Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes

Electroanalytical Chemistry and Interfacial Electrochemistry. 53 (1974) 235 241 t

235

Elsevier Sequoia S.A., Lausanne .. Printed in The Netherlands

ELECTROSTATIC EFFECT OF SPECIFICALLY ADSORBED ELECTROINACTIVE IONS UPON ELECTRODE PROCESSES II. PERBROMATE ELECTROREDUCT1ON IN THE PRESENCE OF CI-, Br-, SCN-, AND N]

MARIA LUISA FORESTI, DANILO COZZI and R O L A N D O G U I D E L L I

Institute qJ Analytical ('hemistry, University t~] Florence, Florence (Italy) (Received 5th February 1974)

In a systematic study of the effect of specific ionic adsorption upon the kinetics of electrode processes carried out in this laboratory ~-3 we found it interesting to examine the influence of halide and pseudohalide adsorption upon the reduction rate of BrO4 ion. The polarographic behaviour of BrO2 is quite similar to that 4 of $402-. Thus in both cases the rising portion of the reduction wave in fluoride medium develops at -0.3 to -0.4 V/SCE and a dip in the polarographic current is observed at potentials just negative of the point of zero charge, Ez. Moreover, the electroreduction rate of BrO2 ion is independent of pH just as is that of $402-- (ref. 5). Also, the effect of halide and pseudohalide adsorption upon BrO2 electroreduction is in many aspects analogous to that described in ref. 3 (henceforth referred to as Paper I) for $40~ - electroreduction. Hence, for brevity, we will limit ourselves to pointing out the differences between the behaviour of these two electro-active oxyanions. Certain advantageous features of BrOg electroreduction over $402- electroreduction are: (1) the streaming maximum exhibited by the BrO;, polarogram around Ez is much less pronounced than that shown by the $402- polarogram and disappears completely upon addition of small amounts of any surface-active halide or pseudohalide ion. This permitted us to study the effect of CI- adsorption on BrOg reduction at potentials anodic of Ez; (2) the monovalency of BrOg reduces the possibility of ion-pair formation with the cation of the supporting electrolyte; (3) the high symmetry of the BrO£ ion allows a much better estimate of its cross-sectional area and hence of the distance of closest approach between BrO2 and the surrounding adsorbed halide ions. On the other hand, as opposed to the $402- ion, BrO~ oxidizes 1- in neutral media, thus preventing us from studying the effect of I- adsorption upon the kinetics of BrOg electroreduction. Moreover, relatively small amounts of TIF shift the rising portion of the BrOw7 polarogram to potentials more positive than +0.0 V/SCE, where no data on the potential ~b~ at the outer Helmholtz plane nor on the charge density q~ due to TI + specific adsorption are available 6. Hence, even though the wave shift is in qualitative agreement with theoretical predictions, we were unable to relate the observed increase in the rate of BrO,/ electroreduction following TI ~ adsorption to the charge density qi of this cation.

236

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Fig. 1. (RT/F) In X vs. E and the rs. E plots for the electroreduction of 5x 10-4 M KBrO4 in the prcsencc of KCI of concentration: (a) 10-2, (b) 1.3x 10-2 , (c) 2x 10..2, (d) 3x 10-2 , (e) 4x 10-2, (f) 0.1, (g) 1, (h) 2.449 M. Curve I shows (RT/F) In Z vs. ~ba for (E-~ba)= -0.950 V. Curve 2 shows (RT/F) In X vs. E for ~ba=-100 mV. Figure la shows [(RT/F) In x-(l+~')q~a] rs. qi for E= -0.325. The experimental set-up and the purification of salts, water, and mercury are described in Paper I. Figures 1, 2, 3, and 4 show plots of ( R T / F ) In l~ vs. E for various bulk concentrations of C l - , B r - , S C N -, and N 3 . As in Paper 1, X designates the quantity {12 td/(7 D)}~'kf, where D is the diffusion coefficient of the reactant, td=3.1 s the drop time, and /q. the rate constant of the electrode process. Measurements of ~Cwere carried out as described in Paper I. Table 1 summarizes the experimental values of the "apparent" charge-transfer coefficient ~' and of the charge z of the reacting particle as obtained by Gierst's graphical method 7 at potentials sufficiently cathodic to justify the neglect of halide and oseudohalide specific adsorption. The ~bd vs. E curves employed in applying Gierst's method

PERBROMATE REDUCTION IN PRESENCE OF CI-, Br , SCN-, N~

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-oA6i I Fig. 2. ( R T / F ) In )~vs. E and q~d VS. E plots h)r the electroreduction of 5x 10 -4 M KBrO4 in the presence of KBr of concentration: (a) 5 x 10-3, (b) 10- 2 (c) 2 x 10 -2, (d) 5 x 10 2 (c) 0.1, (f) 0.2, (g) 0.5, (h) ! M. Curve l shows ( R T / F ) In Z vs. r~a for (E-4),)= -0.860 V. Curve 2 shows ( R T / F ) In Z vs. E for 4),= - 9 0 mV. Figure 2a shows [(RT/F) In Z - ( I +:d)q~,] vs. qi for E= -0.375 V. were k i n d l y supplied to us b y Dr. P a r s o n s a- 11 a n d are r e p o r t e d on the lower half o f Figs. 1-4. T h e z values in T a b l e ! are in satisfactory a g r e e m e n t with the value - 0 . 8 o b t a i n e d in ref. 4 from s o d i u m fluoride s o l u t i o n s a n d are close to the expected value - 1. A close inspection o f the ~' values o f T a b l e 1 reveals that the a p p a r e n t c h a r g e - t r a n s f e r coefficient in the presence o f s o d i u m salts, ~{~a+, is s o m e w h a t s m a l l e r t h a n that, ~k+, in the presence o f p o t a s s i u m salts. T h u s ~ a + ~ 0 . 0 9 , whereas ~ k + ~ 0 . 1 2 . It s h o u l d be noted that the difference between ~ . + a n d ~k+, albeit small, is nonetheless d e t e c t a b l e a n d r e p r o d u c i b l e , as actually verified by c a r r y i n g out all m e a s u r e m e n t s twice. Such a difference can be e x p l a i n e d as in P a p e r I by

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Fig. 3. (RT]/I") In X. vs. E and qSd vs. E plots for the electroreduction of 5 × 10 -4 M KBrO4 in the presence of NaSCN of concentration: (a) 1.224 x 10- 3 (b) 2.136 x 10- 3 (c) 6.105 x 10- 3, (d) 1.223 x 10 - 2. (e) 2.446 x 10 2. (f) 6.105 x 10-z, (g) 0.1169, (h) 0.1982, (i) 0.4943 M. Curve 1 shows (R T/F) In ~' vs. cbd for (E--4bd)=--0.990 V. Curve 2 shows ( R T / F ) In X, vs. E for q~d=-90 mV. Figure 3a shows [(RT/F) In 7, -(1 +~') 4~d] v,s. q~ for E= -0.450 V. t a k i n g into a c c o u n t that the h y d r a t e d r a d i u s of N a ÷ is greater than t h a t o f K +, a n d hence that the d i s t a n c e of the o.h.p, from the e l e c t r o d e surface is likely to be g r e a t e r in the presence of N a + t h a n o f K ÷ ions. T h e theoretical value o f the (~-~,-) difference as derived from eqn. (5) of P a p e r I on using the same p a r a m e t e r s e m p l o y e d therein (with the o b v i o u s e x c e p t i o n o f z, which is set equal to - 1 ) a m o u n t s to +0.11. This value is not t o o far from the e x p e r i m e n t a l value 0.03. T h e g r e a t e r e x p e r i m e n t a l difference (~k+ - c ~ , + ) = 0.10 o b s e r v e d in $4062e l e c t r o r e d u c t i o n 3 is to be a s c r i b e d to the g r e a t e r sensitivity o f the b i a n i o n i c r e a c t a n t to the electric p o t e n t i a l c h a n g e which the a d s o r b e d a c t i v a t e d c o m p l e x experiences in p a s s i n g from N a + to K + u n d e r otherwise identical c o n d i t i o n s . Figures l a - 4 a show plots o f [ ( R T / F ) I n x + ( z - ~ ' ) 4 b a ] =q~, with z = - 1, at

PERBROMATE REDUCTION IN PRESENCE OF CI- Br-, SCN-, N~

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-0.12 Fig. 4. (RT/F) In X vs. F and ~)~ t,x. E plots for the electroreduction of 5 x 10 -4 M KBrO4 in the presence of NaN3 of concentration: (a) 8 x ]0 -3 , (b) 2.25× l0 -2, (c) 3.3× l0 -2, (d) 5.7× l0 -2, (c) 8.0x l0 -2, (f) 0.14, (g) 0.23"/, (h) 0.467, (i) 0.905, (j) 1.4 M. Curvc 1 shows (RT/F) In Z t,s. (~d for (E-q~)=-0.605 V. Curve 2 shows (RT/F) In Z vs. E for ~ d = - - 6 8 inV. Figure 4a shows [(RT/F) In X - ( 1 +st') ~a] vs. qi for E equal to: - 0 . 3 0 0 V (a'), - 0 . 4 0 0 V (b').

constant applied potential E against the charge qi of the specifically adsorbed halide or pseudohalide ion. Values of qi as a function of E were kindly supplied by Dr. Parsons 8-1°. It is readily seen that these q~ vs. qi plots, just as those for $406z- electroreduction, are linear for -q~ >-,~ 10/aC cm -z, whereas they curve tending to become horizontal as qi tends to zero. A possible cause of deviation from linearity at low values of Iqi] was considered in Paper I. Table 1 summarizes the slopes of the linear portions of the q, vs. q~ plots for the various halides and pseudohalides. The slope Acb/Aq~ for CI- ion is less accurately known than

240

M . L . FORESTI, D. C O ZZI, R. G U I D E L L I

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for the remaining surface-active anions, on account of the limited number of chloride concentrations for which 4~d vs. E and q~ vs. E curves are available. Table 1 also summarizes the theoretical slopes as derived from eqns. (3) and (4) of Paper 1 on using the same double-layer parameters employed therein, with the obvious exception of the value a' for the distance of closest approach between BrO,,S and the halide or pseudohalide ion. As usual, a was set equal to the crystal ionic radius ri of the halide or pseudohalide ion (see Table 1) plus the radius r (BrOg) of the BrOg ion. To estimate r (BrOg), the crystal ic, nic radius of K +, 1.33 /~, was first subtracted from the average weighted K - O distance in the KBrO4 crystal ~, 2.92 A, so as to obtain the intermolecular contact radius r(O) of the O atom in the BrOg ion. The radius r (BrOg) of the tetrahedrai BrOg ion was then taken as equal to the sum 3.20 ~ of r(O) and of the Br-O bond distance in KBrO4, 1.61 /~, (ref. 12). The agreement between theoretical and experimental values of the Aq~/Aqi slope is quite satisfactory. Also, the gradual increase in slope predicted by theory, as the radius ri of the surface-active anion decreases, is actually verified experimentally, with the remarkable exception of the N3 ion. The unexpectedly low slope of the tb vs. q~ plot in the presence of N3 ions is probably due to the particular electron-density distribution in this ion, as already pointed out in Paper I. In conclusion, the foregoing investigation on the effect of halide and pseudohalide adsorption upon BrOg electroreduction confirms the results obtained in connection with $4062- electroreduction and lends further support to their interpretation. ACK N O W L E D G E M ENTS

The authors thank Dr. E. H. Appelman of the Argonne National Laboratory for the potassium perbromate, as well as Dr. Roger Parsons for having provided them with the double-layer data pertaining to refs. 8-11. This work was supported by the C.N.R. under contract 72.01135.64. SUMMARY

The reduction rate/q, of BrO£ .in the pre~nce of halides and pseudohalides

PERBROMATE REDUCTION IN PRESENCE OF CI-, Br-, SCN-, N:~

241

at sufficiently negative potentials with respect to the point of zero charge satisfies the well-known Butler-Volmer equation corrected for diffuse-layer effects according to Frumkin. On the other hand at less cathodic potentials, where halide and p~udohalide adsorption becomes appreciable, the rate constant kf at constant applied potential E, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density qi due to the specifically adsorbed halide and pseudohalide ions. Thus in the presence o f C l - , Br-, SCN-, and N3 the logarithm of/q.~,,= 0 (where kf,~,,= 0 denotes the rate constant at constant E corrected for diffuse-layer effects) decreases linearly with Iqil, at least for Iq~l> ~ 10/~C cm -2. The slopes of the various log kr.,,,=o vs. q~ plots are in satisfactory agreement with a theoretical treatment proposed by two of the authors ~'2 which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring specifically adsorbed ions within the compact layer. REFERENCES R. Guidelli and M. |,. Forcsti, Electrochim. Acre, 18 (1973) 301. R. Guidelli, d. Electroanal. Chem., 53 (1974) 205. M. L. Foresti and R. Guidelli. d. Electroanal. Chem., 53 (1974) 219. D. Cozzi, M. L. Foresti and R. Guidelli, d. Electroanal. Chem., 42(1973) App. 31. B. Ja~lskis and J. Huston, Anal. Chem., 43 (1971) 581. P. Delahay and G. G. Susbielles, J. Phys. Chem., 70(1966) 647. L. Gicrst in E. Yeager (Ed.), Trans. Syrup. on Electrode Processes, Philadelphia, 1959, Wiley, .New York, 1961, p. 109. 8 D. C. Grahame and R. Parsons, d. Amer. Chem. Sot., 83 (1961) 1291. 9 J. Lawrence, R. Parsons and R. Paync, d. Electroanal. Chem., 16 (1968) 193. 10 R. Parsons and P. C. Symons, Trans. Faraday Soc., 64 (1968) I{)77. 11 C. V. D'AIkaine, E. R. Gonzalez and R. Parsons, J. Electroanal. Chem., 32(1971) 57. 12 S. Siegel, B. Tani and E. Appelman, Inorg. Chem., 8 (1969) 1190. 1 2 3 4 5 6 7