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The temperature shift of the cathodic background discharge
JElectroanal. Chem., 180 (1980) 375--378
375
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note THE T E M P E R A T U R E SHIFT OF THE CATHODIC B A C K G R O U N D DISCHARGE
Y. MUGNIER and E. L A V I R O N
Laboratoire de Synth~se et d'Electrosynth~se Organom~tallique associ$ au CNRS (L. A. 33), Facult$ des Sciences Gabriel 21100 Dijon (France) (Received 1st April 1980)
In the course of electrochemical studies at low temperature [ 1] in dimethylformamide (DMF) on a dropping mercury electrode (DME), we noticed a shift of the cathodic background potential limit (reduction "wall") towards negative potentials. A similar effect had already been mentioned by Van D u y n e and Reilley [2] in the case of a platinum electrode in DMF. We wish to report here some preliminary results on this phenomenon, which is of general interest to electrochemists; it already enabled us to obtain a wellcharacterized reduction wave for ferrocene [3 ], which was previously reputed n o t to be electrochemically reducible [4]. We also propose an interpretation for it. As shown in Fig. 1, the limiting potential Ew measured at constant current i0 (with respect to the base current, Fig. 2a) varies linearly with the temperature, with a slope equal to 4mV °C-1, when a DME is used. The shift does n o t depend on the anion, within the precision of the experiments. In no case does the anodic background limit vary. The reference electrode was an aqueous saturated calomel electrode, which was maintained at 20°C and which was separated from the solution b y two fritted glass disks. With a DME, the p h e n o m e n o n is also observed (Fig. 3) in acetone (slope "o
20
No
o •
o
20
EIV
Fig. 1. Variations of E w with the temperature. Dropping mercury electrode in dimethylformamide, i0 = 6 ~A. Supporting electrolyte (c = 0.2 M). (e) tetrabutylammonium perchlorate TBA÷C10~, (0) tetrabutylammonium iodide TBA+I -, (D) tetrabutylammonium hexafluorophosphate TBA*PF6 .
376
20
a
,i
-~
----
15
30
-2.2 -Z4
~
0.2
EIV
d n(E_EIh)
'
-0.2
Fig. 2. (a) D e f i n i t i o n o f g w and i 0. D r o p p i n g mercury electrode in acetone, 0.2 M T B A + [ -, - 3 5 ° 0 . (b) P o l a r o g r a m s a t t w o t e m p e r a t u r e s differing b y l°C ( 2 5 a n d 24°C) f o r a r e d o x s y s t e m O + nec-R. It is a s s u m e d t h a t t h e d i f f u s i o n c o e f f i c i e n t s o f O a n d R are equal, a n d t h a t E ° does n o t vary w i t h t h e t e m p e r a t u r e .
0
)
'
-20
-g0 E/v
Fig. 3. Variations of E w with the temperature, i0 = 6 ~A. (m) Pt----aeetone, (0) Hg'--acetone, (0) Pt--THF, (0) Hg---THF, (o) Pt--DMF, (e) Hg--DMF. Supporting electrolyte TBA+I except for Pt--THF, where TBA÷C102 was used. 3.2 m V °C -1) or in t e t r a h y d r o f u r a n e ( T H F ) (slope 2.5 m V °C-1). A shift is also o b s e r v e d w h e n a r o t a t i n g p l a t i n u m disk e l e c t r o d e is used (Fig. 3). In D M F o r T H F o n m e r c u r y , t h e limiting c u r r e n t is d u e t o t h e l e r e d u c t i o n o f t h e c a t i o n R4N ÷ o f t h e s u p p o r t i n g e l e c t r o l y t e t o give a free radical. Subseq u e n t r e a c t i o n s o f t h e radical are n o t c o m p l e t e l y clear. It has b e e n s h o w n [5--7] t h a t it can r e a c t w i t h m e r c u r y t o f o r m an amalgam; it has also been suggested t h a t it dimerizes [ 8, 9] : R4N + ~ R 4 N
• kl o r k 2 .> P
(1)
In t h e case o f a c e t o n e (DME or p l a t i n u m e l e c t r o d e ) , the limit c o r r e s p o n d s p r o b a b l y t o t h e r e d u c t i o n o f t h e solvent; t h e e l e c t r o n u p t a k e c o u l d be fol-
377 lowed by a dimerization [10] : e S~S
.
;2S
. k2
,S--S
(2)
By using linear potential sweep voltammetry, we have ascertained t h a t reactions (1) and (2) are irreversible, even at high sweep rates; in both cases the electron uptake must indeed be followed by a fast irreversible reaction. With platinum in DMF or THF, the limit at room temperature appears at potentials more positive than on mercury, and is constituted by an ill-defined wave which could be due to the presence of residual water. For a reversible reduction followed by a fast irreversible reaction, the theory predicts a shift of the polarographic half-wave potential E1/2 towards positive potentials relatively to the redox potential E ° of the reaction; E , n is given by: E1/2 = E ° + ( R T / 2 n F ) i n k I T
+ ct
(3)
for a first-order reaction [11--13], and Ell2 = E ° + ( R T / 3 n F ) l n c ° k 2 r
+ ct
(4)
for a second order reaction [13, 14]. c o is the concentration of the reactant and T the drop time. If i0 is small enough, the influence of the ohmic drop can be neglected so that the reduction wall can be regarded as the f o o t of a very large polarographic wave. The shift can then be explained on the basis of three main effects: (a) a shift of E °, (b) a shift of E , n due to the action of the temperature on k~ or k2, and (c) a shift due to the fact t h a t the potential Ew is measured at constant current. (a) The standard potential of a redox system varies with the temperature [2,15] negatively or positively; d E ° / d T can reach + 2 mV °C -1. (b) If kl and k2 are written under the classical form k l = k ° e x p ( - A E / R T ) , and k2 = k ° exp ( - A E 2 / R T ) , in which AE1 and AE2 are activation energies and k ° and k ° pre-exponential factors, eqns. (3) and (4) yield, at constant concentration and drop time: E1/2 = E ° + ( R T / 2 n F ) l n k ° + c t
(5)
Ell2 = 'E ° + ( R T / 3 n F ) l n k ° + c t
(6)
If we take for k ° and k ° respectively 1013 s -1 and 1012 1 mo1-1 s -1 [16], we find d E ~ / 2 / d T = 1.3 mV °C-1 for the first order reaction, and 0.79 mV °C-1 for the second-order reaction. (c) A decrease in the temperature causes a decrease of the current at constant potential, due mainly to a decrease of the diffusion coefficient. As Ew is measured at constant current, this means that it corresponds to a higher relative value of the current along the curve; besides, a decrease in the temperature makes the wave steeper; Ew becomes thus more negative when T decreases (Fig. 2b). From the equation of the polarographic wave, it can be easily calculated that d E w / d T corresponding to this effect is about 0.8 mV °C-1 at the f o o t of the wave if it is assumed [17] t h a t the temperature coefficient of i d is 2% °C-1 and t h a t E1/2 does n o t vary.
378
The sum of the three effects mentioned above accounts reasonably well for the experimentally observed shift. The effect of temperature on E 0, E l n and Ew can be illustrated by the following examples (DMF, DME). For 1,1-dimethyl-4,4'-dipyridinium diiodide (c = 10 -3 mol 1-1, 0.2 M TBA+PF~ as supporting electrolyte), which gives a monoelectronic reversible reaction, d E 1 / 2 / d T 1.1 mV/C. In the case of 1-methylpyridinium iodide (c = 10 -`3 mol 1-1, 0.2 M TBA÷PF~ as supporting electrolyte), the l e reduction is followed by a fast dimerisation: d E 1 / 2 / d T = 1.6 mV °C-1. If 1-methyl pyridinium iodide is used as supporting electrolyte (c = 0.2 mol 1-1), d E w / d T = 2.2 mV °C (Ew is measured at 10 #A). We are now investigating the p h e n o m e n o n in more detail, in particular at lower temperatures. ACKNOWLEDGEMENT
We thank Mrs M.T. Compain for her technical assistance.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
E. Laviron and Y. Mugnier, J. ElectroanaL Chem., 93 (1978) 69. R.P. Van Duyne and C.N. Reilley, Anal. Chem., 44 (1972) 142. Y. Mugnier, C. Mo~se, J. Tixouflet and E. Laviron, J. Organometal. Chem., 186 (1980) C49. R.E. Dessy, F.E. Stary, R.B. King and M. Waldrop, J. Am. Chem. Soc., 88 (1966) 471; S.P. Gubin, S.A. Smirnova, L.I. Denisovich and A.A. Lubovich, J. OrganometaL Chem., 30 (1971) 243. B.C. S o u t h w o r t h , R. Osteryoung, K.D. Fleischer and F.C. Nachod, AnaL Chem., 33 (1961) 208. D.C. Grahame, J. Am, Chem. Soc., 63 (1941) 1207. J.D. Littlehailes and B.J. Woodhall, Discuss. Farad. S o c , 45 (1968) 187. H. Lund, M.A. Michel and J. Simonet, A c t a Chem. Sc~md., B29 (1975) 217. J.M. Sav~ant and Su Khac Binh, J. Electroanah Chem., 88 (1978) 27. A.P. Tomilov, S.G. Malranovskii, M.Ya. Fioshin and V.A. Smirnov, E l e c t r o c h e m i s t r y of Organic Compounds, Halsted Press, New York, 1972, p. 208. D.M.H. Kern, J. Am. Chem. Soc., 76 (1954) 1011. J. Kouteck~, Collect. Czech. Chem. Commun., 20 (1955) 116. Z. Galus, F u n d a m e n t a l s of Electrochemical Analysis, Halsted Press, New York, 1976. J. K o u t e c k ~ and V. Hanu~, Collect. Czech. Chem. Commun., 20 (1955) 124. A.J. deBethune, T.S. Lieht and N. Swendeman, J. Electrochem. Soc., 106 (1959) 616. C.H. Bamford and C.F. Tipper (Eds.), Comprehensive Chemical Kinetics, VoL 2, Elsevier, Amsterdam, 1969. I.M. Ko lthoff and J:J. Lingane, Polarography, Interscience, New York, 1952, p. 93.
=
375
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
Preliminary note THE T E M P E R A T U R E SHIFT OF THE CATHODIC B A C K G R O U N D DISCHARGE
Y. MUGNIER and E. L A V I R O N
Laboratoire de Synth~se et d'Electrosynth~se Organom~tallique associ$ au CNRS (L. A. 33), Facult$ des Sciences Gabriel 21100 Dijon (France) (Received 1st April 1980)
In the course of electrochemical studies at low temperature [ 1] in dimethylformamide (DMF) on a dropping mercury electrode (DME), we noticed a shift of the cathodic background potential limit (reduction "wall") towards negative potentials. A similar effect had already been mentioned by Van D u y n e and Reilley [2] in the case of a platinum electrode in DMF. We wish to report here some preliminary results on this phenomenon, which is of general interest to electrochemists; it already enabled us to obtain a wellcharacterized reduction wave for ferrocene [3 ], which was previously reputed n o t to be electrochemically reducible [4]. We also propose an interpretation for it. As shown in Fig. 1, the limiting potential Ew measured at constant current i0 (with respect to the base current, Fig. 2a) varies linearly with the temperature, with a slope equal to 4mV °C-1, when a DME is used. The shift does n o t depend on the anion, within the precision of the experiments. In no case does the anodic background limit vary. The reference electrode was an aqueous saturated calomel electrode, which was maintained at 20°C and which was separated from the solution b y two fritted glass disks. With a DME, the p h e n o m e n o n is also observed (Fig. 3) in acetone (slope "o
20
No
o •
o
20
EIV
Fig. 1. Variations of E w with the temperature. Dropping mercury electrode in dimethylformamide, i0 = 6 ~A. Supporting electrolyte (c = 0.2 M). (e) tetrabutylammonium perchlorate TBA÷C10~, (0) tetrabutylammonium iodide TBA+I -, (D) tetrabutylammonium hexafluorophosphate TBA*PF6 .
376
20
a
,i
-~
----
15
30
-2.2 -Z4
~
0.2
EIV
d n(E_EIh)
'
-0.2
Fig. 2. (a) D e f i n i t i o n o f g w and i 0. D r o p p i n g mercury electrode in acetone, 0.2 M T B A + [ -, - 3 5 ° 0 . (b) P o l a r o g r a m s a t t w o t e m p e r a t u r e s differing b y l°C ( 2 5 a n d 24°C) f o r a r e d o x s y s t e m O + nec-R. It is a s s u m e d t h a t t h e d i f f u s i o n c o e f f i c i e n t s o f O a n d R are equal, a n d t h a t E ° does n o t vary w i t h t h e t e m p e r a t u r e .
0
)
'
-20
-g0 E/v
Fig. 3. Variations of E w with the temperature, i0 = 6 ~A. (m) Pt----aeetone, (0) Hg'--acetone, (0) Pt--THF, (0) Hg---THF, (o) Pt--DMF, (e) Hg--DMF. Supporting electrolyte TBA+I except for Pt--THF, where TBA÷C102 was used. 3.2 m V °C -1) or in t e t r a h y d r o f u r a n e ( T H F ) (slope 2.5 m V °C-1). A shift is also o b s e r v e d w h e n a r o t a t i n g p l a t i n u m disk e l e c t r o d e is used (Fig. 3). In D M F o r T H F o n m e r c u r y , t h e limiting c u r r e n t is d u e t o t h e l e r e d u c t i o n o f t h e c a t i o n R4N ÷ o f t h e s u p p o r t i n g e l e c t r o l y t e t o give a free radical. Subseq u e n t r e a c t i o n s o f t h e radical are n o t c o m p l e t e l y clear. It has b e e n s h o w n [5--7] t h a t it can r e a c t w i t h m e r c u r y t o f o r m an amalgam; it has also been suggested t h a t it dimerizes [ 8, 9] : R4N + ~ R 4 N
• kl o r k 2 .> P
(1)
In t h e case o f a c e t o n e (DME or p l a t i n u m e l e c t r o d e ) , the limit c o r r e s p o n d s p r o b a b l y t o t h e r e d u c t i o n o f t h e solvent; t h e e l e c t r o n u p t a k e c o u l d be fol-
377 lowed by a dimerization [10] : e S~S
.
;2S
. k2
,S--S
(2)
By using linear potential sweep voltammetry, we have ascertained t h a t reactions (1) and (2) are irreversible, even at high sweep rates; in both cases the electron uptake must indeed be followed by a fast irreversible reaction. With platinum in DMF or THF, the limit at room temperature appears at potentials more positive than on mercury, and is constituted by an ill-defined wave which could be due to the presence of residual water. For a reversible reduction followed by a fast irreversible reaction, the theory predicts a shift of the polarographic half-wave potential E1/2 towards positive potentials relatively to the redox potential E ° of the reaction; E , n is given by: E1/2 = E ° + ( R T / 2 n F ) i n k I T
+ ct
(3)
for a first-order reaction [11--13], and Ell2 = E ° + ( R T / 3 n F ) l n c ° k 2 r
+ ct
(4)
for a second order reaction [13, 14]. c o is the concentration of the reactant and T the drop time. If i0 is small enough, the influence of the ohmic drop can be neglected so that the reduction wall can be regarded as the f o o t of a very large polarographic wave. The shift can then be explained on the basis of three main effects: (a) a shift of E °, (b) a shift of E , n due to the action of the temperature on k~ or k2, and (c) a shift due to the fact t h a t the potential Ew is measured at constant current. (a) The standard potential of a redox system varies with the temperature [2,15] negatively or positively; d E ° / d T can reach + 2 mV °C -1. (b) If kl and k2 are written under the classical form k l = k ° e x p ( - A E / R T ) , and k2 = k ° exp ( - A E 2 / R T ) , in which AE1 and AE2 are activation energies and k ° and k ° pre-exponential factors, eqns. (3) and (4) yield, at constant concentration and drop time: E1/2 = E ° + ( R T / 2 n F ) l n k ° + c t
(5)
Ell2 = 'E ° + ( R T / 3 n F ) l n k ° + c t
(6)
If we take for k ° and k ° respectively 1013 s -1 and 1012 1 mo1-1 s -1 [16], we find d E ~ / 2 / d T = 1.3 mV °C-1 for the first order reaction, and 0.79 mV °C-1 for the second-order reaction. (c) A decrease in the temperature causes a decrease of the current at constant potential, due mainly to a decrease of the diffusion coefficient. As Ew is measured at constant current, this means that it corresponds to a higher relative value of the current along the curve; besides, a decrease in the temperature makes the wave steeper; Ew becomes thus more negative when T decreases (Fig. 2b). From the equation of the polarographic wave, it can be easily calculated that d E w / d T corresponding to this effect is about 0.8 mV °C-1 at the f o o t of the wave if it is assumed [17] t h a t the temperature coefficient of i d is 2% °C-1 and t h a t E1/2 does n o t vary.
378
The sum of the three effects mentioned above accounts reasonably well for the experimentally observed shift. The effect of temperature on E 0, E l n and Ew can be illustrated by the following examples (DMF, DME). For 1,1-dimethyl-4,4'-dipyridinium diiodide (c = 10 -3 mol 1-1, 0.2 M TBA+PF~ as supporting electrolyte), which gives a monoelectronic reversible reaction, d E 1 / 2 / d T 1.1 mV/C. In the case of 1-methylpyridinium iodide (c = 10 -`3 mol 1-1, 0.2 M TBA÷PF~ as supporting electrolyte), the l e reduction is followed by a fast dimerisation: d E 1 / 2 / d T = 1.6 mV °C-1. If 1-methyl pyridinium iodide is used as supporting electrolyte (c = 0.2 mol 1-1), d E w / d T = 2.2 mV °C (Ew is measured at 10 #A). We are now investigating the p h e n o m e n o n in more detail, in particular at lower temperatures. ACKNOWLEDGEMENT
We thank Mrs M.T. Compain for her technical assistance.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
E. Laviron and Y. Mugnier, J. ElectroanaL Chem., 93 (1978) 69. R.P. Van Duyne and C.N. Reilley, Anal. Chem., 44 (1972) 142. Y. Mugnier, C. Mo~se, J. Tixouflet and E. Laviron, J. Organometal. Chem., 186 (1980) C49. R.E. Dessy, F.E. Stary, R.B. King and M. Waldrop, J. Am. Chem. Soc., 88 (1966) 471; S.P. Gubin, S.A. Smirnova, L.I. Denisovich and A.A. Lubovich, J. OrganometaL Chem., 30 (1971) 243. B.C. S o u t h w o r t h , R. Osteryoung, K.D. Fleischer and F.C. Nachod, AnaL Chem., 33 (1961) 208. D.C. Grahame, J. Am, Chem. Soc., 63 (1941) 1207. J.D. Littlehailes and B.J. Woodhall, Discuss. Farad. S o c , 45 (1968) 187. H. Lund, M.A. Michel and J. Simonet, A c t a Chem. Sc~md., B29 (1975) 217. J.M. Sav~ant and Su Khac Binh, J. Electroanah Chem., 88 (1978) 27. A.P. Tomilov, S.G. Malranovskii, M.Ya. Fioshin and V.A. Smirnov, E l e c t r o c h e m i s t r y of Organic Compounds, Halsted Press, New York, 1972, p. 208. D.M.H. Kern, J. Am. Chem. Soc., 76 (1954) 1011. J. Kouteck~, Collect. Czech. Chem. Commun., 20 (1955) 116. Z. Galus, F u n d a m e n t a l s of Electrochemical Analysis, Halsted Press, New York, 1976. J. K o u t e c k ~ and V. Hanu~, Collect. Czech. Chem. Commun., 20 (1955) 124. A.J. deBethune, T.S. Lieht and N. Swendeman, J. Electrochem. Soc., 106 (1959) 616. C.H. Bamford and C.F. Tipper (Eds.), Comprehensive Chemical Kinetics, VoL 2, Elsevier, Amsterdam, 1969. I.M. Ko lthoff and J:J. Lingane, Polarography, Interscience, New York, 1952, p. 93.
=