AKIKOARAMATA AND PAUL DELAHAY
880
Electrode Kinetics with Adsorbed Foreign Neutral Substance
by Akiko Aramata and Paul Delahay Coates Chemical Laboratory, Louisiana State University, Baton Rouge, Louisiana (Received November 9 , 1968)
The kinetics of discharge of Zn(I1) on a hanging Zn(Hg) drop was studied by the galvanostatic method in perchlorate medium without and with n-amyl alcohol. The Ii’rumkin correction for the double layer structure was verified in the absence of n-amyl alcohol for Mg(C104)z and Ba(C104)2,but departure was considerable for NaC104 and A1(C10,)3. This discrepancy is tentatively explained. The variations of apparent exchange current density in the presence of n-amyl alcohol and R/Ig(C104)2 were essentially accounted for up to coverages of 0.5 by the change of current density with coverage and by the change of potential across the diffuse double layer. At higher coverage, exchange current densities appear to be somewhat smaller than the values computed on the basis of the above two corrections. An explanation is offered, and the significance of coverage, as computed from thermodynamic surface excesses, is discussed. The transfer coefficient was not affected by the presence of n-amyl alcohol.
Electrode processes in the presence of an adsorbed neutral substance, which is not directly involved in the charge-transfer reaction, have been considered from two points of ~ i e w , l -namely ~ the effects of adsorption (a) on processes controlled by mass transfer and (b) on kinetics of charge transfer. Adsorption kinetics is also relevant, particularly with the dropping mercury elect r ~ d e . ~This , ~ paper deals with the second problem in a somewhat novel way. Conditions were selected to simplify as much as possible the interpretatjon of experimental results and to eliminate effects which are only incidental to the problem. The discharge of Zn(1I) on zinc amalgam was selected because its kinetics can be investigated by a relaxation method (e.g., the galvanostatic method) over a narrow interval of potentials in which coverage by the adsorbed additive hardly varies. Furthermore, this process occurs a t sufficiently negative potentials to preclude anion specific adsorption. Complexation was avoided by the use of a perchlorate medium,6 and the charge of the discharged species was unambiguously known. Complications due to adsorption kinetics, as would have been encountered with a dropping amalgam electrode, were eliminated by the use of a hanging zinc-amalgam electrode. The Journal of Physical Chemistry
Experimental Solutions. Solutions of Ba(C10J2, Mg(C104)2, and Zn(C104)2 were prepared by dissolution of the corresponding oxides. Al(C104)3 was prepared by dissolution of aluminum foil in HCIOI (in the presence of Hg t o speed up metal attack), and the solution was analyzed by an ion-exchange technique. Analytical grade SaC104 was utilized without further purification. Solutions were prepared with double-distilled water (once over KMn04) and were treated with purified activated charcoal to remove adsorbable impurities.’ (1) For a review see A. N. Frumkin, paper presented a t the C.1.T.C.E. Moscow meeting, Aug., 1963; abstract in Electrochim. Acta, 8 , iii (1963). (2) R. Parsons, “Advances in Electrochemistry and Electrochemical Engineering,” Vol. I, P. Delahay, Ed., Interscience Publishers, New York, N. Y., 1961, pp. 54-58. (3) I n polarography: (a) J. Weber, J. Koutecky, and J. Koryta, 2. Elektrochem., 63, 583 (1959); J. Koutecky and J. Weber, Collection Czech. Chem. Commun., 25, 1423 (1960); (c) J. Kuta, J. Weber, and J. Koutecky, ibid., 25, 2376 (1960); (d) J. Weber and J, Koutecky, ibid., 25, 2993 (1960). (4) For a review in polarography see C. N. Reilley arid W. Stumm, “Progress in Polarography,” Vol. I, P. Zuman, Ed., Interscience Publisher, New York, N. Y., 1962, pp. 81-121. (5) See ref. 2, pp. 20-26. (6) 3. Bjerrum, G. Schwaraenbach, and L. G. Sill&, “Stability Constants,” The Chemical Society, London, 1958, p. 111.
881
ELECTRODE KINETICS WITH ADSORBED FOREIGN NEUTRAL SUBSTANCE
I
Zinc amalgam was prepared by electrolysis of zinc perchlorate in acidic solution and was stored, with cathodic protection, under a layer of 0.01 M HC104. The zinc concentration was determined by treatment of the amalgam with perchloric acid and subsequent polarographic analysis. n-Amyl alcohol was redistilled a t 135-1 36 O . Cell for Galvanostatic Measurements. An all-glass H-cell was used with a fritted glass disk between the arms of the cell. Both compartments were filled with the solution being studied. One compartment contained a zinc-amalgam pool, a dropping amalgam electrode, and a hanging amalgam drop, which was prepared according to DeMars and Shain.8 The zincamalgam pool served as auxiliary electrode in galvanostatic measurements. The other compartment was connected by a 1 M KaC1 bridge to a calomel electrode prepared with 1 M NaC1. The latter electrode was part of the potential stabilization circuit, described below, and the liquid junction potential was of no consequence. The solution in the compartment with the zinc-amalgam electrodes was freed of oxygen by nitrogen bubbling. Nitrogen was purified over activated charcoal a t Dry Ice temperature and by a vanadous sulfate solution according to standard polarographic practice. Galvanostatic Measurements. The conventional bridgeg was fed by a Tektronix pulse generator, Type 161. The potential of the zinc-amalgam drop was somewhat unstable with n-amyl alcohol in solution, possibly because of slow hydrogen gas evolution, and the resulting error in galvanostatic measurements was minimized by polarization of the electrode a t the equilibrium potential with the 1 M NaC1-calomel electrode as counter electrode. The polarization circuit was opened with a Clare mercury relay, Type 1010, just before the galvanostatic determination of the overvoltage-time curve. Overvoltages a t t = 0 were determined by extraplolation against t/’ in the interval t = 2 to 50 msec. Surface Tension. Measurements were made with a Gouy electrometer after substitution of Zn(1J) by Mg(I1) in equivalent amount, regardless of the supporting electrolyte. Conditions corresponding to an ideal polarized electrode were thus practically achieved. The resulting error was negligible since, a t any rate, the Zn(I1) concentration was small in comparison with that of the supporting electrolyte. Charge densities and surfaces excesses were computed from electrocapillary curves. lo Double layer capacity data, which lead to more precise values of the charge density in the absence of adsorbed organic substance, were not determined here because their interpretation is com-
plicated by a frequency-dependent mass-transfer component5 when n-amyl alcohol is present.
Discussion and Results Z n ( I I ) Discharge without n-Amyl Alcohol. The transfer coefficient CY for Zn(I1) discharge was determined by application ofll
(1) where 1,0 is the apparent exchange current density; n = 1 2 ; ICo is the standard rate constant for the reaction Zn(1T) 4- 2e = Zn(Hg) in the medium being studied; the C values are the concentrations; A q is the difference of potential from the plane of closest approach to the bulk of the solution a t the equilibrium potential; and F , R, and T are as usual. One had I,O = 1.75, 2.66, 4.52, and 7.47 ma. cm.-2, respectively, for 1, 2, 4, and 8 mmoles 1.-’ of Zn(I1) ; other conditions I ) 0.048 mole 1.-’ for Zn(Hg), C = 0.25 were C Z ~ ( I = M Mg(C104)2 for supporting electrolyte. A plot12 of log Ia0against log Czn(I1) yielded a = 0.30. The same value of CY was also computed from measurements with 0.05 A4 A!tg(C104)2. Validity of the Frumkin correction was ascertained from the linearity of a plot of log I,O against A p for a varying concentration of supporting electrolyte and for fixed concentrations of Zn(I1) and Zn(Hg). Results for .1/g(C104)2and Ba(C10& (Table I) yielded CY = 0.30, in good agreement wit,h the above determination of CY. Results for NaC104 gave the absurd value of a = -0.38; likewise, CY = 0.53 for Al(C104)3 was abnormal. This indicated a definite departure from the Frumkin correction for the A p values calculated from the plane of closest approach to the bulk of the solution. The disagreement may be ascribed to the differences in ionic charges13 or, more likely, to differences in ionic radii of Zn(I1) and Ka(1) or Al(I1I). One has the following radii in solution according to Monk1*: 1.83 [Na(I)I, 2.88 [Ba(II)], 3.45 [Xg(II)], ~
(7) G. C. Barker. “Transactions of the Svrnoosium on Electrode Processes,” E. Yeager, Ed., John Wiley and Sons, Inc., New York, N. Y., 1961, pp. 325-365. (8) R. D. DeMars and I. Shain, Anal. Chem.. 29, 1825 (1957). (9) See ref. 2, pp. 305-306. (10) D. C. Grahame, Chem. Rev., 41, 441 (1947). (11) For & review see ref. 2, pp. 237-247. (12) The more rigorous plot of log IO , t8. log Czn(rx) - (0.484nF/ RT)Aq yielded 01 = 0.28, but variations of a resulting from t h e change of equilibrium potential, as C of Zn(I1) varied, were so small (0.4 mv.) t o hardly affect the determination of a. (13) K. Asada, P. Delahay, and A. K. Sundaram, J. Am. Chem. Soc., 83, 3396 (1961).
Volume 68, Number 4
A p r i l , 1964
AKIKOARAMATA AND
882
Table I : Variations of I,O with Supporting Electrolyte Concentration in the Absence of n-Amyl Alcohol" at 26 Supporting electrolyte, mole l . - l
mv.
0.025 M Mg( C10& 0.05 0.125 0.25 0,025 M Ba(C10& 0.05 0.125 0.250
-63.0 -56.8 -46.3 -41.1 -60.8 -52.7 -42.8 -36.0
A%
I$, ma. om.-'
12.0 9.0 4.7 2.7 9.1 5.7 3.2 2.1
zk
1"
Io, ma. cm.-a
0.40 0.43 0.37 0.38 0.33 0.39 0.31 0.39
a For C z n ( ~ = ~ )2 mmoles k1, C z n ( ~ a=) 0.048 mole 1.-'. The exchange current density Io, defined as I o = IO , for A'p = 0, w a ~ computed for 'I! = 0.30.
and 3.47 A. [Zn(II)]. The difference in ionic radii in mixed electrolytes, as pointed out by Joshi and Parsons,16is a cause of departure from the Gouy-Chapman theory. Specific cation effects might possibly play a role.'6 At any rate, all work with adsorbed n-amyl alcohol was done with NIg(ClO,), since the Frumkin correction did hold in that case. Zn(II) Discharge with Constant n-Amyl Alcohol Concentration and Varying Supporting Electrolyte Concentration. Before applying eq. 1 to Zn(I1) discharge in the presence of adsorbed n-amyl alcohol, one must consider the main assumptions made in the derivation of this equation. These assumptions are: (a) The discharge process is of the first order with respect to all reactants, and its rate-determining step involves the same number of electrons as the over-all reaction. (b) The double layer correction is made by using Acp, as calculated from the plane of closest approach and the bulk of the solution; and the concentration of Zn(I1) in the plane of closest approach is correlated to that in the bulk of the solution on the assumption of purely electrostatic interactions. We shall assume that these assumptions can also be made when namyl alcohol is adsorbed on the dectrode. The difference of potential Acp is now calculated by combination of the theoretical dependence of the charge density q on the electrode as a function of Acp with the experimental dependence of q on the electrode potential. The former relationship is given by the Gouy-Chapman theory, and the function q = f(E) is obtained from electrocapillary curves. The procedure is the same as for solutions free of adsorbed neutral organic species, and its extension to electrodes with adsorption was made by Parsons. The rate constant ICo in eq. 1 is some unknown funcThe Journal of Physical Chemistry
PAUL
DELAHAY
tion of the n-amyl alcohol coverage 6, but one deduces from this equation that a plot of log I,O against E Acp is linear and has the slope 0.434(1 - a)(nF/RT) provided 6 and the Zn(Hg) concentration are kept constant. This relationship provides one means of ascertaining the validity of the above method of calculation of Ap, especially if a is constant whether or not n-amyl alcohol is present. At constant n-amyl alcohol concentration, A s can be changed by variation of the supporting electrolyte concentration. The namyl alcohol coverage then varies slightly but could be kept constant, in principle, by adjustment of the potential E a t which Isois measured. The Zn(I1) concentration therefore should be adjusted t o maintain 6 constant, as the supporting electrolyte concentration varies. In practice, however, variations of 6 due to the change of the supporting electrolyte concentration are smaller than the experimental error on 0. Measurements thus were made a t constant E, ie., with constant concentration of Zn(I1) and Zn(Hg), and data were plotted as log I,O against Acp. Results in the presence of 0.1 M n-amyl alcohol (Table 11) gave a linear plot with a slope corresponding to a = 0.32. This value of a for a high electrode coverage (0 = 0.78) is the same within experimental error as that determined without n-amyl alcohol ( a = 0.30). It appears that a, in this case, is not affected by the presence of n-amy117alcohol and that the Frumkin correction holds for the value of A p calculated by the above method. Zn(IZ) Discharge with Constant Supporting Electrolyte Concentration and Varying n-Amyl Alcohol Concentration. Values of I 2 were determined for different n-amyl alcohol concentrations in order to determine the influence of coverage. Since it was shown above that the correction for Acp can be made in presence of namyl alcohol, one can calculate the value of (I,o)o=o that would be measured if only Acp varied by addition of n-amyl alcohol and 6 remained equal to zero (Table 111). A plot of the ratio I,o(measured)/(.Io)o=~ then gives the influence of coverage (Fig. 1). The (14) C. B. Monk, "Electrolytic Dissociation," Academic Press, Inc., New York, N. Y., 1961, p. 271. (15) K. M . Joshi and R. Parsons, Electrochim. Acta, 4, 129 (1961). (16) (a) A. Aramata and P. Delahay, J . Phys. Chem., 66, 2710 (1962); (b) F. Lamy, J. J. Tondeur, and L. Gierst, paper presented a t the C.I.T.C.E. Moscow meeting, August, 1963; abstract in Electrochinz. Acta, 8 , xii (1963). (17) The method for the determination of a in the absence of mamyl alcohol cannot be applied here because 0 varies too rapidly with the equilibrium potential as a result of the change in the Zn(I1) concentration. An attempt to apply this method for 0.1 n-amyl alcohol gave the abnormal value a = 0.63. Thus, when Czn(rr) increases, being constant, E becomes more positive and 0 increases; hence I,o is lower than the value expected for the same Czn(rr) and a constant 0. The plot of log 1 ,' against log CZ.(II) yields too low a value of 1 - a. This is indeed observed.
8133
ELECTRODE KINETICSWITH ADSORBEDFOREIGN NEUTRAL SUBSTANCE
Table 111: Variation of 1,o with n-Amyl Alcohol Concentration in 0.05 M Mg(CIOa)zaa t 26 3t 1"
Table I1 : Variations of ZO, with Supporting Electrolyte Concentration in the Presence of 0.10 M %-Amyl Alcohol" at 26 =k 1" mole 1. - 1
mv.
I,O, ma. cm.-%
0.05 0.125 0.25
-40.0 -30.5 -23.5
0.32 0.20 0.14
CMpCC 104) 2,
AV>
10,
ma. cm.-a
0.037 0.037 0,039
a For C z n ( ~ = ~ ) 2 mmole l.-l, C Z ~ ( R=~ )0.048 mole 1.-l. computed for (Y == 0.30.
10
ri--
Ii n ecorrnction a r c o v e rag e
0.5 a le 0
m
E m
Iso (measured), ma. cm.-a
0 0.02 0.04 0.08 0.12
-55.8 -53.5 -49.2 -43.6 -37.5
9.0 7.7 4.2 0.65 0.23
(Ia0)8=n, ma. cm.-P
9.0 8.1 6.4 4.5 3.4
fao
-
molecule to vary with 6, as pointed out by Frumkin.' Keeping in mind these reservations, one concludes froin Fig. 1 that, in the present case, the influence of adsorbed n-amyl alcohol is essentially accounted for up to 6 = 0.5 by consideration of two effects: variation of I,O with A p and the concomitant variation of current density with coverage. ,4t high coverages (6 > 0.5), exchange current densities are somewhat smaller than the values expected after correction for the above effects. Reservations made about the definition of 6, made above, may be invoked to explain results for 0 > 0.5. Further work is now in progress to examine the validity of this interpretation for other systems. The validity of the Ap-calculation for a variable 0 is also being re-examined.
Acknowledgment. This work was supported by the Office of Naval Research. We thank Dr. D. M, Mohilner (University of Pittsburgh), formerly of this laboratory, for discussion of the effect of ionic size on the Frurnkin correction. Comments by Dr. R. Parsons, University of Bristol, are also gladly acknowl.. edged.
'0 Y
e
AV,
my.
a For Czn(1~) = 2 mmoles l.-l, Czn(ag) = 0.048 mole 1.-'. (for e = 0) computed from eq. 1 for 01 = 0.30.
significance of the coverage plotted in Fig. 1, however, must be scrutinized. Thus, 6 was defined as I'/I',,? were I' is the surface excess of n-amyl alcohol and rm is the maximum value of .'I One had I' = 1.26 X: mole a t E = -0.55 v. us. n.c.e. for 0.12 M n-amyl alcohol. 'This value of r was adopted13 as being rmim the preparation of Fig. 1. There results some uncertainty on rm,as determined in this way, and, furthermore, experimental errors on I' can be as large as 10% under the best experimental conditions.21 One should also expect the covered area per
I
Caleohol,
mole 1. --I
Y
-
H
0 U, 3
8
Figure 1. Influence of coverage after correction for Arp.
I
(18) It is somewhat larger than rm 1.03 X lo-* mole cm.-2 computed by Parsons1* for 0.1 M namyl alcohol in 1 M NeClOd from data from this laboratory.a0 Differences in conditions and experi.. mental errors may aacount for the difference. (19) R. Parsons, J. Electroanal. Chem.. 5, 397 (1963). (20) M .Breiter and P. Delahay, J . Am. Chem. doc., 81, 2939 (1959). (21) E. Blomgren, J. O'M. Bockris, and C. Jesch, J . Phys. Chem., 6 5 , 2000 (1961).
Volume 68, Number 4
April, 196.4
