J. Electroanal. Chem., 96 (1977) 245--247 245 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary note

DETECTION OF AN ELECTRON TRANSFER ACROSS THE INTERFACE BETWEEN TWO IMMISCIBLE ELECTROLYTE SOLUTIONS BY CYCLIC VOLTAMMETRY WITH FOUR-ELECTRODE SYSTEM

Z. SAMEC, V. MARE~EK and J. WEBER

J. Heyrovsk$ Insti tute o f Physical Chemistry and Electrochemistry, Czechoslovak Academy o f Sczences, U tov6ren 254, 102 000 Prague 10 -- HostivaY (Czechoslovakia)

(Received 30th November 1978)

The ion transfer across the interface between two immiscible electrolyte solutions has been investigated by several electrochemical methods inclusive of steady-state polarization measurements [1 ], chronopotentiometry [2--5], polarography with the electrolyte dropping electrode [6, 7] and cyclic voltammetry [8, 9]. Electron transfer, which represents an alternative way of the charge transfer through the liquid/liquid interface, has not been detected yet with the exception of the metal deposition at the water/ethylene chloride interface [10]. In this communication we report the preliminary results of the investigation of the electron transfer reaction between

3- 4- Fe(CN)6 /Fe(CN)6 redox couple in water and ferrocene in nitrobenzene by cyclic voltammetry with a four-electrode system.

The electrolytic cell and the electronic circuit used were similar to those described previously [8] except that positive feedback was introduced in order to eliminate the ohmic drop between the tips of Luggin capillaries [9]. In the electrolytic cell an interface between the aqueous and the nitrobenzene phases was formed which had an area of 1.8 cm 2. In all experiments the base electrolytes were 0.05 M LiC1 in the aqueous phase and 0.05 M tetrabutyl- ammonium tetraphenylborate (TBATPB) in the nitrobenzene phase. When no hexacyanoferrates were present in the aqueous phase the Ag/AgC1 reference electrodes were connected to each phase close to the interface by means of a Luggin capillary. The Ag/AgC] reference electrode connected to the aqueous phase was immersed in the 0.05 M aqueous solution of LiC1 while the Ag/AgC1 reference electrode connected to the nitrobenzene phase was immersed in the 0.05 M aqueous solution of tetrabutylammonium chloride in order to ensure a defined Nernst-Donnan potential difference between this solution and the TBATPB solution in nitrobenzene [8]. When the hexacyanoferrate redox system was present in the aqueous phase the Ag/AgC1 reference elec- trode connected to the aqueous phase was replaced by a platinum electrode which was dipped directly in the test aqueous solution. Because only negligible current may flow through this platinum electrode in the four-electrode con- figuration, it serves as the reference electrode whose potential is controlled

3- N 4- by the Fe(CN) 6 /Fe(C )6 redox couple. The potential difference A V applied to the reference electrodes from a

triangular voltage pulse source through the potentiostatic four-electrode

246

circuit [8] is defined as the (inner) potential of the metallic contact to the reference electrode connected to the aqueous phase from which the (inner) potential of the metallic contact to the reference electrode connected to the nitrobenzene phase is subtracted. Using the value of the formal electrode potential for the Fe(CN)3-/Fe N 4- ( C ) 6 redox couple in the presence of Li ÷ ions [11], the correction was made for the shift of the potential of the refer- ence electrode connected to the aqueous phase after the Ag/AgC1 reference electrode had been replaced by the platinum electrode immersed in the

F 3 - 4 - aqueous solution of e(CN)6 and Fe(CN)6 ions. Then, in all experiments, A V is considered as the potential difference AnW~ = ~(w) - ~(n) at the water(w)/ ni trobenzene(n) interface under investigation related to the formal potential

W e difference for te t rabutylammonium ion, A n ~TBA ÷ [ 9 ]

AV w w o = A n ~ - An~0TBA+

The latter quanti ty is defined by the equation

W O W O A n ~TBA ÷ = A n ~TBA + + (RT/F) In [~fTBA+(n)/~/TBA+(W) ]

W O where A n ~TBA + is the standard potential difference [7] and 7TBA+(n) or 7TBA+(W) are the activity coefficients of TBA + ion in the nitrobenzene or the

A W O aqueous phase, respectively. The value ~n ~°TBA+ = - 0 . 2 4 8 V can be deduced [7] from the extraction data [12].

The electric current I connected with the transfer of positive charge from the aqueous phase to the nitrobenzene phase will be considered as positive.

In all experiments the triangular voltage pulse was applied to the interface starting at AV = 0.150 V towards more positive values of AV. Curve 1 of Fig. 1 shows the cyclic vol tammogramme of the base electrolyte, 0.05 M LiC1 in water and 0.05 M TBATPB in nitrobenzene. By comparison of the standard potential difference values for the individual ions present in solutions ( -0 .324 V for CI-, - 0 .248 V for TBA +, 0.372 V for TPB- and 0.395 V for Li ÷ [7]), it can be concluded that the positive current in the increasing potential sweep at about 0.450 V corresponds to the transfer of te traphenylborate anion from the nitrobenzene to the aqueous phase. On the other hand, the negative cur- rent in the decreasing potential sweep at about 0.150 V should correspond to the transfer of TBA ÷ ion from the nitrobenzene to the aqueous phase.

As expected, practically no change is observed in the voltammetric behav- iour when the electrically neutral molecules of ferrocene are present in the nitrobenzene phase (curve 2). The transfer of the highly charged Fe(CN)~-

F 4- and e(CN)6 ions from the aqueous phase to nitrobenzene is obviously much more difficult than that of the base electrolyte ions, so that no current connected with it should be detected in the cyclic voltammogram. The positive current in the increasing potential sweep at about 0.350 V (curve 3) in the case that the salts K3Fe(CN)6 and K4Fe(CN)6 are dissolved in the aqueous phase is apparently due to the transfer of K + ion from the aqueous phase to nitrobenzene. The standard potential difference for K ÷ ion An~K+W o = 0.242 V [7] is by0 .130 V more negative than that of TPB- ion.

Finally, when Fe(CN)36 - and Fe (CN) 4- ions are present in the aqueous phase and at the same time the nitrobenzene phase contains ferrocene, the polariza-

/ j / ~0 4 :3 1,2

-50 -- /

2 4 7

Fig. 1. Cyc l i c v o l t a m m o g r a m m e s o f t h e b a s e e l e c t r o l y t e , 0 . 0 5 M T B A T P B in: n i t r o b e n z e n e (1) ; in t h e p r e s e n c e o f 0 .1 M f e r r o c e n e in t h e n i t r o b e n z e n e p h a s e (2) ; in t h e p r e s e n c e o f 10 -~ M K3Fe (CN)6 + 10 -4 M K 4 F e ( C N ) ~ in t h e a q u e o u s p h a s e (3) ; a n d in t h e p r e s e n c e o f b o t h 0 .1 M f e r r o c e n e in t h e n i t r o b e n z e n e p h a s e a n d o f 10 -3 M K 3 F e ( C N ) 6 + 10 -4 M K 4 F e ( C N ) 6 in t h e a q u e o u s p h a s e (4) . T h e r a t e o f p o l a r i z a t i o n was 0 . 0 1 V s -1.

t ion of the interface by the triangular voltage pulse gives rise to the new charge transfer process (curve 4). This process is clearly the electron transfer reaction

Fe(CN)~-(w ) + (CsHs)2Fe(n) ~ Fe(CN)~-(w ) + (CsHs)2Fe+(n)

which takes place at the water /ni t robenzene interface and whose rate is effected by the potential difference w A n ~. The cyclic vo l t ammogramme is at first sight similar to that observed with the ion transfer reaction [ 8, 9], bu t more detailed inspection reveals some new features in the vol tammetr ic behaviour which are presumably connected with the second order kinetics of the electron transfer reaction at the interface [13]. The investigation of this p h e n o m e n o n is in progress and the results will be reported in a subsequent publication.

R E F E R E N C E S

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Anal. Chem., in press. 6 J. Koryta, P. Van~sek and M. B~ezina, J. Electroanal. Chem., 67 (1976) 263. 7 J. Koryta, P. Van~sek and M. B~ezina, J. Electroanal. Chem., 75 (1977) 211. 8 Z. Samec, V. MareSek, J. Koryta and M.W. Khalil, J. Electroanal. Chem., 83 (1977) 393. 9 Z. Samec, V. MareSek and J. Weber, J. Electroanal. Chem., 100 (1979) in press.

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