concentration, a limiting current from ferrocene oxidation was observed (Fig. 1A). The limiting current was directly proportional to the ferrocene concentration (Fig. 1B). At a microdisk electrode, under spherical diffusion conditions, the limiting current (Il) is given by the following equation [4]: Il = 4nFDc a
(1)
One electron (n=1) is exchanged during the electrochemical oxidation of ferrocene. F is the Faraday constant (96500 C mol − 1). D, the diffusion coefficient of ferrocene in acetonitrile is 2.2×10 − 9 m2 s − 1 [22]. c indicates the concentration of ferrocene in the bulk of the solution. If all the parameters of Eq. (1) are known, the radius (a) of the microelectrode can be deduced. In our case a was 1090.5 mm. This result is in good agreement with the radius as measured by SEM.
3.1.2. Oxidation of K4Fe(CN)6 in buffered aqueous medium In the same way, Pt microelectrodes were calibrated using the oxidation of hexacyanoferrate (II) in aqueous solution. The oxidation of K4Fe(CN)6 was performed
in aqueous medium (pH 10) in order to confirm that Eq. (1) could be used for the determination of the diffusion coefficient. The validity of Eq. (1) requires that a spherical diffusion is maintained at the microelectrode interface. As for ferrocene, a steady state is reached for slow potential scan rates (20 mV s − 1) [23,24]. A large range of K4Fe(CN)6 concentration (from 4.1×10 − 4 to 0.01 M) was explored. For each concentration, a limiting current from Fe(CN)46 − oxidation was observed. The limiting current was directly proportional to the Fe(CN)46 − concentration. Since the microelectrode radius is known (10 mm), the diffusion coefficient of Fe(CN)46 − can be deduced. The value found, (8.29 0.5)× 10 − 10 m2 s − 1, is in good agreement with the literature [23,24]. The fact that the diffusion coefficient of Fe(CN)46 − was confirmed, suggests that a stationary state of diffusion is obtained for a scan rate of 20 mV s − 1.
3.2. Protons in liquid ammonia After these calibration experiments in common solvents, Pt microelectrodes were tested in liquid ammonia by studying proton reduction. Without proton (NH4Br) in solution, no reduction current was observed from 0 V to − 2.3 V versus SRE (Fig. 2A). From −2.4 V versus SRE (Fig. 2A) a reduction current was detected resulting from potassium salt reduction [25]. When NH+ 4 ions were added, a current was observed from −1.2 to − 2.0 V versus SRE resulting from proton reduction [12]. Liquid ammonia indeed provides a large potential range for studying proton reduction. On varying the scan rate, two different voltammogram shapes were distinguished (Fig. 2A): a stationary current from −1.6 to − 2 V versus SRE and the typical shape as observed at a macroelectrode with a current peak at −1.6 V versus SRE. Since little is known of proton reduction in liquid ammonia, this electrochemical mechanism must be wholly understood. Assuming that proton reduction is not limited by the transfer of electrons, then current is limited by the mass transport of protons. As a rule, mass transport depends essentially on the diffusion of the species. The current at a microelectrode (I) results from transient and steady state diffusion current contributions: I= nFDc pa 2(pDt) − 1/2 + 4anFDc
Fig. 1. Oxidation of ferrocene in acetonitrile at a Pt microelectrode with a radius of 10 mm and with a scan rate of 20 mV s − 1. Concentration of conducting salt: (Bu)4NPF6 = 10 − 3 M. A: Current–potential curves for different concentrations of ferrocene. (a) 3.22× 10 − 4 M; (b) 6.45 × 10 − 4 M; (c) 0.0010 M; (d) 0.0014 M. B: Limiting current (Il) versus concentration of ferrocene. Determination of the radius (a) of the Pt microelectrode at E= 0.4 V versus SRE according to the relation: Il = 4nFDc a.
(2)
According to Eq. (2), it is possible to recognise at least two types of diffusion regime with a different time scale. At short times (or high scan rate), the first term in Eq. (2) will be much larger than the second term and the current is given by the Cottrell equation [19]: Id = nFDc pa 2(pDt) − 1/2
(3)
Therefore in a potentiodynamic experiment with a short scan rate, a transient diffusion current (Id) will be
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Fig. 2. Current – potential curves in liquid ammonia at −50°C with 0.1 M KBr as conducting salt. A: Current – potential curves for different scan −3 rates. (a) Without protons addition (NH4Br), (b)–(h): with [NH+ M. The scan rate was for (a): 20 mV s − 1. (b): 1 mV s − 1; (c) 4 ]= 5.25 × 10 10 mV s − 1; (d): 20 mV s − 1; (e): 50 mV s − 1; (f) 100 mV s − 1; (g): 200 mV s − 1; (h): 400 mV s − 1. B: Current density of NH+ 4 reduction (at − 1.6 V versus SRE) versus the square root of the scan rate (mV s − 1) for different concentrations of proton: (a) 5.25 ×10 − 3 M; (b) 8.8× 10 − 2 M.
observed with a current – potential curve of the same shape as that observed at a macroelectrode, i.e. with a current peak. For a slow scan rate, the transient current density will become negligible and the current reaches a steady state value given by Eq. (1). Eq. (1) is valid in liquid ammonia only for a suitable range of scan rate. In order to determine this range, the reduction of protons was carried out for several scan rates and for different proton concentrations. For each proton concentration, the current measured at − 1.6 V versus SRE was plotted versus the square root of the scan rate (6 1/2) (Fig. 2B). Two ranges of scan rate were observed. At a slow scan rate (from 5 to 30 mV s − 1), the limiting current did not depend on scan rate. This result suggests that a steady state current was reached at the microelectrode interface, and consequently Eq. (1) can be used. For the second range (from 30 to 400 mV s − 1), the limiting current was proportional to the square root of the scan rate. Since the square root of the scan rate is directly proportional to the inverse square root of the time, the Cottrell Eq. (3) can be used. This linear variation results from the fact that the current depends only on the diffusion of protons according to the Cottrell equation. It is clear that only the first range of scan rate is available for studying the proton reduction in liquid ammonia since the limiting current depends only
slightly on the scan rate. A scan rate of 20 mV s − 1 was chosen for the rest of the experiments. The limiting current of proton reduction was measured at −1.2 V [12] for a large range of proton concentrations (from 7× 10 − 4 to 0.1 mol l − 1) (Fig. 3A). These limiting currents were constant from − 1.4 to 2 V. Then, several reduction plateaux were observed according to the concentration of proton (Fig. 3A). These reduction plateaux demonstrate that these reduction currents were limited only by the diffusion of protons at the microelectrode liquid ammonia interface. As a rule, for weak acids, the reaction of proton dissociation must be taken into account during the proton reduction. Then, the whole reaction process is ruled by a CE mechanism [26–28]. In this study, ammonium ion (NH+ 4 ) is the strongest acid in liquid ammonia, then its dissociation from NH4Br is complete in liquid ammonia [18]. As for strong acids in aqueous media, the dissociation process is fast enough to assume that the ammonium ion reduction in liquid ammonia is ruled only by an electrochemical process. These limiting currents were directly proportional to the proton concentration. The plot of these limiting currents against proton concentration is useful for getting information on proton diffusion coefficients in liquid ammonia (Fig. 3B).
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The proton diffusion coefficient in liquid ammonia was deduced from the plot of limiting current versus proton concentration using Eq. (1) (Fig. 3B). The value −9 obtained was D(NH+ m2 s − 1. 4 )NH3 =(3.8 90.4) ×10 This value is in good agreement with the literature values [14,29] obtained by proton resonance at 50°C. In liquid ammonia, the diffusion coefficient of the proton is surprisingly low in comparison to that in aqueous medium. The diffusion coefficient of protons is higher in aqueous medium than in liquid ammonia, since its value is equal to 9.34×10 − 9 m2 s − 1 at 25°C although the viscosity of the water (hWater) is higher than that of in liquid ammonia (hNH3):hWater =1 ×10 − 2 poise at 25°C [14]. By polynomial regression on data obtained by Jander [18] the viscosity of liquid ammonia with 1 M of NH4Cl was deduced: h(NH3) =0.40 ×10 − 2 poise at −60°C. Despite the fact that the viscosity of water is at least a factor of two higher than that of liquid ammonia at −60°C, the diffusion coefficient of the proton in water is more than twice that in liquid ammonia. DNH4+ (NH3) h(H2O) " DH O + (H2O) h(NH3) 3
This result explains the high molar conductivity of H3O+ in water (25°C) in comparison to NH+ 4 in liquid ammonia [14,30]: l0 (H3O+)water =350 V − 1 cm2 mol − 1 −1 l0 (NH+ cm2 mol − 1 4 )NH3 = 142 V
The anomalously high mobility of H3O+ in water is well known to result from a special proton conduction mechanism which is known as the Grotthuss (or proton ‘switching’) mechanism. In water, as a result of hydrogen-bonding, the proton travels a distance of several H2O molecule without having to knock them out of its path. The proton solvated by water takes advantage of the hydrogenbonded structure with low frictional resistance. This Grotthuss mechanism is clearly not applicable to a liquid ammonia solution of NH+ 4 since the conductivity of NH+ 4 in liquid ammonia is three times lower than H3O+ conductivity in water. In contrast to the proton diffusion coefficient in aqueous medium, the low value of the proton diffusion coefficient in liquid ammonia results essentially from the absence of the Grotthus mechanism for proton conductivity in liquid ammonia. The difference between these two diffusion coefficients does not result from the different viscosity of the media but essentially from different mechanisms of proton diffusion: DNH4+ (NH3) l0(NH4+ )(NH3) : DH O + (H2O) l0(H O + )(H2O) 3
3
Even if a Grotthus mechanism does not occur during proton diffusion in liquid ammonia, the diffusion coefficient of the proton is still high enough to ensure a stationary state at the microelectrode with a suitable scan rate potential (6= 20 mV s − 1). This result points
−1 Fig. 3. Limiting current of NH+ . 4 reduction in liquid ammonia ( −50°C) with 0.1 M KBr as conducting salt and with a scan rate of 20 mV s −3 + −3 A: Current – potential curves for different concentrations of protons. (a): without NH4Br; (b): [NH+ ]= 3.21 × 10 M; (c): [NH ]= 5.25×10 4 4 −3 M; (d): [NH+ M. B: Limiting current (Il) versus concentration of NH4Br. Determination of diffusion coefficient of NH+ 4 ] =9.62 ×10 4 according to the relation: Il =4nFDc a.
A.-M. Gonc¸al6es et al. / Journal of Electroanalytical Chemistry 477 (1999) 140–145
to the fact that a suitable time scale experiment could not be deduced simply from the viscosity of the solvent. In liquid ammonia at − 50°C, a value of 4.4× 10 − 9 2 −1 m s was determined for the diffusion coefficient of oxygen [13]. In aqueous medium at 25°C, a value of 1.90×10 − 9 m2 s − 1 was also found for the diffusion coefficient of oxygen [31]. A comparison between these two values emphasises the fact that the diffusion of uncharged species depends essentially on the viscosity of the medium. In contrast to proton diffusion, the following relation was indeed observed for oxygen diffusion: DO2(NH3) h(H2O) : DO2(H2O) h(NH3) 4. Conclusions The study of proton reduction in liquid ammonia at a Pt microelectrode showed that a stationary state can be maintained at the microelectrode liquid ammonia interface using a suitable experimental scan rate (20 mV s − 1). This result is significant since Pt microelectrodes with a radius of 10 mm can indeed be used instead of rotating disk electrodes without significant experimental difficulties. In terms of analysis, for the same experimental conditions, a steady state was still observed for a proton concentration of 5×10 − 5 M. The plot of the limiting current against proton concentration can be used to detect any electrochemical contribution of protons. Since the diffusion coefficients of proton and oxygen are quite similar in liquid ammonia, this solvent offers new opportunities to study, under stationary conditions, the contribution of protons during oxygen reduction in liquid ammonia. These results will be correlated with those obtained on semiconductors as a function of the pH of the medium [12]. Obviously, these microelectrodes can be also used to detect traces of other compounds; an example is water from moisture, which behaves like a weak acid in liquid ammonia.
Acknowledgements It is a pleasure to acknowledge the contribution of Dr M.G. Boutelle, from the Department of Chemistry
.
of King’s College discussions.
London,
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to
many
valuable
References [1] C. Amatore, L. Thouin, J.S. Warkocz, Chem. Eur. J. 5 (1999) 456. [2] C. Amatore, C.R. Acad. Sci. Electrochem. Photochem., Paris 323 (1996) 757. [3] C. Amatore, A.R. Brown, L. Thouin, J.S. Warkocz, C.R. Acad. Sci. Electrochem. Photochem. Paris 323 (1998) 509. [4] M.I. Montenegro, M.A. Queiros, J.L. Daschbach, Microelectrodes: Theory and Applications., Natos ASI Series. Series E: Applied Sciences. 197 (1990). [5] P. Simon, G. Farsang, C. Amatore, J. Electroanal. Chem. 435 (1997) 165. [6] K.B. Oldham, T.J. Cardwell, J.H. Santos, A.M. Bond, J. Electroanal. Chem. 430 (1997) 25. [7] M. Fatima Bento, L. Thouin, C. Amatore, J. Electroanal. Chem. 446 (1998) 91. [8] C. Privat, S. Trevin, F. Bedioui, J. Devynck, J. Electroanal. Chem. 436 (1997) 261. [9] D.H. Evans, M.D. Geraldo, M.I. Montenegro, D. Pletcher, L. Slevin, J. Electroanal. Chem. 439 (1997) 115. [10] R. Muller, J. Heinze, L. Lamberts, L. Christiaens, J. Electrochem. Soc. 145 (1998) 541. [11] A.J. Appleby, J. Electroanal. Chem. 357 (1993) 117. [12] A.M. Gonc¸alves, C. Mathieu, M. Herlem, A. Etcheberry, J. Electroanal. Chem. 462 (1998) 88. [13] F.A. Uribe, A.J. Bard, Inorg. Chem. 21 (1982) 3160. [14] K.F. Purcell, J.C. Kotz, Inorganic Chemistry, Saunders Golden Sunburst Series (1977) 241. [15] W.L. Jolly, J. Phys. Chem. 58 (1954) 250. [16] W.L. Jolly, J. Chem. Educ. 33 (1956) 512. [17] M. Herlem, A. Thie´bault, Bull Soc. Chim. Fr. (1969) 383. [18] J. Jander, Anorganische und allgemeine Chemie influssigen Ammoniak. Part I, Friedr. Vieweg and Sohn. Braunschweig. (1966) 466. [19] A.J. Bard, L.R. Faulkner, in: Masson (Ed.), Electrochimie Principes, me´thodes et applications, 1983. [20] H.A. Laitinen, C.E. Shoemaker, Polarography of Thallium, Copper, Ammonium and Oxygen. 72 (1950) 4975. [21] D. Guyomard, C. Mathieu, M. Herlem, J. Electroanal. Chem. 246 (1988) 29. [22] J.E. Baur, R.M. Wightman, J. Electroanal. Chem. 305 (1991) 73. [23] N. Oyama, T. Ohsaka, N. Yamamoto, J. Matsui, O. Hatozaki, J. Electroanal. Chem. 265 (1989) 297. [24] M. Opallo, J. Electroanal. Chem. 444 (1998) 187. [25] A. Etcheberry, A.-M. Gonc¸alves, C. Mathieu, M. Herlem, J. Electrochem. Soc. 44 (1997) 928. [26] A. Jaworski, M. Donten, Z. Stojek, J.G. Osteryoung, Anal. Chem. 71 (1999) 167. [27] S. Daniele, I. Lavagnini, M.A. Baldo, F. Magno, Anal. Chem. 70 (1998) 285. [28] S. Daniele, M.A. Baldo, C. Bragato, I. Lavagnini, Anal. Chim. Acta. 361 (1998) 141. [29] D.R. Clutter, T.J. Swift, J. Am. Chem. Soc. 31 (1968) 601. [30] V.A. Pleskov, Acta Physicochim. URSS 5 (1936) 509. [31] K.E. Gubbins, R.D. Walker Jr, J. Electrochem. Soc. 12 (1965) 469.
Using Pt microelectrodes in liquid ammonia for studying proton reduction A-M. Gonc¸alves *, C. Mathieu, M. Herlem, A. Etcheberry IREM (Institut La6oisier), Uni6ersite´ de Versailles St Quentin, Baˆt. La6oisier, UMR CNRS CO173, 45 A6. des Etats Unis, 78035 Versailles Cedex, France Received 21 June 1999; received in revised form 24 September 1999; accepted 24 September 1999
Abstract The electrical response of Pt microelectrodes, with a radius of 10 mm, was studied in liquid ammonia ( − 50°C). The question of a suitable time scale for steady state measurements in liquid ammonia is important. The proton reduction currents were measured for different scan rates. A suitable experimental time scale of 20 mV s − 1 was determined to reach a stationary state. The diffusion coefficient of protons in liquid ammonia was deduced from these electrochemical results: D(NH+ 4 )NH3 =(3.8 9 0.4)× 10 − 9 m2 s − 1. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Microelectrodes; Stationary state; Liquid ammonia; Diffusion coefficient
1. Introduction Microelectrode techniques have been used widely in electrochemistry during the past decade in many scientific areas, in for example monitoring fundamental biological events or electrochemical processes [1 – 5]. In comparison to electrodes of millimetre size, microelectrodes have at least three unique advantages resulting from their reduced size [4,6]. The first is that mass transport to and from the electrode is enhanced because of spherical diffusion due to edge effects. A second advantage is that the double-layer capacitance is reduced due to the reduction in surface area. A third advantage is that microelectrodes are almost free from ohmic drop (IR) phenomena. In contrast to macroelectrodes, it is not necessary to correct the data for IR drop. This property allows one to detect weak Faradaic currents which can be also observed using a solvent of low dielectric constant, in the absence or with a low concentration of supporting electrolyte. All these properties make microelectrodes very useful for studying intermediate species in non-aqueous solvents [7 – 10].
* Corresponding author. Fax: +33-1-39-254381. E-mail address: [email protected] (A.-M. Gonc¸alves)
Protons are often implied in many fundamental electrochemical processes like the oxygen reduction [11– 13]. An obvious way to understand these complex electrochemical processes is to isolate the effects of protons. Thus, a specific study of protons becomes crucial. Among all non-aqueous solvents, liquid ammonia appears to be unique both for its similarities to water (both are protic solvents) and its own chemical properties [14–18]. The interesting point in using liquid ammonia is that a large range of pH can be obtained: 33 pH units. This large pH range allows the study of electrochemical processes with and without proton contributions. Whatever the electrochemical reaction, the knowledge of the number of electrons exchanged is crucial for understanding the process. Steady state diffusion at the electrode solution interface gives a limiting current, which is directly proportional to the number of electrons exchanged and also to the concentration of electroactive species in the bulk solution. If the number of electrons is known, the limiting current can be related to a specific concentration. Electrochemical processes which are controlled by mass transport are useful for chemical analysis. Two classical electrochemical techniques are used usually to achieve a stationary state in both aqueous
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 3 9 8 - 8
A.-M. Gonc¸al6es et al. / Journal of Electroanalytical Chemistry 477 (1999) 140–145
and common non-aqueous solvents [19]. The first is related to forced convection techniques in, for example, the rotating disk electrode. The second technique makes use of spherical diffusion obtained, for instance, at a dropping mercury electrode. These two classical electrochemical techniques provide controlled diffusion at the electrode solution interface. The number of electrons involved in the electrochemical process can be deduced from the limiting current. Since experiments in liquid ammonia must be carried out with great care, rotating disk and dropping mercury electrodes have been hardly used. Even when a steady state could be reached with the dropping mercury electrode, the limiting diffusion current was not well-defined and consequently the number of electrons exchanged was uncertain [20]. Microelectrodes should provide new opportunities for obtaining a well-defined limiting diffusion current in liquid ammonia without involving significant experimental difficulties. In contrast to the two other classical techniques, there is no doubt about the practicality of microelectrodes resulting from their reduction in size. In spite of the fact that microelectrodes are used usually in both aqueous and non aqueous solvents, nothing is known about their properties in liquid ammonia. In this work, before using a microelectrode in liquid ammonia, we checked each electrode in usual solvents. For this purpose two reactions were used; the oxidation of ferrocene in acetonitrile and the oxidation of potassium ferrocyanide (K4Fe(CN)6) in aqueous solvent. These two electrochemical couples have been chosen since they are well studied systems and are commonly used to characterize microelectrodes [4]. Studies of electrochemical reduction of protons in liquid ammonia are relevant for at least two reasons. The first is that the basic medium allows the exclusion of all contributions from protons during an electrochemical process. The second point is that providing a steady state at a microelectrode, the limiting diffusion current due to proton reduction is directly proportional to the concentration of protons in acidic medium. Protons, in liquid ammonia, (NH+ 4 ) are solvated by the ammonia molecule because of the strong basicity of liquid ammonia. In contrast to aqueous media, ammonium ion (NH+ 4 ) is the strongest acid in liquid ammonia [18]. Even if the electrical conductivities of many ammonium salts were measured since 1900 in liquid ammonia [18], no data were given concerning the electrical conductivity of either cations or anions of the salt. This study gives an opportunity to compare electrical conductivity of ammonium ion (NH+ 4 ) to NMR results. A relation between the limiting currents of proton reduction (NH+ 4 ) and a large range of proton concentration becomes really relevant for detecting any contribution of protons during an electrochemical process.
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Since nothing is known of the interface between a microelectrode and liquid ammonia, the question of a suitable time scale for steady state measurements in liquid ammonia is important. This time scale depends on the potential scan rate: the longer is the time scale, the slower must be the scan rate. A stationary state is reached if the scan rate is slow enough to ensure spherical diffusion due to edge effects at the microelectrode. What kind of time scale can we expect in liquid ammonia? If the time scale is too long, corresponding to scan rates of less than 1 mV s − 1, variations of temperature could occur during the electrochemical processes and lifetimes of species and kinetic processes could be modified. The aim of this work is then to define a suitable time scale for steady state experiments using a platinum microelectrode with a radius of 10 mm.
2. Experimental Ammonia (‘electronic grade’) was obtained from Air Liquid. The device for condensing ammonia and the cell have already been described [21]. The volume of liquid ammonia is about 150 ml and the cell was maintained at − 50°C in a cryostat. An acidic medium is obtained by addition of NH4Br (from Aldrich). Platinum microelectrodes have been made using a platinum wire with a radius of 10 mm. The microelectrode was prepared by sealing the Pt wire into soft glass tubing. The microelectrode was polished on successively finer grades of alumina and finally cleaned in an ultrasonic water bath. The radius of the microdisk electrode was determined by linear voltammetry from the plot of steady state current against concentration of Fe(CN)46 − . We get complementary information by studying the oxidation of ferrocene (from Aldrich) in a volume of 10 ml of acetonitrile and also in studying the oxidation of K4Fe(CN)6 (from Aldrich) in a volume of 100 ml in a buffered basic aqueous medium (pH 10). These two electrochemical couples were chosen because they are model systems in these solvents [4,22]. A Pt wire (diameter l mm) was used as an auxiliary electrode. All potentials were measured against a silver reference electrode (SRE) in non-aqueous solvent (acetonitrile and liquid ammonia). In aqueous medium, all potentials were measured against the mercury sulphate electrode (MSE). Since the radius of our microelectrode was not small enough to allow us to omit a supporting electrolyte, 10 − 3 M of (Bu)4NPF6 was added in acetonitrile and 10 − 3 M of KBr was added to liquid ammonia to ensure sufficient conductivity. All electrochemical measurements were carried out under an argon stream to avoid electrochemical reactions with oxygen.
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3. Results and discussion
3.1. Microelectrode calibration 3.1.1. Ferrocene oxidation in acetonitrile The oxidation of ferrocene was performed in acetonitrile. A large range of ferrocene concentrations (from 3.2 ×10 − 4 to 1.4 × 10 − 3 M) was explored. A potential scan rate of 20 mV s − 1 provides a steady state at a 10 mm radius Pt microelectrode [22]. For each concentration, a limiting current from ferrocene oxidation was observed (Fig. 1A). The limiting current was directly proportional to the ferrocene concentration (Fig. 1B). At a microdisk electrode, under spherical diffusion conditions, the limiting current (Il) is given by the following equation [4]: Il = 4nFDc a
(1)
One electron (n=1) is exchanged during the electrochemical oxidation of ferrocene. F is the Faraday constant (96500 C mol − 1). D, the diffusion coefficient of ferrocene in acetonitrile is 2.2×10 − 9 m2 s − 1 [22]. c indicates the concentration of ferrocene in the bulk of the solution. If all the parameters of Eq. (1) are known, the radius (a) of the microelectrode can be deduced. In our case a was 1090.5 mm. This result is in good agreement with the radius as measured by SEM.
3.1.2. Oxidation of K4Fe(CN)6 in buffered aqueous medium In the same way, Pt microelectrodes were calibrated using the oxidation of hexacyanoferrate (II) in aqueous solution. The oxidation of K4Fe(CN)6 was performed
in aqueous medium (pH 10) in order to confirm that Eq. (1) could be used for the determination of the diffusion coefficient. The validity of Eq. (1) requires that a spherical diffusion is maintained at the microelectrode interface. As for ferrocene, a steady state is reached for slow potential scan rates (20 mV s − 1) [23,24]. A large range of K4Fe(CN)6 concentration (from 4.1×10 − 4 to 0.01 M) was explored. For each concentration, a limiting current from Fe(CN)46 − oxidation was observed. The limiting current was directly proportional to the Fe(CN)46 − concentration. Since the microelectrode radius is known (10 mm), the diffusion coefficient of Fe(CN)46 − can be deduced. The value found, (8.29 0.5)× 10 − 10 m2 s − 1, is in good agreement with the literature [23,24]. The fact that the diffusion coefficient of Fe(CN)46 − was confirmed, suggests that a stationary state of diffusion is obtained for a scan rate of 20 mV s − 1.
3.2. Protons in liquid ammonia After these calibration experiments in common solvents, Pt microelectrodes were tested in liquid ammonia by studying proton reduction. Without proton (NH4Br) in solution, no reduction current was observed from 0 V to − 2.3 V versus SRE (Fig. 2A). From −2.4 V versus SRE (Fig. 2A) a reduction current was detected resulting from potassium salt reduction [25]. When NH+ 4 ions were added, a current was observed from −1.2 to − 2.0 V versus SRE resulting from proton reduction [12]. Liquid ammonia indeed provides a large potential range for studying proton reduction. On varying the scan rate, two different voltammogram shapes were distinguished (Fig. 2A): a stationary current from −1.6 to − 2 V versus SRE and the typical shape as observed at a macroelectrode with a current peak at −1.6 V versus SRE. Since little is known of proton reduction in liquid ammonia, this electrochemical mechanism must be wholly understood. Assuming that proton reduction is not limited by the transfer of electrons, then current is limited by the mass transport of protons. As a rule, mass transport depends essentially on the diffusion of the species. The current at a microelectrode (I) results from transient and steady state diffusion current contributions: I= nFDc pa 2(pDt) − 1/2 + 4anFDc
Fig. 1. Oxidation of ferrocene in acetonitrile at a Pt microelectrode with a radius of 10 mm and with a scan rate of 20 mV s − 1. Concentration of conducting salt: (Bu)4NPF6 = 10 − 3 M. A: Current–potential curves for different concentrations of ferrocene. (a) 3.22× 10 − 4 M; (b) 6.45 × 10 − 4 M; (c) 0.0010 M; (d) 0.0014 M. B: Limiting current (Il) versus concentration of ferrocene. Determination of the radius (a) of the Pt microelectrode at E= 0.4 V versus SRE according to the relation: Il = 4nFDc a.
(2)
According to Eq. (2), it is possible to recognise at least two types of diffusion regime with a different time scale. At short times (or high scan rate), the first term in Eq. (2) will be much larger than the second term and the current is given by the Cottrell equation [19]: Id = nFDc pa 2(pDt) − 1/2
(3)
Therefore in a potentiodynamic experiment with a short scan rate, a transient diffusion current (Id) will be
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Fig. 2. Current – potential curves in liquid ammonia at −50°C with 0.1 M KBr as conducting salt. A: Current – potential curves for different scan −3 rates. (a) Without protons addition (NH4Br), (b)–(h): with [NH+ M. The scan rate was for (a): 20 mV s − 1. (b): 1 mV s − 1; (c) 4 ]= 5.25 × 10 10 mV s − 1; (d): 20 mV s − 1; (e): 50 mV s − 1; (f) 100 mV s − 1; (g): 200 mV s − 1; (h): 400 mV s − 1. B: Current density of NH+ 4 reduction (at − 1.6 V versus SRE) versus the square root of the scan rate (mV s − 1) for different concentrations of proton: (a) 5.25 ×10 − 3 M; (b) 8.8× 10 − 2 M.
observed with a current – potential curve of the same shape as that observed at a macroelectrode, i.e. with a current peak. For a slow scan rate, the transient current density will become negligible and the current reaches a steady state value given by Eq. (1). Eq. (1) is valid in liquid ammonia only for a suitable range of scan rate. In order to determine this range, the reduction of protons was carried out for several scan rates and for different proton concentrations. For each proton concentration, the current measured at − 1.6 V versus SRE was plotted versus the square root of the scan rate (6 1/2) (Fig. 2B). Two ranges of scan rate were observed. At a slow scan rate (from 5 to 30 mV s − 1), the limiting current did not depend on scan rate. This result suggests that a steady state current was reached at the microelectrode interface, and consequently Eq. (1) can be used. For the second range (from 30 to 400 mV s − 1), the limiting current was proportional to the square root of the scan rate. Since the square root of the scan rate is directly proportional to the inverse square root of the time, the Cottrell Eq. (3) can be used. This linear variation results from the fact that the current depends only on the diffusion of protons according to the Cottrell equation. It is clear that only the first range of scan rate is available for studying the proton reduction in liquid ammonia since the limiting current depends only
slightly on the scan rate. A scan rate of 20 mV s − 1 was chosen for the rest of the experiments. The limiting current of proton reduction was measured at −1.2 V [12] for a large range of proton concentrations (from 7× 10 − 4 to 0.1 mol l − 1) (Fig. 3A). These limiting currents were constant from − 1.4 to 2 V. Then, several reduction plateaux were observed according to the concentration of proton (Fig. 3A). These reduction plateaux demonstrate that these reduction currents were limited only by the diffusion of protons at the microelectrode liquid ammonia interface. As a rule, for weak acids, the reaction of proton dissociation must be taken into account during the proton reduction. Then, the whole reaction process is ruled by a CE mechanism [26–28]. In this study, ammonium ion (NH+ 4 ) is the strongest acid in liquid ammonia, then its dissociation from NH4Br is complete in liquid ammonia [18]. As for strong acids in aqueous media, the dissociation process is fast enough to assume that the ammonium ion reduction in liquid ammonia is ruled only by an electrochemical process. These limiting currents were directly proportional to the proton concentration. The plot of these limiting currents against proton concentration is useful for getting information on proton diffusion coefficients in liquid ammonia (Fig. 3B).
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The proton diffusion coefficient in liquid ammonia was deduced from the plot of limiting current versus proton concentration using Eq. (1) (Fig. 3B). The value −9 obtained was D(NH+ m2 s − 1. 4 )NH3 =(3.8 90.4) ×10 This value is in good agreement with the literature values [14,29] obtained by proton resonance at 50°C. In liquid ammonia, the diffusion coefficient of the proton is surprisingly low in comparison to that in aqueous medium. The diffusion coefficient of protons is higher in aqueous medium than in liquid ammonia, since its value is equal to 9.34×10 − 9 m2 s − 1 at 25°C although the viscosity of the water (hWater) is higher than that of in liquid ammonia (hNH3):hWater =1 ×10 − 2 poise at 25°C [14]. By polynomial regression on data obtained by Jander [18] the viscosity of liquid ammonia with 1 M of NH4Cl was deduced: h(NH3) =0.40 ×10 − 2 poise at −60°C. Despite the fact that the viscosity of water is at least a factor of two higher than that of liquid ammonia at −60°C, the diffusion coefficient of the proton in water is more than twice that in liquid ammonia. DNH4+ (NH3) h(H2O) " DH O + (H2O) h(NH3) 3
This result explains the high molar conductivity of H3O+ in water (25°C) in comparison to NH+ 4 in liquid ammonia [14,30]: l0 (H3O+)water =350 V − 1 cm2 mol − 1 −1 l0 (NH+ cm2 mol − 1 4 )NH3 = 142 V
The anomalously high mobility of H3O+ in water is well known to result from a special proton conduction mechanism which is known as the Grotthuss (or proton ‘switching’) mechanism. In water, as a result of hydrogen-bonding, the proton travels a distance of several H2O molecule without having to knock them out of its path. The proton solvated by water takes advantage of the hydrogenbonded structure with low frictional resistance. This Grotthuss mechanism is clearly not applicable to a liquid ammonia solution of NH+ 4 since the conductivity of NH+ 4 in liquid ammonia is three times lower than H3O+ conductivity in water. In contrast to the proton diffusion coefficient in aqueous medium, the low value of the proton diffusion coefficient in liquid ammonia results essentially from the absence of the Grotthus mechanism for proton conductivity in liquid ammonia. The difference between these two diffusion coefficients does not result from the different viscosity of the media but essentially from different mechanisms of proton diffusion: DNH4+ (NH3) l0(NH4+ )(NH3) : DH O + (H2O) l0(H O + )(H2O) 3
3
Even if a Grotthus mechanism does not occur during proton diffusion in liquid ammonia, the diffusion coefficient of the proton is still high enough to ensure a stationary state at the microelectrode with a suitable scan rate potential (6= 20 mV s − 1). This result points
−1 Fig. 3. Limiting current of NH+ . 4 reduction in liquid ammonia ( −50°C) with 0.1 M KBr as conducting salt and with a scan rate of 20 mV s −3 + −3 A: Current – potential curves for different concentrations of protons. (a): without NH4Br; (b): [NH+ ]= 3.21 × 10 M; (c): [NH ]= 5.25×10 4 4 −3 M; (d): [NH+ M. B: Limiting current (Il) versus concentration of NH4Br. Determination of diffusion coefficient of NH+ 4 ] =9.62 ×10 4 according to the relation: Il =4nFDc a.
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to the fact that a suitable time scale experiment could not be deduced simply from the viscosity of the solvent. In liquid ammonia at − 50°C, a value of 4.4× 10 − 9 2 −1 m s was determined for the diffusion coefficient of oxygen [13]. In aqueous medium at 25°C, a value of 1.90×10 − 9 m2 s − 1 was also found for the diffusion coefficient of oxygen [31]. A comparison between these two values emphasises the fact that the diffusion of uncharged species depends essentially on the viscosity of the medium. In contrast to proton diffusion, the following relation was indeed observed for oxygen diffusion: DO2(NH3) h(H2O) : DO2(H2O) h(NH3) 4. Conclusions The study of proton reduction in liquid ammonia at a Pt microelectrode showed that a stationary state can be maintained at the microelectrode liquid ammonia interface using a suitable experimental scan rate (20 mV s − 1). This result is significant since Pt microelectrodes with a radius of 10 mm can indeed be used instead of rotating disk electrodes without significant experimental difficulties. In terms of analysis, for the same experimental conditions, a steady state was still observed for a proton concentration of 5×10 − 5 M. The plot of the limiting current against proton concentration can be used to detect any electrochemical contribution of protons. Since the diffusion coefficients of proton and oxygen are quite similar in liquid ammonia, this solvent offers new opportunities to study, under stationary conditions, the contribution of protons during oxygen reduction in liquid ammonia. These results will be correlated with those obtained on semiconductors as a function of the pH of the medium [12]. Obviously, these microelectrodes can be also used to detect traces of other compounds; an example is water from moisture, which behaves like a weak acid in liquid ammonia.
Acknowledgements It is a pleasure to acknowledge the contribution of Dr M.G. Boutelle, from the Department of Chemistry
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of King’s College discussions.
London,
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to
many
valuable
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