Electrocatalysis of the Oxidation of Antimony(ll1) at Platinum Electrodes in 2M Hydrogen Chloride by Adsorbed Iodide Ronald J. Davenport and Dennis C. Johnson' Department of Chemistry, lowa State University, Ames, lowa 50070
Several examples of the electrocatalysis of electrochemical reactions by halide ions adsorbed at Hg electrodes have been documented and discussed (1-4). There has been less activity involving solid electrodes, probably because of the greater difficulty of preparing a reproducible surface and determining adsorption isotherms a t a solid material. Anson described the electrocatalysis of the oxidation of ethylenediaminetetraacetocobalt(I1) a t Pt electrodes by Br- present in the solution of supporting electrolyte ( 5 ) . Experimental and theoretical evidence was presented to support a proposed mechanism involving adsorbed Br-. Hubbard and Lau (6, 7) studied the electrocatalysis of the electroreactions of various complexes of Pt(I1, IV) a t Pt by the presence of excess halide ions in the solution of supporting electrolyte. They determined that the reactions at an I - treated electrode were accelerated for anionic complexes and decelerated for cationic complexes. We have discovered that I- adsorbed at a Pt electrode catalyzes the electrooxidation of Sb(II1) in 2M HCl. Hubbard et a!. ( 8 ) and Johnson (9) determined that I- is adsorbed at P t electrodes in acidic media by irreversible processes. The adsorbed I- is not desorbed even if the electrode surface is rinsed with water. We conclude that the heterogeneous electron transfer involves a bridging mechanism. The feasibility of using the electrocatalyzed reaction as the basis for the quantitative determination of Sb(II1) was investigated.
EXPERIMENTAL Apparatus. The three-electrode potentiostat used was one-half of a four-electrode potentiostat constructed according to the schematic diagram described in Ref (IO).The portion of the circuit used was that for control of electrode 1-1 and this was modified according to the description in Ref ( 1 2 ) . The platinum disk electrode was obtained from Pine Instrument Co. of Grove City, Pa., and had a disk area of 0.3135 cm2. The electrode surface was polished according to standard metallographic procedures, 0.3 Fm alumina on Buehler Microcloth with water as the lubricant was used for the final polishing step. The electrolysis cell was of all glass construction with a volume of approximately 600 ml. The Pt-wire counter electrode was placed in a chamber containing supporting electrolyte and separated from the test solution by a fritted glass disk. A commercial saturated calomel electrode (SCE) was used as the reference elec'To whom correspondence should be addressed. A. F r u m k i n , Trans. Symp. Electrode Processes. 1 (1959). R. Parsons. J. ElectroanaL Chern., 21, 35 (1969). S. Trasattt, Chim. lnd. (Milan). 51, 1063 (1969). R. De Levie. J. Electrochem. SOC.,118, 185C (1971). F. Anson. J. Electrochem. SOC., 110, 436 (1963). A. L. Y . Lau and A. T. Hubbard, J. Electroanal. Chem., 24, 237 (1970) A . L. Y Lau and A . T Hubbard. J. Electroanal. Chem., 33, 77 (1971) A. T. Hubbard, R . A . Osteryoung, and F. C. Anson, Anal. Chem., 38. 692 (1966). D. C. Johnson, J. Electrochem. SOC., 119, 331 (1972). D. T. Napp, D. C. Johnson, and S. Bruckenstein, Anal. Chern., 39, 481 (1967). S. Bruckenstein and B. Miller, J. Electrochem. SOC.,117, 1040 (1970).
trode. It was placed in a chamber containing supporting electrolyte with electrical contact to a Luggin capillary made through the wetted joint of a glass stopcock. In use, the RDE was positioned close to the tip of the Luggin capillary. All potentials were measured and are reported in volts (V) us. SCE. Reagents. All chemicals were reagent grade from commercial sources. Water was triply distilled with a deionization following the first distillation and the second being from alkaline permanganate solution. The supporting electrolyte was 2M HC1 and was deareated using prepurified (99.997%) Nz from Air Products. A stock solution of Sb(1II) was prepared by dissolving sufficient Sbz03 to make a 500-ml solution of 5.00mM Sb(II1) using 25 ml of concentrated HCl. This solution was transferred to a 500-ml flask and diluted with 2M HC1. Procedure. The electrode was pretreated a t the beginning of each experimental day by polishing with 0.3 pm alumina, rinsing thoroughly with water, and potentiostating in the supporting electrolyte for 3 min a t 1.50, -1.50, and 0.00 V. The electrode was rotated using a synchronous, variable speed rotator from Pine Instrument Co. Current-potential (I-E) curves were recorded using a Model 815, Plotamatic X-Y recorder from Bolt, Beranek, and Newman, Inc., of Santa Ana, Calif.
RESULTS AND DISCUSSION
I-E curves are shown in Figure 1 for 2M HC1 (curve 1) and 2M HCl containing l . O d Sb(II1j (curve 2 ) . The value of E112 of the anodic wave for Sb(II1) on curve 2 is 0.90 V and the wave has the appearance of being very irreversible. The limiting-current plateau cannot be resolved from the current for oxidation of C1- at 1.05 v, and analytical application of a measurement of limiting current is hopeless. Curve 3 in Figure l was obtained for 2M HC1 containing 1pM KI and l.OmM Sb(I1Ij. In preparing this solution, the Sb(II1) was added after addition of KI and pretreatment of the electrode. The electrocatalytic effect of' the trace of KI on the anodic wave for Sb(II1) is very dramatic. Substitution of NaI for KI did not result in any change in the subsequent I-E curve from that shown by curve 3 and the electrocatalysis is clearly due to the I - , The E112 for the electrocatalyzed wave is 0.72 V and a broad current plateau indicates the limiting anodic process is controlled by convective-diffusional mass transport of Sb(II1). A similar catalysis resulted when I- was absent from the solution of supporting electrolyte but the electrode was dipped in a solution of KI, followed by thorough rinsing with triply distilled water, prior to insertion in the test solution. The catalytic role of the I- was determined to persist indefinitely with repeated cyclic scans of E d within the potential limits shown for Figure 1. The electrocatalytic behavior is clearly due to I- adsorbed at the surface of the platinum electrode. The limiting current in a RDE for an electrolytic reaction controlled by processes of convective-diffusional mass transport is predicted to be proportional to the square root of the angular velocity of electrode rotation, d / 2 (12). A plot of I, us. w1/2 is shown in Figure 2 for the electrocatalyzed oxidation of Sb(II1). All values of ZIwere measured at 0.85 V on the anodic scan of potential and are corrected (12) V. G . Levich, "Physicochemical Hydrodynamics," Prentice-Hall,Englewood Cliffs,N.J. 1962, p 69.
ANALYTICAL CHEMISTRY. VOL. 45, NO. 9. AUGUST 1973
1755
I
I
I
I
0 -
I
-
900
Figure 1. /-E
curves
Anodic scan of E at 0.2 V/min; w = 261 rad/sec; (1) 2M HCI-lpM KI, (2) 2M HCI, 1.0 x 10-3M Sb(lll), and (3) 2M HCI-lpM KI, 1.0 X 10-3M Sb(lll)
loo
for the residual current. The plot is linear for o1I2 < 22 (rad/sec)l/2. The positive intercept was determined to be a function of potential scan rate; the value decreased with the use of a lower scan rate. The value of Zl for the electrocatalyzed oxidation of Sb(II1) in 2M HC1-1WM KI was plotted as a function of Cbsb(III)for o = 41.9 rad/sec. The plot was h e a r for 1.00 X M X CbSb(lII) I 1.00 x M with a slope of 202 pA/mM and an intercept of 4 PA. The beneficial consequence of the electrocatalysis by adsorbed I- for the quantitative determination of Sb(JII) is very apparent. This is particularly true since the catalytic double layer can be prepared at the electrode prior to its use in the solution of Sb(1II) and no I- need be added to the test solution. Application of this phenomenon is presently being made in our laboratory for the quantitative determination of Sb(1II) by high-speed liquid chromatography using electrochemical detection (13). The use of HC104 or HzS04 as the supporting electrolyte was found to be unsatisfactory for the electrocatalysis even though adsorbed I- at the platinum electrode is stable in those solutions (8, 9). In 2M HC1, ionic antimony exists only as SbC14- or SbC16- (14) whereas in HC104 or HzS04 the oxyanions probably exist (7, 15). The exchange (13) L. R. Taylor and D. C. Johnson, Iowa State University, unpublished data. (14) G. P. Haight, Jr., J. Arner. Chern. SOC., 75, 3848 (1953). (15) M. V. Vojnovic and D. 8 . Sepa, J. Electroam/. Chern., 31, 413 (1971).
1756
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1
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 9,
t/
I
I
I
I
I
I
I
5
IO
15
20
25
30
35
LIA
Figure 2.
(rod-sec-')i
/ l - ~ ' / ~
Anodic scan of E at 1.0 V/min; I measured at E l p M K I , 1.0 X 10-3MSb(lII)
=
+0.85 V; 2M HCI-
rate of the halide is much greater than that of the hydroxy ligands (15). Explanation of the increase in the rate of the heterogeneous reaction due to adsorbed I- can hardly be based on electrostatics. Since both I- and SbC14- are anionic species, any electrostatic argument would conclude the reaction should be slowed by adsorbed I-. We propose that the mechanism of the electrocatalysis involves a bridging role by the adsorbed I- to facilitate the electrontransfer reaction as described by Equations 1-3. SbC1,-
Pt(I-),,j,
Pt(I-SbCls-),d,
+
c1-
(1)
We are convinced that the results described here are the most convincing proof of the existence of true electrocatalysis by adsorbed halides at platinum electrodes through electron-transfer bridging. Received for review January 26, 1973. Accepted March 19, 1973:
AUGUST 1973
