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Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-d -gluconate complex by cyclic voltammetry
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 580 (2005) 173–178 www.elsevier.com/locate/jelechem
Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-D-gluconate complex by cyclic voltammetry Bechtold Thomas *, Turcanu Aurora Institute for Textile Chemistry and Textile Physics, Leopold-Franzens-University Innsbruck, Hoechsterstrasse 73, A-6850 Dornbirn, Austria Received 31 January 2005; received in revised form 13 March 2005; accepted 30 March 2005 Available online 5 May 2005
Abstract Electron transfer reactions between ironII/III-complexes are of general interest for corrosion processes, biological processes and technical applications. The cathodic reduction of FeIII-triethanolamine (TEA) complex to form the FeIITEA-form of the complex has been examined by cyclic voltammetry. In the presence of FeIII-D-heptagluconate (HDGL), complex catalytic currents are observed due to homogenous chemical reaction between FeIITEA and FeIII(HDGL)2 which regenerates FeIIITEA. This method can be used to determine the rate constant of the redox reaction between FeIITEA and FeIII(HDGL)2 with k 0f ¼ 88 12 dm3 mol1 s1 . 2005 Elsevier B.V. All rights reserved. Keywords: Triethanolamine; Hepta-D-gluconic acid; Iron; Catalytic current; Cyclic voltammetry; Reaction rate
1. Introduction In aqueous alkaline solutions amino compounds and sugar-acids are excellent complexing agents for FeII- and FeIII-ions [1–3]. Iron complexes with sugar acid type ligands are of importance as corrosion inhibitors and in biological systems [4,5]. Alkali stable ironII/III-complexes are also of interest for the indirect cathodic reduction of vat dyes in different dyeing processes [6,7]. The electrochemical behaviour of the FeIII/IITEA complexes has been studied by different groups (TEA = triethanolamine) [1,8–10]. The reversible cathodic reduction of the Fe(III)-triethanolamine complex is observed at (Ep)c = 1050 mV (vs. Ag/AgCl, 3 M KCl) as reversible redox process [1,8,10]. The limited stability of the FeIII/IITEA complexes at lower pH led to the use of mix-
tures of complexing agents of amino-ligands and sugar acids, e.g., TEA and D-gluconate (DGL) [6]. In analogy to the behaviour of D-gluconate (DGL) which forms a 1:2 complex Fe(DGL)2, the Fe(HDGL)2 can be assumed to be present in alkaline solution investigated (HDGL = D-hepta-gluconate) [11]. In this study, the reaction between cathodically formed FeIITEA and the FeIIIHDGL-complex present in excess has been investigated with cyclic voltammetry. As a result of the redox reaction between FeIITEA and FeIIIHDGL-complex, a catalytic current is observed in the CV experiments. Catalytic processes in electrochemistry occur when a substance O is reduced to R (Eq. (1)) and R regenerates O by a chemical reaction with a substance Z (Eq. (2)). The reduction of Z yields the product P O þ n e ¢ R
ð1Þ
*
Corresponding author. Tel.: +43 5572 28533; fax: +43 5572 28629. E-mail addresses: [email protected], Thomas.Bechtold@uibk. ac.at (B. Thomas). 0022-0728/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.03.033
k 0f
R þ Z!O þ P
ð2Þ
174
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
In the time scale of the CV experiments performed, the cathodic formation of FeIITEA can be considered as fast compared to the reaction of FeIITEA with FeIII(HDGL)2. Thus, a calculation of the rate constant k 0f for the reaction between FeIITEA and FeIII(HDGL)2 is possible using a large scale plot of the working curve given by Nicholson and Shain for an irreversible catalytic reaction following a reversible charge transfer [12,13]. In this paper, an analysis of the catalytic currents measured in cyclic voltammetry in the FeIIITEA/ FeIII(HDGL)2 system is presented and the pseudofirst-order rate constant k 0f for the reaction according to Eq. (2) has been estimated.
2. Experimental The NaOH, Fe(NO3)3 Æ 9H2O and triethanolamine (TEA, tris-(2-hydroxy-ethyl)-amine) were analytical grade chemicals (Merck, Riedl-de-Haen). An aqueous solution of Na-hepta-D-gluconate (HDGL, C7H13O8Na) was used, the exact concentration of the solution was determined by potentiometric titration with 1 M HCl. The CV experiments were performed at a HMDEelectrode (EG&G potentiostat 264A with 303A polarographic analyser EG&G, Princeton, NJ) equipped with a Rikadenki X-Y recorder. A mercury drop of 0.96 · 102 cm2 area was used (HMDE drop size small). Potential values are related to a Ag/AgCl, 3 M KCl reference electrode. Samples were deoxygenated for at least 8 min with He. Experiments were performed at room temperature.
The pH of the solutions was measured with a glass electrode and a potentiometer (Metrohm 654 pH meter, Herisau, Switzerland). Spectrophotometric experiments were performed with a diode array photometer using a 10 mm pathlength cuvette (Zeiss CLH 500/MCS 521 UV–vis, Carl Zeiss (Jean), Germany). The complexes were prepared 24 h before the analysis to permit sufficient time for equilibration between the Fe-complexes. During equilibration time samples were stored in the dark to avoid light induced oxidation.
3. Results and discussion The composition of the solutions investigated by cyclic voltammetry is given in Table 1. A stock solution containing the components of solution 1 was prepared and the sample solutions were then prepared by dilution, respectively, by addition of the required amount of TEA to obtain solutions 1–4. According to Eq. (3), an equilibrium between the two ligands and the corresponding FeIIIcomplexes establishes in the solution TEA þ FeIII ðHDGLÞ2 ¢ FeIII TEA þ 2HDGL K¼
cðFeIII ðHDGLÞ2 ÞcðTEAÞ cðFeIII ðTEAÞÞcðHDGLÞ
ð3Þ ð4Þ
2
The equilibrium between the two FeIIIcomplexes could be observed by cyclic voltammetry. Table 2 shows the cathodic peak current (ip)d of the FeIIITEA complex in the presence of various concentrations of both ligands
Table 1 Composition of solutions investigated for determination of k 0f No.
c(Fe3+) (·102 mol dm3)
c(TEA) (·104 mol dm3)
c(HDGL) (·102 mol dm3)
c(NaOH) (mol dm3)
pH
1 2 3 4
2.40 2.40 2.40 0.25
0 2.44 4.93 2389
5.73 5.73 5.73 0
0.344 0.344 0.344 0.356
13.31 13.28 13.32 13.42
Table 2 Composition of solutions 5–11, pH and cathodic current (ip)c at (Ep)c = 1050 mV at various scan rates No.
5 6 7 8 9 10 11
c(Fe3+) (·104 mol dm3)
c(TEA) (·102 mol dm3)
c(HDGL) (·102 mol dm3)
c(NaOH) (mol dm3)
pH
24.4 24.4 24.4 24.4 24.4 24.4 24.4
0.24 2.38 23.6 0.24 2.38 23.6 23.6
0.48 0.48 0.48 1.0 1.0 1.0 0.0
0.34 0.34 0.34 0.34 0.34 0.34 0.34
13.21 13.22 13.22 13.23 13.23 13.23 13.21
Cathodic peak current (ip)c (lA) Scan rate (mV s1) 5
10
20
50
100
200
1.5 1.4 1.7 1.3 1.8 1.9 2.0
2.0 2.4 2.3 1.6 2.4 2.5 2.5
2.8 3.7 3.2 2.3 3.2 3.4 3.4
4.6 4.8 4.9 3.5 4.9 5.7 5.2
5.0 6.7 7.0 5.0 6.8 7.3 7.3
7.2 9.4 9.5 6.8 9.5 10.1 9.1
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
TEA and HDGL. On basis of the cathodic peak current (ip)d as a measure for the concentration of FeIIITEA present in solutions 5 and 8, the equilibrium constant K can be determined in 0.34 mol dm3 NaOH according to Eqs. (3) and (4) with K = 5.1 ± 0.4. Using these data, the distribution between the complexes FeIIITEA and FeIII(HDGL)2 in the solutions 2 and 3 can be determined. As a result the concentration of uncomplexed TEA remains below 3% of the total concentration of TEA present in solutions 2 and 3, respectively. Thus, the concentration of uncomplexed TEA can be neglected and c(FeIIITEA) can be set equivalent to c(TEA). In photometry the formation of the TEA complex can also be studied by the decrease of the absorbance of the FeIII(HDGL)2 complex followed by an addition of TEA. Fig. 1 shows the absorption spectra in the wavelength range of 350–550 nm of a solution of 2.39 · 102 mol dm3 FeIII(HDGL)2 in 0.34 mol dm3 aqueous NaOH in the presence of 0, 2.65 · 104, 5.11 · 104, 13.54 · 104, 124.1 · 104 and 4 3 2389 · 10 mol dm TEA. Lowering of the absorbance between 400 and 420 nm (Fe(HDGL)2 and formation of the TEA complex can be observed with increased concentration of TEA. The FeIII/IITEA complex shows the shape of a voltammogram for reversible redox reaction (Fig. 2) [1,8–11]. The cathodic peak current of the diffusion controlled process (ip)d is directly proportional to the square root of the scan rate. In the CV of a solution of 24.5 · 104 mol dm3 Fe(NO3)3 in 0.239 mol dm3 TEA and 0.356 mol dm3 NaOH, a cathodic peak is observed at (Ep)c = 1050 mV and in the reverse scan an anodic peak is observed at (Ep)a = 990 mV. The cathodic peak current (ip)d at scan rates between 5 and
175
Fig. 2. Cyclic voltammograms of 0.25 · 102 mol dm3 FeIIITEA in 0.239 mol dm3 TEA and 0.356 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5, 10, 20, 50, 100 and 200 mV s1 (solution 4).
200 mV s1 ranges from 2.1 lA at 5 mV s1 to 8.4 lA at 200 mV s1. In Fig. 3, the CVs of a solution of 2.40 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV are shown for scan rates of 5 and 200 mV s1. Despite the rather high concentration of the FeIII(HDGL)2 complex, no distinct cathodic current peak is observed in the
1.8 1.6 1.4
0 2.65 5.11 13.54 124.1 2389
1.2
absorbance
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 360
380
400
420
440
460
480
500
520
540
λ (nm)
Fig. 1. Absorption spectra of 2.39 · 102 mol dm3 FeIII(HDGL)2 in the presence of 0, 2.65 · 104, 5.11 · 104, 13.5 · 104, 124 · 104 and 2389 · 104 mol dm3 TEA in the wavelength range of 350–550 nm.
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B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
Fig. 3. Cyclic voltammograms of 2.40 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 1).
potential range between 400 and 1100 mV. No anodic peak which could indicate cathodically formed FeII(HDGL)2 is observed during the reverse scan. This behaviour is similar to the results given in the literature for FeIII(DGL)2 [6,11]. In the presence of low concentrations of TEA, the concentration of TEA limits the concentration of FeIIITEA complex formed in solution. The reduction of FeIIITEA to FeIITEA is observed in the CV near 1050 mV. Due to a redox reaction between cathodically formed FeIITEA and FeIII(HDGL)2 present in high excess, a catalytic current with regeneration of FeIIITEA is observed according to Eqs. (5) and (6).
As expected from theoretical considerations given by Nicholson and Shain [13] and Saveant and Vianello [12], a cathodic current plateau is observed when co cz, c(FeTEA) c(Fe(HDGL)2), respectively. The process according to Eq. (6) can be taken as irreversible reaction due to the absence of an anodic current peak for the reoxidation of Fe(II)TEA. Typical results of CV experiments are shown in Figs. 4 and 5. In Fig. 4, CVs of 2.44 · 104 mol dm3 FeIIITEA in the presence of 2.37 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 are shown. In Fig. 5, the concentration of FeIIITEA complex was raised to 4.93 · 104 mol dm3. The detailed conditions and results of the experiments are given in Table 3.
Fig. 4. Cyclic voltammogram of 2.44 · 104 mol dm3 FeIIITEA in the presence of 2.37 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 2).
Fig. 5. Cyclic voltammogram of 4.93 · 104 mol dm3 FeIIITEA in the presence of 2.35 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 3).
As can be seen in the cyclic voltammograms, a catalytic peak current is observed at (Ep)c 1050 mV which corresponds to the cathodic reduction of FeIIITEA. Due to the reaction according to Eq. (6), regeneration of FeIIITEA occurs and the catalytic peak current (ip)c is observed. In cases of low scan rates of 5, 10 and 20 mV s1, a small anodic peak is observed in the reverse scan, which indicates limitations of the assumption
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
177
Table 3 Catalytic current (ip)c and rate constant k 0f for the reaction between Fe2+TEA (2.44 · 104, 4.93 · 104 mol dm3) and Fe3+(HDGL)2 (solutions 2 and 3) Scan rate (mV s1)
c(Fe3+TEA) (·104 mol dm3)
c(Fe3+HDGL2) (·102 mol dm3)
c(HDGL) (·102 mol dm3)
(ip)c (lA)
(ip)d (lA)
(ip)c/(ip)d
k 0f ðdm3 mol1 s1 Þ
5 10 20 50 100 200
2.44 2.44 2.44 2.44 2.44 2.44
2.37 2.37 2.37 2.37 2.37 2.37
1.01 1.01 1.01 1.01 1.01 1.01
0.80 0.75 0.73 0.93 1.25 1.28
0.21 0.25 0.32 0.46 0.63 0.84
3.81 3.00 2.27 2.01 1.98 1.52
18 26 37 72 142 134
5 10 20 50 100 200
4.93 4.93 4.93 4.93 4.93 4.93
2.35 2.35 2.35 2.35 2.35 2.35
1.11 1.11 1.11 1.11 1.11 1.11
1.63 1.78 1.90 2.03 2.18 2.10
0.42 0.50 0.65 0.93 1.27 1.69
3.84 3.52 2.95 2.18 1.71 1.24
18 31 48 85 101 78
of co cz due to relative high turn over in the diffusion layer of the cathode. Thus, experiments at scan rates of 5–20 mV s1 were not considered for the final calculation of kf. From theoretical considerations for a given cz, the height of the catalytic current (ip)c is independent of the scan rate, however, in the experiments presented in Table 3 (ip)c a certain dependence on the scan rate was detected. To eliminate the influence of the baseline slope, for determination of (ip)c the base line current of the pure Fe(HDGL)2 system was subtracted from the cathodic current measured at (Ep)c. On basis of the method given by Nicholson and Shain, a large scale plot of (ip)c/(ip)d vs. (kf/a) can be used to determine the rate constant for the reaction according to Eq. (6) using Eqs. (7)–(9). Spherical correction was introduced into Eq. (7) to correct the current function for plane electrodes to spherical geometry of the Hg-drop [13]. Due to the introduction of the spherical correction, the graph of (ip)c/(ip)d vs. (kf/a) becomes dependent on the scan rate of the voltammogram. Thus, to obtain more accurate graphs the graph was calculated for each scan rate 1=2
1=2
ðip Þc ðapÞ vðatÞc þ D0 r1 0 /ðatÞ ¼ ; 1=2 1=2 ðip Þd ðapÞ vðatÞd þ D0 r1 0 /ðatÞ
ð7Þ
a ¼ nF mR1 T 1 ;
ð8Þ
k f ¼ k 0f cz ;
ð9Þ
where v(at)c denotes current function for cyclic voltammogram with catalytic regeneration of species [13] and /(at) is a term for spherical correction of electrode [13]. The other symbols have their usual meaning as defined in [13]. According to Polcyn and Shain [9] the diffusion coefficient of the FeIIITEA-complex was set 7 · 106 cm2 s1. The catalytic current was corrected
by subtraction of the baseline current observed in the FeIII(HDGL)2 system in the absence of any TEA. The calculated values for k 0f are given in Table 2. Generally, the experiments at lower concentration of FeIIITEA 2.44 · 104 mol dm3 suffer from increased experimental errors for the determination of the cathodic currents due to the low concentration of FeIIITEA. Thus, the data obtained at a concentration of 4.93 · 104 mol dm3 (solution 3) at scan rates of 50– 200 mV s1 were used for the calculations. From these data, the rate constant k 0f for the reaction (4) can be calculated with 88 ± 12 dm3 mol1 s1. An increase of the concentration of FeIIITEA concentration is limited by the presumption co cz, while the application of higher concentrations of FeIII(HDGL)2 is prohibited by the height of the observed cathodic baseline current due to slow cathodic reduction of FeIII(HDGL)2.
4. Conclusions In alkaline solutions FeIIITEA complexes show diffusion controlled cathodic reduction to yield FeIITEA and cyclic voltammograms of a reversible redox couple are observed. In cyclic voltammograms of FeIII(HDGL)2 complexes, much lower cathodic currents are measured in the potential region between 400 and 1200 mV. Addition of a relative small concentration of TEA to a solution containing excess FeIII(HDGL)2 results in the formation of FeIIITEA and in CV experiments a catalytic current between cathodically formed FeIITEA and FeIII(HDGL)2 is observed. This catalytic current permits the calculation of the rate constant k 0f for the homogenous electron transfer between FeIITEA and FeIII(HDGL)2. The presented chemical system can be seen as a general for numerous redox reactions
178
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
occurring between FeII/III-amino-complexes and FeII/IIIcarbohydrate complexes.
References [1] J.G. Ibanez, C. Choi, R.S. Becker, J. Electrochem. Soc. 134/12 (1987) 3083. [2] R.L. Pecksock, J. Sandera, J. Am. Chem. Soc. 77 (1955) 1489. [3] W. Traube, F. Kuhbier, Chem. Ber. 69 (1936) 2655. [4] G. Gunasekaran, N. Palaniswamy, B.V. Apparo, V.S. Muralidharan, Proc. Indian Acad. Sci.-Chem. Sci. 108 (4) (1996) 407.
[5] R.E. Shepherd, Y. Isaacson, L. Chensny, S. Zhang, R. Kortes, K. John, J. Inorg. Biochem. 49 (1993) 23. [6] S. Mohr, T. Bechtold, J. Appl. Electrochem. 31 (2001) 363. [7] T. Bechtold, A. Turcanu, J. Electrochem. Soc. 149 (2002) D7. [8] T. Bechtold, E. Burtscher, A. Amann, O. Bobleter, J. Chem. Soc., Faraday Trans. 89 (1993) 2541. [9] D.S. Polcyn, I. Shain, Anal. Chem. 38 (1966) 376. [10] T. Bechtold, E. Burtscher, D. Gmeiner, O. Bobleter, J. Electroanal. Chem. 306 (1991) 169. [11] T. Bechtold, E. Burtscher, A. Turcanu, J. Chem Soc., Dalton Trans. (2002) 2683. [12] J.M. Saveant, E. Vianello, Electrochim. Acta 10 (1965) 905. [13] R.S. Nicholson, I. Shain, Anal. Chem. 36 (1964) 706.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 580 (2005) 173–178 www.elsevier.com/locate/jelechem
Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-D-gluconate complex by cyclic voltammetry Bechtold Thomas *, Turcanu Aurora Institute for Textile Chemistry and Textile Physics, Leopold-Franzens-University Innsbruck, Hoechsterstrasse 73, A-6850 Dornbirn, Austria Received 31 January 2005; received in revised form 13 March 2005; accepted 30 March 2005 Available online 5 May 2005
Abstract Electron transfer reactions between ironII/III-complexes are of general interest for corrosion processes, biological processes and technical applications. The cathodic reduction of FeIII-triethanolamine (TEA) complex to form the FeIITEA-form of the complex has been examined by cyclic voltammetry. In the presence of FeIII-D-heptagluconate (HDGL), complex catalytic currents are observed due to homogenous chemical reaction between FeIITEA and FeIII(HDGL)2 which regenerates FeIIITEA. This method can be used to determine the rate constant of the redox reaction between FeIITEA and FeIII(HDGL)2 with k 0f ¼ 88 12 dm3 mol1 s1 . 2005 Elsevier B.V. All rights reserved. Keywords: Triethanolamine; Hepta-D-gluconic acid; Iron; Catalytic current; Cyclic voltammetry; Reaction rate
1. Introduction In aqueous alkaline solutions amino compounds and sugar-acids are excellent complexing agents for FeII- and FeIII-ions [1–3]. Iron complexes with sugar acid type ligands are of importance as corrosion inhibitors and in biological systems [4,5]. Alkali stable ironII/III-complexes are also of interest for the indirect cathodic reduction of vat dyes in different dyeing processes [6,7]. The electrochemical behaviour of the FeIII/IITEA complexes has been studied by different groups (TEA = triethanolamine) [1,8–10]. The reversible cathodic reduction of the Fe(III)-triethanolamine complex is observed at (Ep)c = 1050 mV (vs. Ag/AgCl, 3 M KCl) as reversible redox process [1,8,10]. The limited stability of the FeIII/IITEA complexes at lower pH led to the use of mix-
tures of complexing agents of amino-ligands and sugar acids, e.g., TEA and D-gluconate (DGL) [6]. In analogy to the behaviour of D-gluconate (DGL) which forms a 1:2 complex Fe(DGL)2, the Fe(HDGL)2 can be assumed to be present in alkaline solution investigated (HDGL = D-hepta-gluconate) [11]. In this study, the reaction between cathodically formed FeIITEA and the FeIIIHDGL-complex present in excess has been investigated with cyclic voltammetry. As a result of the redox reaction between FeIITEA and FeIIIHDGL-complex, a catalytic current is observed in the CV experiments. Catalytic processes in electrochemistry occur when a substance O is reduced to R (Eq. (1)) and R regenerates O by a chemical reaction with a substance Z (Eq. (2)). The reduction of Z yields the product P O þ n e ¢ R
ð1Þ
*
Corresponding author. Tel.: +43 5572 28533; fax: +43 5572 28629. E-mail addresses: [email protected], Thomas.Bechtold@uibk. ac.at (B. Thomas). 0022-0728/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.03.033
k 0f
R þ Z!O þ P
ð2Þ
174
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
In the time scale of the CV experiments performed, the cathodic formation of FeIITEA can be considered as fast compared to the reaction of FeIITEA with FeIII(HDGL)2. Thus, a calculation of the rate constant k 0f for the reaction between FeIITEA and FeIII(HDGL)2 is possible using a large scale plot of the working curve given by Nicholson and Shain for an irreversible catalytic reaction following a reversible charge transfer [12,13]. In this paper, an analysis of the catalytic currents measured in cyclic voltammetry in the FeIIITEA/ FeIII(HDGL)2 system is presented and the pseudofirst-order rate constant k 0f for the reaction according to Eq. (2) has been estimated.
2. Experimental The NaOH, Fe(NO3)3 Æ 9H2O and triethanolamine (TEA, tris-(2-hydroxy-ethyl)-amine) were analytical grade chemicals (Merck, Riedl-de-Haen). An aqueous solution of Na-hepta-D-gluconate (HDGL, C7H13O8Na) was used, the exact concentration of the solution was determined by potentiometric titration with 1 M HCl. The CV experiments were performed at a HMDEelectrode (EG&G potentiostat 264A with 303A polarographic analyser EG&G, Princeton, NJ) equipped with a Rikadenki X-Y recorder. A mercury drop of 0.96 · 102 cm2 area was used (HMDE drop size small). Potential values are related to a Ag/AgCl, 3 M KCl reference electrode. Samples were deoxygenated for at least 8 min with He. Experiments were performed at room temperature.
The pH of the solutions was measured with a glass electrode and a potentiometer (Metrohm 654 pH meter, Herisau, Switzerland). Spectrophotometric experiments were performed with a diode array photometer using a 10 mm pathlength cuvette (Zeiss CLH 500/MCS 521 UV–vis, Carl Zeiss (Jean), Germany). The complexes were prepared 24 h before the analysis to permit sufficient time for equilibration between the Fe-complexes. During equilibration time samples were stored in the dark to avoid light induced oxidation.
3. Results and discussion The composition of the solutions investigated by cyclic voltammetry is given in Table 1. A stock solution containing the components of solution 1 was prepared and the sample solutions were then prepared by dilution, respectively, by addition of the required amount of TEA to obtain solutions 1–4. According to Eq. (3), an equilibrium between the two ligands and the corresponding FeIIIcomplexes establishes in the solution TEA þ FeIII ðHDGLÞ2 ¢ FeIII TEA þ 2HDGL K¼
cðFeIII ðHDGLÞ2 ÞcðTEAÞ cðFeIII ðTEAÞÞcðHDGLÞ
ð3Þ ð4Þ
2
The equilibrium between the two FeIIIcomplexes could be observed by cyclic voltammetry. Table 2 shows the cathodic peak current (ip)d of the FeIIITEA complex in the presence of various concentrations of both ligands
Table 1 Composition of solutions investigated for determination of k 0f No.
c(Fe3+) (·102 mol dm3)
c(TEA) (·104 mol dm3)
c(HDGL) (·102 mol dm3)
c(NaOH) (mol dm3)
pH
1 2 3 4
2.40 2.40 2.40 0.25
0 2.44 4.93 2389
5.73 5.73 5.73 0
0.344 0.344 0.344 0.356
13.31 13.28 13.32 13.42
Table 2 Composition of solutions 5–11, pH and cathodic current (ip)c at (Ep)c = 1050 mV at various scan rates No.
5 6 7 8 9 10 11
c(Fe3+) (·104 mol dm3)
c(TEA) (·102 mol dm3)
c(HDGL) (·102 mol dm3)
c(NaOH) (mol dm3)
pH
24.4 24.4 24.4 24.4 24.4 24.4 24.4
0.24 2.38 23.6 0.24 2.38 23.6 23.6
0.48 0.48 0.48 1.0 1.0 1.0 0.0
0.34 0.34 0.34 0.34 0.34 0.34 0.34
13.21 13.22 13.22 13.23 13.23 13.23 13.21
Cathodic peak current (ip)c (lA) Scan rate (mV s1) 5
10
20
50
100
200
1.5 1.4 1.7 1.3 1.8 1.9 2.0
2.0 2.4 2.3 1.6 2.4 2.5 2.5
2.8 3.7 3.2 2.3 3.2 3.4 3.4
4.6 4.8 4.9 3.5 4.9 5.7 5.2
5.0 6.7 7.0 5.0 6.8 7.3 7.3
7.2 9.4 9.5 6.8 9.5 10.1 9.1
B. Thomas, T. Aurora / Journal of Electroanalytical Chemistry 580 (2005) 173–178
TEA and HDGL. On basis of the cathodic peak current (ip)d as a measure for the concentration of FeIIITEA present in solutions 5 and 8, the equilibrium constant K can be determined in 0.34 mol dm3 NaOH according to Eqs. (3) and (4) with K = 5.1 ± 0.4. Using these data, the distribution between the complexes FeIIITEA and FeIII(HDGL)2 in the solutions 2 and 3 can be determined. As a result the concentration of uncomplexed TEA remains below 3% of the total concentration of TEA present in solutions 2 and 3, respectively. Thus, the concentration of uncomplexed TEA can be neglected and c(FeIIITEA) can be set equivalent to c(TEA). In photometry the formation of the TEA complex can also be studied by the decrease of the absorbance of the FeIII(HDGL)2 complex followed by an addition of TEA. Fig. 1 shows the absorption spectra in the wavelength range of 350–550 nm of a solution of 2.39 · 102 mol dm3 FeIII(HDGL)2 in 0.34 mol dm3 aqueous NaOH in the presence of 0, 2.65 · 104, 5.11 · 104, 13.54 · 104, 124.1 · 104 and 4 3 2389 · 10 mol dm TEA. Lowering of the absorbance between 400 and 420 nm (Fe(HDGL)2 and formation of the TEA complex can be observed with increased concentration of TEA. The FeIII/IITEA complex shows the shape of a voltammogram for reversible redox reaction (Fig. 2) [1,8–11]. The cathodic peak current of the diffusion controlled process (ip)d is directly proportional to the square root of the scan rate. In the CV of a solution of 24.5 · 104 mol dm3 Fe(NO3)3 in 0.239 mol dm3 TEA and 0.356 mol dm3 NaOH, a cathodic peak is observed at (Ep)c = 1050 mV and in the reverse scan an anodic peak is observed at (Ep)a = 990 mV. The cathodic peak current (ip)d at scan rates between 5 and
175
Fig. 2. Cyclic voltammograms of 0.25 · 102 mol dm3 FeIIITEA in 0.239 mol dm3 TEA and 0.356 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5, 10, 20, 50, 100 and 200 mV s1 (solution 4).
200 mV s1 ranges from 2.1 lA at 5 mV s1 to 8.4 lA at 200 mV s1. In Fig. 3, the CVs of a solution of 2.40 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV are shown for scan rates of 5 and 200 mV s1. Despite the rather high concentration of the FeIII(HDGL)2 complex, no distinct cathodic current peak is observed in the
1.8 1.6 1.4
0 2.65 5.11 13.54 124.1 2389
1.2
absorbance
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 360
380
400
420
440
460
480
500
520
540
λ (nm)
Fig. 1. Absorption spectra of 2.39 · 102 mol dm3 FeIII(HDGL)2 in the presence of 0, 2.65 · 104, 5.11 · 104, 13.5 · 104, 124 · 104 and 2389 · 104 mol dm3 TEA in the wavelength range of 350–550 nm.
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Fig. 3. Cyclic voltammograms of 2.40 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 1).
potential range between 400 and 1100 mV. No anodic peak which could indicate cathodically formed FeII(HDGL)2 is observed during the reverse scan. This behaviour is similar to the results given in the literature for FeIII(DGL)2 [6,11]. In the presence of low concentrations of TEA, the concentration of TEA limits the concentration of FeIIITEA complex formed in solution. The reduction of FeIIITEA to FeIITEA is observed in the CV near 1050 mV. Due to a redox reaction between cathodically formed FeIITEA and FeIII(HDGL)2 present in high excess, a catalytic current with regeneration of FeIIITEA is observed according to Eqs. (5) and (6).
As expected from theoretical considerations given by Nicholson and Shain [13] and Saveant and Vianello [12], a cathodic current plateau is observed when co cz, c(FeTEA) c(Fe(HDGL)2), respectively. The process according to Eq. (6) can be taken as irreversible reaction due to the absence of an anodic current peak for the reoxidation of Fe(II)TEA. Typical results of CV experiments are shown in Figs. 4 and 5. In Fig. 4, CVs of 2.44 · 104 mol dm3 FeIIITEA in the presence of 2.37 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 are shown. In Fig. 5, the concentration of FeIIITEA complex was raised to 4.93 · 104 mol dm3. The detailed conditions and results of the experiments are given in Table 3.
Fig. 4. Cyclic voltammogram of 2.44 · 104 mol dm3 FeIIITEA in the presence of 2.37 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 2).
Fig. 5. Cyclic voltammogram of 4.93 · 104 mol dm3 FeIIITEA in the presence of 2.35 · 102 mol dm3 FeIII(HDGL)2 in 0.344 ml dm3 NaOH solution in the potential range of 400 to 1200 mV at a scan rate of 5 and 200 mV s1 (solution 3).
As can be seen in the cyclic voltammograms, a catalytic peak current is observed at (Ep)c 1050 mV which corresponds to the cathodic reduction of FeIIITEA. Due to the reaction according to Eq. (6), regeneration of FeIIITEA occurs and the catalytic peak current (ip)c is observed. In cases of low scan rates of 5, 10 and 20 mV s1, a small anodic peak is observed in the reverse scan, which indicates limitations of the assumption
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177
Table 3 Catalytic current (ip)c and rate constant k 0f for the reaction between Fe2+TEA (2.44 · 104, 4.93 · 104 mol dm3) and Fe3+(HDGL)2 (solutions 2 and 3) Scan rate (mV s1)
c(Fe3+TEA) (·104 mol dm3)
c(Fe3+HDGL2) (·102 mol dm3)
c(HDGL) (·102 mol dm3)
(ip)c (lA)
(ip)d (lA)
(ip)c/(ip)d
k 0f ðdm3 mol1 s1 Þ
5 10 20 50 100 200
2.44 2.44 2.44 2.44 2.44 2.44
2.37 2.37 2.37 2.37 2.37 2.37
1.01 1.01 1.01 1.01 1.01 1.01
0.80 0.75 0.73 0.93 1.25 1.28
0.21 0.25 0.32 0.46 0.63 0.84
3.81 3.00 2.27 2.01 1.98 1.52
18 26 37 72 142 134
5 10 20 50 100 200
4.93 4.93 4.93 4.93 4.93 4.93
2.35 2.35 2.35 2.35 2.35 2.35
1.11 1.11 1.11 1.11 1.11 1.11
1.63 1.78 1.90 2.03 2.18 2.10
0.42 0.50 0.65 0.93 1.27 1.69
3.84 3.52 2.95 2.18 1.71 1.24
18 31 48 85 101 78
of co cz due to relative high turn over in the diffusion layer of the cathode. Thus, experiments at scan rates of 5–20 mV s1 were not considered for the final calculation of kf. From theoretical considerations for a given cz, the height of the catalytic current (ip)c is independent of the scan rate, however, in the experiments presented in Table 3 (ip)c a certain dependence on the scan rate was detected. To eliminate the influence of the baseline slope, for determination of (ip)c the base line current of the pure Fe(HDGL)2 system was subtracted from the cathodic current measured at (Ep)c. On basis of the method given by Nicholson and Shain, a large scale plot of (ip)c/(ip)d vs. (kf/a) can be used to determine the rate constant for the reaction according to Eq. (6) using Eqs. (7)–(9). Spherical correction was introduced into Eq. (7) to correct the current function for plane electrodes to spherical geometry of the Hg-drop [13]. Due to the introduction of the spherical correction, the graph of (ip)c/(ip)d vs. (kf/a) becomes dependent on the scan rate of the voltammogram. Thus, to obtain more accurate graphs the graph was calculated for each scan rate 1=2
1=2
ðip Þc ðapÞ vðatÞc þ D0 r1 0 /ðatÞ ¼ ; 1=2 1=2 ðip Þd ðapÞ vðatÞd þ D0 r1 0 /ðatÞ
ð7Þ
a ¼ nF mR1 T 1 ;
ð8Þ
k f ¼ k 0f cz ;
ð9Þ
where v(at)c denotes current function for cyclic voltammogram with catalytic regeneration of species [13] and /(at) is a term for spherical correction of electrode [13]. The other symbols have their usual meaning as defined in [13]. According to Polcyn and Shain [9] the diffusion coefficient of the FeIIITEA-complex was set 7 · 106 cm2 s1. The catalytic current was corrected
by subtraction of the baseline current observed in the FeIII(HDGL)2 system in the absence of any TEA. The calculated values for k 0f are given in Table 2. Generally, the experiments at lower concentration of FeIIITEA 2.44 · 104 mol dm3 suffer from increased experimental errors for the determination of the cathodic currents due to the low concentration of FeIIITEA. Thus, the data obtained at a concentration of 4.93 · 104 mol dm3 (solution 3) at scan rates of 50– 200 mV s1 were used for the calculations. From these data, the rate constant k 0f for the reaction (4) can be calculated with 88 ± 12 dm3 mol1 s1. An increase of the concentration of FeIIITEA concentration is limited by the presumption co cz, while the application of higher concentrations of FeIII(HDGL)2 is prohibited by the height of the observed cathodic baseline current due to slow cathodic reduction of FeIII(HDGL)2.
4. Conclusions In alkaline solutions FeIIITEA complexes show diffusion controlled cathodic reduction to yield FeIITEA and cyclic voltammograms of a reversible redox couple are observed. In cyclic voltammograms of FeIII(HDGL)2 complexes, much lower cathodic currents are measured in the potential region between 400 and 1200 mV. Addition of a relative small concentration of TEA to a solution containing excess FeIII(HDGL)2 results in the formation of FeIIITEA and in CV experiments a catalytic current between cathodically formed FeIITEA and FeIII(HDGL)2 is observed. This catalytic current permits the calculation of the rate constant k 0f for the homogenous electron transfer between FeIITEA and FeIII(HDGL)2. The presented chemical system can be seen as a general for numerous redox reactions
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occurring between FeII/III-amino-complexes and FeII/IIIcarbohydrate complexes.
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