Electroanatytical Chemistry and Interfacial Electrochemistry, 59 (1975) 295002
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
KINETICS OF ANILINE ADSORPTION-DESORPTION ON MERCURY
G. C. BARKER and D. McKEOWN
Atomic Energy Research Establishment, Harwell, Didcot, Oxon. (England) (Received 6th November 1974)
Few attempts 1,2 have been made to study interfacial'impedance at frequencies above 0.1 MHz and often little is known about the kinetics of adsorption~tesorption for specifically adsorbed species other than that the system is diffusion-controlled at low frequencies. A difficulty when a DME is used is the small contribution of the electrode-solution interface at high frequencies to the measured cell impedance. The intermodulation technique devised by Niki and Kyoya 3, that has been termed modulation polarography 4, largely avoids this difficulty and the technique is potentially a powerful tool for the study of interracial impedance at very high frequencies, making feasible the detection of small effects that are undetectable by conventional bridge methods. The modulation technique is essentially an intermodulation technique involving the study of the potential dependence of slight amplitude modulation (low frequency) of the small high frequency component of interfacial potential when the test electrode is simultaneously polarized by an unmodulated high frequency current and a low frequency voltage. No modulation signal is produced by the internal resistance of the cell, and it is this fact which makes~interfacial impedance measurements (of an unusual type) feasible when the cell impedance is dominated by solution resistance. It has been shown earlier that high frequency modulation polarography 5 can provide information about the non-faradaic part of the interfacial impedance. In the present communication we consider the application of the technique to an organic adsorption-desorption system. It is known that the mercury-solution interface in the absence of orga.nic adsorption is reversible in behaviour at frequencies up to at least 5 MHz when the electrolyte concentration is large but kinetic effects are to be expected when organic species are strongly adsorbed at the interface. Kinetic effects have been reported by Lorenz 9-1~ and a system influenced by reorientation of the adsorbed organic molecules as tlae potential changes could well exhibit kinetic effects at high frequencies due to slow molecular reorientation and perhaps also due to slow penetration of the adsorbed organic layer by the ions of the electrolyte. Thus we have studied with a 2 MHz modulation polarograph the modulation signals produced by aniline, it having been shown by Damaskin 6 that when mercury is positively charged the surface excess of aniline is markedly affected by potential due to changes in molecular orientation at the interface. A qualitative understanding of the kinetics of the aniline system was needed for the interpretation of photopotential data for aniline-N20 solutions using a laser light source.
G.C. BARKER,D. McKEOWN
EXPERIMENTAL Recordings were made showing the variation during a delayed (4 s) 0.5 V cathodic potential scan (duration 6.3 s) of the output voltage of the 2 MHz modulation polarograph 5. This voltage was proportional to the low frequency amplitude modulation of the 2 MHz voltage appearing across a "monitor circuit" consisting of the series arrangement (at 2 MHz) of a fixed resistance (10 ~), the inductance of the leads to the cell and the cell inductance, the non-inductive part of the cell impedance and a variable capacity (normally set at ca. 1600 pF in the present work). In the present work the apparatus was made almost completely insensitive to amplitude modulation of the voltage across the monitor circuit just described connected with the capacitative part of the interfacial impedance (series circuit) by making the inductive part of the circuit resonate at 2 MHz with the capacitative part. This adjustment was made with aniline absent. Lack of perfect resonance throughout the entire voltage scan with or without aniline present was not an important factor due to the dominance of the monitor circuit impedance by the fixed resistance and the internal resistance of the cell. For reasons outlined previously5 the 2 MHz current source was made entirely resistive as was the input impedance of the circuit used to filter out and amplify the studied modulation. The cell was a conventional polarographic cell of low self-inductance. The test electrode unusually5 was a blunt DME with a direct connection to the mercury just above the capillary. Potentials were measured with respect to a SCE. The electrode surface area at the start of the potential scan was 0.96 x 10-2 cm 2 and the 2 MHz sinusoidal current at this time was about 7 mA amplitude. The low frequency component of potential invariably was a slightly distorted square wave voltage of peak to peak amplitude 22 mV. The instrumental bandwidth was ca. 3 Hz and the noise factor not better than 34 db due, it is believed, to low frequency noise modulation of the 2 MHz current. The overall sensitivity of the instrument could be determined with a transistor chopper circuit that produced a periodic variation in the resistive part of the monitor circuit of 0.018s f~ peak to peak. Low frequency, real or apparent, differential capacity data were obtained using the non-faradaic output of the square wave mode of a multi-mode polarograph v. All measurements were made at the ambient temperature (20_+ 2°C). RESULTS AND DISCUSSION Tracings of recordings for potential scans starting at -0.016 and -1.116 V and 1 M KC1, with and without aniline present, are given in Fig. 1. These and other results show that the derivative dRs/dE (Rs is the resistive part of the interracial impedance) changes sign three times in the potential range 0 to - 1.5 V being negative in sign at the positive end of the range. Thus as E becomes more negative the modulation signal produced by aniline exhibits a maximum in the vicinity of - 0.25 V, a minimum, a second maximum and a second minimum before eventually gradually falling to zero as the surface excess falls to zero. The system behaves fairly linearly for aniline concentrations below 5 x 10 -2 M in the region 0 to -0.5 V, where the surface excess, according to Damaskin 6, increases markedly as the potential becomes more negative as a consequence of molecular reorientation. In this region the
KINETICS OF ANILINE ADSORPTION-DESORPTION ON Hg -I 2
. No Ph NH2 Zero
/ 2-5 IO - 3 ohm /-
4 . IO - ~ PhNH2
[ -O 3
I -O 4
Fig. 1. Variation with potential of AR~ fo~ 1 M KC1 with and without amline present; 22 mV square wave amplitude (peak to peak); cathodic potential scans starting at (a) -0.016 V v s . SCE, (b) -1.116 V vs. SCE.
aniline modulation signal is roughly proportional to the aniline concentration and the potential at which the aniline signal changes sign is not markedly concentration dependent. Lacking surface excess data for E < - 0.46 V a comparison of theory with experiment is at present only possible for a small part of the potential range in which modulation signals are observed. For adsorption~tesorption of aniline that is diffusion-controlled the relevant equivalent circuit is the one in Fig. 2. Cr is the differential capacity for constant F, the surface excess of aniline and
Cr = ( q/OE)r
while the aniline adsorption capacity C, as is given by Cads = - flF ( ar/OE)co
where fl is the (fractional) number of electrons transferred to the electrode side of the interface when a molecule of aniline is adsorbed at constant potential and Co is the aniline concentration in the solution. The input impedance (2 MHz) of the uniform resistive transmission line TL(0) needed to describe mass transport of aniline by linear diffusion is 8
G. C. BARKER, D. McKEOWN
Fig. 2. Small signal aperiodic form of the equivalent electrical circuit for the interface (reversible adsorption~lesorption).
ZTa0) = RT(1-j)/[fl2FZCo(2o~Do)i ]
where angular frequency co refers to the 2 MHz polarising current and Do is the diffusion coefficient for aniline in the solution. For the potential range in which a comparison of theory with experiment is possible the impedance of Cads is small relative to ZTL(0) and thus will be ignored. With this simplification the peak to peak variation in Rs is given approximately by ARs = AEFZ(20)Do)~Cod/dE(flz/2)/Aco2C2rRT (4) where A is the interfacial area, and AE is the peak to peak value of the square wave voltage. Parameter /3 we have approximately evaluated using Damaskin's surface excess data 6 and the expression*
/3 = R T / F @ In Co/OE)r
an expression that has precise thermodynamic meaning provided the introduction of the organic compound into the solution has a negligible effect on the chemical potentials of other solution components. For 1 M KC1 4 x 10 -2 M aniline the resulting approximate data are plotted in Fig. 3./3 passes through a maximum at a potential close to -0.32 V. Figure 5 shows the experimental variation of AR~ for 1 M KC1 4 x 10- 2 M aniline and also gives some calculated values of ARs based on (4), the data in Fig. 3 and taking D o = 0.75 x 10-5 canz s-1. Values for C v were obtained from the broken curve in Fig. 4 which is supposed to describe the variation of C r with potential. This curv6 was drawn to link up with the normal low frequency capacity curve in regions where F is not changing rapidly with potential, the shape in the intervening region being that suggested by the potential dependence of the (nonfaradaic) change in the modulation signal on making the monitor circuit mentioned earlier appreciably non-resistive 5. From (4) it follows that the modulation signal should change sign at the * Eqn. (5) most easily is derived by treating adsorption as a type of charge transfer reaction and making use of the appropriate variant on the Nernst equation. Alternatively it can be deduced from the Gibbs equation.
KINETICS OF ANILINE ADSORPTION-DESORPTION ON Hg
[I"/ I0 - I 0 2 25
mole /cm - 2 2-75
Fig. 3. Dependence of electron transfer number on potential and F for 1 M KCI 4 x 10 -2 M aniline.
Fig. 4. Dependence of capacity on potential for 1 M KC1 with varying C O(data obtained using SW mode of MMP). The dashed curve describes approximately the variation of Cr with E.
G. C. BARKER, D. McKEOWN
300 -3 ~8 I0 ohm
5 10-2 ohm I~ ~7 lO-2ohm
~_0 0~ 
 A RS (calcd) ~ -2-
Fig. 5. Comparison of theory (reversible adsorption) with experiment for potential range in which F is large and influenced by molecular reorientation.
potential at which/3 is largest in value and this expectation is borne i~ut in practice but the observed value of ARs in the potential range - 0.1 to - 0.4 V is smaller than the calculated value by a factor increasing from about 5 at - 0 , 1 V to a value of the order of 100 at -0.36 V. The position of the maximum value of AR~ is roughly that suggested by the curve in Fig. 3 but the observed maximum signal is smaller than the calculated value by about an order of magnitude. The discrepancies in signal size presumably arise mainly from irreversibility as it seems unlikely that the derivatives of f12 with respect to potential calculated from the curve in Fig. 3, on the average, can be greatly in error although individual values could be wrong by a factor conceivably as large as 2 or more. r Without more precise information concerning factors influencing the potential dependence of/3 it is not profitable to discuss in detail the causes of the discrepancies mentioned above*. However, it should be pointed out that while * There seems to be fair agreement between reversible theory la and experiment at very negative potentials where F is small.
KINETICS OF ANILINE ADSORPTION DESORPTION ON Hg
Fig. 6. More exact form of the small-signal aperiodic circuit for the interface taking account of slow adjustment of orientation, slow penetration of the adsorbed layer by supporting electrolyte ions and slow exchange of the organic solute between the interface and the solution at the interface.
irreversibility often is formally allowed in the equivalent circuit by introducing a kinetic resistance in series with C=e~and TL(0), it may be doubted whether, in the case of a system influenced by molecular reorientation, this modification alone will serve to describe the behaviour of the system. Elementary reasoning suggests that the circuit should then take the form shown in Fig. 6. In this circuit account is taken of the fact that at high frequency it may no longer be possible to describe the behaviour of the interface at constant F by a simple condenser. Thus C r has been replaced by the parallel combination of (a) the differential capacity for constant 0 and F (0 denotes the average orientation of the absorbed organic molecules) defined by (6)
Co,r : (~q/OE)o.r
with (b) the series arrangement of a capacity C~,r connected with the dependence of q on 0 for constant E and £ and defined by
c=,F = (
and a resistance Ro connected with the kinetics of molecular orientation within the absorbed film. Slow exchange of ions between the interface and the solution is allowed for by a resistance R: in series with. Co.r*. Slow exchange of the organic species between the interface (constant F) and the solution at the interface is represented by resistance R0 in series with C,d=. These circuit changes tend to influence the interracial impedance at high frequency and therefore may affect the size and potential dependence of the high frequency modulation signal. Obviously the evaluation of circuit elements by any method becomes a formidable task when the circuit is as complex as the one in Fig. 6. The present work suggests that the modulation technique, using perhaps frequencies higher than 2 MHz, could prove powerful for the study of faster adsorption~tesorption systems than the aniline system provided that accurate thermo* Formal expressions for R, and circuits (e.g. ref. 8).
we believe to be obvious from earlier work on aperiodic
G.C. BARKER, D. McKEOWN
dynamic data are available. We shall show elsewhere that the method can usefully be employed to study the kinetics of the adsorption of halide ions on mercury. SUMMARY
A preliminary study has been made of the amplitude modulation signal connected with adsorption-desorption of aniline on mercury using a 2 MHz modulation polarograph. The signal tends to be appreciably smaller than that expected for reversible adsorption~tesorption in the range of potential in which molecular reorientation is an important effect. The discrepancies between theory and experiment are thought to be caused by lack of equilibrium between the interface and the solution at the interface. REFERENCES 1 W. Lorenz and G. Kriiger, Z. Phys. Chem., 221 (1961) 231. 2 W. Lorenz, Z. Phys. Chem., 224 (1963) 145. 3 E. Niki and T. Kyoya, Rev. Polarogr. Jap., 14 (1967) 141. 4 G. C. Barker and A. W. Gardner, Chem. Ing. Tech., 44 (1972) 211. 5 G. C. Barker, J. A. Bolzan and A. W. Gardner, J. Electroanal. Chem., 52 (1974) 193. 6. B. Damaskin, Electrochim. Acta, 9 (1964) 231. 7 G. C. Barker, A. W. Garc[ner and M. J. Williams, J. Electroanal. Chem., 42 (1973) App. 21. 8 G. C. Barker, Pure Appl. Chem., 15 (1967) 364. 9 W. Lorenz and F. M~Sckel, Naturwiss., 43 (1956) 197. 10 W. Lorenz and F. Mifckel, Z. Elektrochem., 60 (1956) 939. 11 W. Lorenz, F. M6ckel and W. Muller, Z. Phys. Chem., NF, 25 (1960) 145, 161. 12 G. C. Barker, unpublished results.