0.0 V for 15 minutes in 1M HCl, oxidizing with a constant current to +0.71 V, and heating the electrode and solution to dissolve any fdm. They report finding an amount of Pt(I1) equivalent to the amount calculated on the basis of the anodic transition time in the potential region +0.4 to f0.6 V-Le., 0.13 m e . We have taken a 60 cmz platinum gauze electrode, potentially cycled it in 0.1M HC104 between 0 and 1.2 V ten times, stopping at f1.2 V in the last cycle. This electrode was then submerged in helium bubbled 0 . 5 HCl, ~ heated for two hours, and the dissolved platinum content determined spectrophotometrica~~yusing the procedure of peters and Lingane (5). The ratio of R(IV) to pt (11) was 4:1 i9 mole Pt(I1) and 2.4 X mole Pt(IV)]. This electrode was then reheated in helium bubbled 0.5M HC1 for hours. Spectrophotometr~c analysis indicated that no further dissolution of platinum was occurring. Cu(I1) (10-6M) was now added to the solution, and the electrode treated in the 0.5M HC1 as above. Spectrophotometric analysis showed an increase of 9 X lo-* mole of Pt (11) and 1.8 X mole of Pt(1V).
Reheating the electrode in 0.5M HC1 produced only a slight increase in the amount of Pt(I1) and Pt(1V) which could be dissolved. Hence, we believe that in the presence of trace Cu(II), our results can be explained by the following reaction Platinum Metal
+ Cu(I1) +.Pt(I1) + Pt(1V) + Cu(1) (adsorbed + solution)
(6)
All the above experiments support the hypothesis that the anomalous oxidation process results from the presence of a monolayer of adsorbed cU(I), which is deposited by potentiastating the platinum electrode at 0.0 V for a time which is a function of the concentration of Cu(I1) present in the 0.5M hydrochloric acid. We find no evidence for the electrochemical oxidation of platinum in 0.5M hydrochloric acid at potentials less than +0.6 V us. SCE. RECEIVED for review January 8, 1968. Accepted March 11, 1968. Work supported in part by a National Science Foundation Faculty Fellowship and the University of Minnesota Space Science Center.
Voltammetry of lodine(1) Chloride, Iodine, and Iodate at Rotated PIatinum Disk and RingDisk Electrodes Piemysl Beran' and Stanley Bruckenstein Department of Chemistry, University of Minnesota, Minneapolis, Minn. 55455
The voltammetry of I + , Iz, I-, and IO3- has been studied using platinum-ring disk electrodes. The I+/& (in acid chloride medium and 12/1- systems are reversible and the diffusion coe ficients ( X 105, cm2/sec) of these species are: IC1 -1.16, l2 -1.05 and I - -1.72 at 25 O C , = 0.5. The oxidation of I - to IC1 is rapidly inhibited by the formation of a film of the electrode surface, while the oxidation of I - to I2 is not. During a large scale electrolysis of an I - solution, in C104- medium, l2 is the first product and then Iz is irreversibly oxidized to IO3-. Using a ring-disk electrode it was demonstrated that oxidation of I- to IO3- at a platinum electrode becomes significant at otentials more positive than +1.15 V v5. SCE in 0.1hfHCI04 (+0.4M NaCIO!). IO3- is reduced to I - at a platinum electrode in acid chloride medium via a catalytic process. I - formed by the highly irreversible reduction of IO3- reacts with IOs- to form 12, which is then electroreduced to I-.
THE ELECTROCHEMICAL BEHAVIOR of the various oxidation states of iodine at platinum electrodes has been the subject of many studies (1-15). The work of Kolthoff and Jordan (2, 3) describes the first systematic study of the anodic behavior of Iz and I- at platinum electrodes. Later workers have verified many experimental results and elaborated considerably on some aspects of Kolthoff and Jordan's work. It is generally agreed that in acid perchlorate and other noncomplexing media, IZ or 13- is reduced to I- in a reversible fashion, under the usual voltammetric conditions (6). The reverse process, the oxidation of I- to Iz or 13- may be complicated by the precipitation of solid Iz at the electrode, as reported by Averbukh et al. (12) and by Toren and Driscoll 1 Present address, Department of Analytical Chemistry, Charles University, Prague.
1044
ANALYTICAL CHEMISTRY
(14). The effect of 13- formation upon the wave equation
for the oxidation of I- is also discussed in the latter work. Anson and Lingane (4) demonstrated that a second anodic process observed by Kolthoff and Jordan in the absence of halide or cyanide ion corresponded to the oxidation to Io3rather than to I+, as first postulated by Kolthoff and Jordan. Anson and Lingane's chronopotentiometric experiments made it possible to distinguish between the mass transfer controlled oxidation to Io3- and the surface oxidation of the platinum electrode, which cannot be done at rotated platinum wire electrodes, as used by Kolthoff and Jordan. Zakharov and Songina (7)also verified that I- can be electrochemically (1) E. Brunner, 2.Physik. Chem., 56, 321 (1906). (2) I. M. Kolthoff and J. Jordan, J. Am. Chem. Soc., 75, 1571 (1953). (3) I. M. Kolthoff and J. Jordan, ANAL.CHEM.,25, 1833 (1953). (4) F. C. Anson and J. J. Lingane, J. Am. Chem. Soc., 79, 1015 (1957). (5) A. I. Beilby and A. L. Crittenden, J . Phys. Chem., 64, 177 (1 960). (6) J. D. Newson and A. C. Riddiford, J . Elecfrochem.Soc., 7 , 699 (1961). (7) V. A. Zakharov and 0. A. Songina, Russ. J . Phys. Chem. English Transl., 36, 649 (1962). (8) S.V. Gorbachev and Yu. A. Korostelin, ibid., 39, 777 (1965). (9) Yu. A. Korostelin and S. V. Gorbachev, ibid., p 942. (10) F. J. Miller and H. E. Zittel, J . Electroanal. Chem., 11, 85 (1 966). (11) N. A. Zakhodyakina, M. A. Novitskii, L. A. Sokolov, and P. D . Lukovtsev, Elektrokhimiya, 1, 138 (1965). (12) A. M. Averbukh, M. A. Novitskii, L. A. Sukolov, and P. D. Lukovtsev, ibid., p 251. (13) A. T. Hubbard, R. A. Osteryoung, and F. C. Anson, ANAL. CHEM. 38,692 (1966). (14) E. C. Toren, Jr. and C. P. Driscoll, ibid., 38, 872 (1966). (15) G. Dryhurst and P. J. Elving, ibid., 39, 606 (1967).
oxidized to IO3- by performing a large scale electrolysis at a platinum electrode.
0
EXPERIMENTAL Chemicals. All chemicals were of reagent grade and triply distilled water was used. Apparatus. The ring-disk electrode was fabricated of platinum and Teflon (made by Du Pont) as described elsewhere (16). The four-electrode potentiostat design previously used (17) was kept, but solid state operational amplifiers were substituted for the vacuum tube types previously used. The parts list given in Figure 1 of Reference (17) was changed as follows: Amplifiers I, A2, and F-2-Analog Devices Model 108; Amplifiers A-1 and CF--Analog Devices Model 108 plus a Philbrick P66A booster; Amplifier F-1-Analog Devices Model 301; Resistors R1, R l ' , R2, R3, R5, R710K, 1 %; R4, R4'-10 turn potentiometer, 0.5K; R C l O K , 1 selected to be less than R7; R8-100 to 100K, 0.1 %; and R9-10 to l K , 0.1%. All potentials are reported cs. the saturated calomel electrode. All experiments were performed at 25.0 "C. The physical dimensions of the platinum ring-platinum disk electrodes used were: Electrode 1, disk radius-0.3874 cm, ring inner radius-0.3949 cm, ring outer radius -0.4054 cm; Electrode 2, disk radius-0.2410 cm, ring inner radius-0.4758 cm, ring outer radius-0.4858 cm. Repolishing of Disk and Ring-Disk Electrodes. The final stage in the fabrication of a disk or a ring-disk electrode is polishing the surface to a mirror-like finish, as has been described elsewhere (16). It has been found that regular repolishing with 0.05-micron alumina on a conventional metallographic polishing apparatus results in an electrode surface which yields highly reproducible results (18). The same electrodes are often used by different workers in our laboratories. Quite frequently, the use of an electrode in different supporting electrolytes, or with various metal ions, results in changes in the electrode characteristics. It is our experience that chemical and electrical treatments are frequently unsatisfactory in restoring the electrode to its original state, while the repolishing technique is always satisfactory. RESULTS AND DISCUSSION Voltammetry and Controlled Potential Electrolysis of the I-/Iz and I,/I Couples. The voltammetry of solutions con-
taining I- and solutions containing Iz in 0.1M + 0.3M H + in the presence and absence of C1- was studied using the rotated disk electrode. The ionic strength of supporting electrolytes was adjusted to 0.5M using sodium perchlorate. In addition the voltammetry of IC1 in acid chloride medium was studied. Figures 1, 2, and 3 give the current potential curves for I-, 12,and I+ in a supporting electrolyte containing 0.1M H+ and 0.1M C1- (curves labelled iD-ED). These curves are in substantial agreement with those reported by Kolthoff and Jordan (2, 3) using a rotating platinum wire electrode, considering the differences in supporting electrolyte. The current potential curves for I- and Iz in 0.1M H+ containing no chloride do not show any wave corresponding to the oxidation to the +1 state, though a peak anodic current is observed at quite positive potentials (see below, Figure 7). (16) J. Albery and S. Bruckenstein, Trans. Faraday Soc., 62, 1920 (1966). (17) D. T. Napp, D. C . Johnson, and S. Bruckenstein, ANAL. CHEM., 39,481 (1967). (18) D. C . Johnson, G. W. Tindall, and D. T. Napp, University of
Minnesota, private communication, 1968.
I-
I-
z
z
U
U
W
W
e
E 3
3
0
0
0 0.8 0.6 0.4 0.2 Electrode Potential (volts)
?.
1.0
(
0
Figure 1. Current-potential curves of 1.33 X 10-3M I - at a ring-disk electrode
+
0.1M HCl 0.4M NaC104 w = 3600 rpm, voltage scan rate = 0.4 V/min Electrode 1. iD-ED = disk current-disk potential curve, iR-ER = ring current-ring potential curve, I'R-ED = ring current-disk potential curve at the specified ring potential (En)
The stability of the +1 oxidation state of I2 in acid chloride media is evident from the second oxidation wave at +0.74 V shown in Figures 1, 2, and 3. The two waves observed at the disk electrode in these figures are of equal height only in IC1 solutions. In the case of I- solutions, the wave corresponding to the oxidation of I- to I2 is considerably larger than the wave corresponding to the oxidation to IC1. The second wave is approximately 60% the height of the first wave and varies with the experimental conditions, but we have never observed equal wave heights for these two waves. A number of other workers ( 2 , 5 , 7-9) have also reported the inequality of waveheights. Ring-disk electrode studies were conducted in the same solutions for which disk electrode current potential curves are given in Figures 1, 2, and 3. Plots of the ring current as a function of disk potential (iR - ED) at selected values of ring potential, ER, are shown in these three figures, as are ring current, ring potential (iR - ER) curves. In all cases, the iR - ED curves exhibit the electrochemical behavior expected of a system which can exist in three soluble oxidation states, Beilby and Crittenden (5) proposed for stationary electrodes that the inequality in the wave heights for the two oxidation waves of I- in acid chloride medium results from the reaction between IC1 within ward diffusing I- to forms lower diffusing I*. This explanation is not inconsistent with our results obtained under the conditions used to obtain Figure 1. In particular, consider the iB - ED curve for VOL. 40, NO. 7, JUNE 1968
1045
.....
L . .
1.2
I
1.0
:
I
1
I
1
0.6 0.4 0.2 0 E lectrode Potential (volts)
0.8
Figure 2. Current-potential curves of 6.7 X lO-‘M Iz at a ring-disk electrode
+
0.1M HCI 0.4M NaCIOr w = 3600 rpm, voltage scan rate = 0.4 V/min Electrode 1. See Figure 1 for key to symbols
12
1.0
I 0.6
I I 0.4 0.2 Electrode Potential (volts) I
0.8
C 3
Figure 3. Current-potential curve of 1.33 X 10-3MIC1 at a ring-disk electrode
which ER = 0.6 V. In the voltage range +0.5 < < 0 no reaction occurs at the disk electrode, and the current at the ring electrode is caused by the oxidation of I- to Iz. When +0.8 < E D < 0.6, I- is oxidized to Iz at the disk electrode, and the anodic current at the ring electrode decreases because of the decreased flux of I- to the ring electrode. The disk electrode shields the ring electrode from a fraction of the I- in solution by preelectrolyzing the portion of solution which contacts the disk electrode. The ring current is given by iR (ER = 0.6 V, ED = 0.7 V) = ( p 2 / 3- miD, where p and N are determined solely by the radius of the disk electrode and the inner and outer radii of the ring electrode (16, 17, 19). When E D > 0.8 V, IC1 is produced at the disk electrode, and if it reached the ring electrode is reduced to 12. This cathodic current would add to the existing anodic current, producing a current more cathodic by NiD. However, exactly the same ring current behavior would be observed if I+ reacted with I- before reaching the ring electrode, because additional I- present in the diffusion layer and the gap between the ring and disk electrodes would be prevented from reaching the ring electrode. Thus the ring electrode would be further shielded by a current equal to NiD, provided the electrode geometry was appropriate (pz’3 2 2N). If some I+ reached the ring electrode, because of either kinetic or geometric considerations, the magnitude of the ring current would be identical to that calculated for the two extreme cases just discussed, Uzi3 2N)iDand the ring current would be cathodic, as is the case in Figure 1.
Because it was not possible to draw any conclusions concerning the rate of reaction of If and I- from the ring-disk experiments, 10-bM solutions of each in 0.1M HC1 were mixed rapidly. They appeared to react as fast when added to each other, lending weight to Beilby and Crittenden’s hypothesis. However, their explanation cannot be applied to the experiments shown in Figures 2 and 3. I+ and I- never coexist in the diffusion layer of an Iz solution, while the two waves obtained for the reduction of I+ are of equal height even though the Beilby and Crittenden hypothesis would predict that the second wave should be smaller than the first wave. The lesser height of the wave which corresponds to the production of IC1 in Figure 1 and 2 is largely the result of the formation of a film on the platinum electrode. Figure 4 shows the effect of reversing the potential scan at various positive potentials in millimolar I- solutions containing 1M Hf and 1M C1-, p = 0.5M. The more positive the potential at which the scan is reversed, the more the wave corresponding to the production of IC1 is distorted on reversed scan, virtually vanishing if the potential is reversed at +1.2 V, If the potential scan is halted in the potential region 0.9 to 1 V, the current decreases considerably, but still exceeds the limiting current for the oxidation of I- to Iz. Napp (20) has shown that the oxidation of a platinum electrode in the potential region 0.8 to 1.0 V is relatively slow in 0.5M HCl, taking several minutes to completely oxidize
(19) S. Bruckenstein, Elektrokhimiya, 2, 1085 (1966).
(20) D. T. Napp, Ph.D Thesis, University of Minnesota, Minneapolis, Minn., 1967.
ED
1046
ANALYTICAL CHEMISTRY
O.lMHCl+ 0.4MNaC10a w = 3600 rpm, voltage Scan rate = 0.4 Vjmin Electrode 1. See Figure 1 for key to Figures
the electrode surface. In addition, little soluble platinum is formed in this potential region. Reduction of such an oxidized electrode yields a current-potential curve similar to that which is obtained in acid perchlorate or sulfate medium, strongly suggesting that the oxidized platinum surface is similar in all three media--i.e., R(OH)2, PtO, or platinumadsorbed oxygen. It seems probable that the decrease in the current caused by the oxidation of I- to I+ results from the formation of this oxidized platinum surface. The possibility of the formation of an insoluble platinum-iodine species is not ruled out. However, a controlled potential electrolysis at +0.9 V, using a massive platinum electrode, demonstrated that no soluble platinum species was formed under the conditions corresponding to Figure 1. In a millimolar I- solution less than 10-7M soluble platinum was detected after exhaustive electrolysis. Table I lists half wave potentials and slopes of the appropriate logarithmic plots used to test for reversibility for the disk electrode experiments shown in Figures 1, 2, and 3, and analogous experiments varying [H+] and [Cl-1. The specific logarithmic plots used are indicated in Table I. In I+ solutions the reduction of I+ to IP is reversible. The shift in half wave potential as a function of [Cl-] indicates that the value of n in 21C1n('+1)2e + Iz 2nC1is 1.8--i.e., there is a substantial amount of IClz- present. Considering the shift in half wave potential caused by changing [H+], approximately 0.5 H+ is also involved in the electrode process. Hence, in the media studied HIClz and IC12- are present in comparable amounts, along with a small quantity of IC1. The wave corresponding to the reduction of IZ to I- is reversible, except in the most acid solution. In I- solution containing chloride ion, the oxidation wave corresponding to the formation of Iz is reversible; the wave
0
a
I
I IO0
u?
e
200
U
2 .I 0
300
+
+
- ., , ,.,,,.,, .,.. ,,.. .... . .
400 I.2
0.4
0.8
0.0
Volts Figure 4. Effect of anodic potential limit on the currentpotential curve of I- in 0.1M HCl [I-]= 5 X 1 0 - P M
1050 rpm, disk electrode area = 0.39 cm2, scan rate = 0.5 Vjmin. residual i - E curve; i - E curve scanning to f;- - - - i Ecurvescanning to w =
...
+
-
-
Table I. Reversibility of Various Iodine Couples" Depolarizer in soln
Ell2 CE'
Cc1-
I2
o. 1
...
I-
0.1
I+
0.1
I2
0.1
0.1 0.1
I- 2 12
I2
* I+
12
0.49
0.033
0.49
0.031
0.495 0.49
0.780
0.780
E plotted us. log
Slope
I+
0.032 0.031
I2
I+
* I*
I-
i2 id - i (id - ')i i i2 id - i
0.032 0.028b
i2 id - i
- i)2
I-
0.1
0.1
0.49
0.790
0.031
0.026b
(id
I+
0.1
0.5
0.486
0,705
0.032
0.032
i2 -
I2
i
-
(id i)z id i -
-
i2 id - i __
i
-i
id I2
0.1
0.5
0.49
0.705
0.031
0.0306
i2 -
-i (id - i)2
i2 i
(id id
0.1
0.5
0.48
0.726
0.032
0.029~
I+
0.3
0.1
0.48
0.766
0.040
0.032
I2
0.3
0.1
0.47
0.760
0.033
0.029d
i i2 id - i i2 ~
id
-i
-i i2
-i iz i id
(id
- i)l
id - i i2
id - i ___ i i2 a Determined using a rotating platinum disk electrode and millimolar I- or IC1 or 0.5 millimolar 12 in medium made to 0.5M ionic strength using NaC104. T = 25 "C. Reference electrode = SCE. Line deviates from linearity when E > -0.78 V, and the slope increases. c Line deviates from linearity when E > -0.73 V, and the slope increases. dLine deviates from linearity when E > -0.75 V, and the slope increases.
I-
0.3
0.1
0.48
0.770
0.032
0.028
(id
- ')i
- i)2
~
id
I-
* I+
VOL 40, NO. 7,JUNE 1968
1047
Table 11. Diffusion Coefficientsof I-, 12, and I + at 25 “C 01 = 0.5) Supporting Diffusion coefficienta X IO6,cm*/sec of II2 I+ electrolyte 0.1M HClO4 0.4MNaC104 1.73 f 0.03 1.04 i 0.03 ... 0.1MHCl 0.4MNaC104 1.72 & 0.03 1.07 f 0.03 1.16 f 0.03 a Determined using a rotating platinum disk electrode of 0.36 cm2area.
8oot L
+
800
+
600
p.
E400 0
200
is apparent for the greater discrepancy found in the case of I- solutions. Table I1 gives diffusion coefficients for I-, 11, and I+ in various media, as calculated from the Levich equation in those solutions in which the limiting current is proportional to the square root of the angular velocity. Controlled Potential Electrolysis of Iodide Solutions. Controlled potential electrolyses of millimolar I- dissolved in 0.1M H+ 0.1M chloride ( p = 0.5 with sodium perchlorate) were performed using a large area platinum electrode at +0.9, +0.7, and +0.6 V. A 5 X 10-4M solution of IZ in the same supporting electrolyte was electrolyzed at 0.9 V. A plot of log i us. time for these electrolyses is given in Figure 6. The plot for the If solution is a straight line as predicted by theory (21). A straight line is obtained for the electrolysis of I- at f0.6 V and +0.7 V. The electrolysis at +0.9 V of the I- solution yields a curve which has two straight line portions, the slope in the early stages of electrolysis corresponding to the one obtained at +0.6 V (and $0.7 V) in I- solution. The limiting slope obtained at longer times corresponds to the slope obtained when an I2 solution is electrolyzed at +0.9 V. The mechanism for the large scale electrochemical oxidation of I- to I+ at +0.9 V evidently proceeds in two steps, the first involving the oxidation of 1to I2 until most of the I- has been oxidized, followed by the oxidation of I2 to I+. The direct oxidation of I- to I+, followed by the diffusion of I+ into the bulk of the solution where it reacts with excess I- to form 1 2 , is not a significant pathway in the large scale electrochemical oxidation of I-. This is readily seen by considering the different time scales of the electrochemical processes occurring at a disk and at a large area platinum electrode. The direct oxidation of I- to form If at a disk electrode is rapid until the formation of the film decreases the rate of this process. Under the conditions of the controlled potential electrolysis at a large area platinum electrode, a film is present for nearly all of the electrolysis time. The time scale in Figure 6 is such that the details of the current variation early in the electrolysis for the curve representing the oxidation of I- at +0.9 V are not apparent. However, during the first few minutes of electrolysis, the observed currents are considerably higher than those which would be extrapolated from the first linear region of the log current-time curve. Initially, the direct oxidation of I- to I+ does occur to a significant extent, but the formation of the film on the platinum surface reduces the rate of this process to a small value without appreciably affecting the oxidation of I- to 1 2 . The electrochemical production of I+ becomes significant only when the Iz concentration becomes much larger than the concentration of I- in solution. Further evidence for the existence of a film on a platinum
+
+
20
40 60 VAngdar Velocity, RPM
Figure 5. Angular velocity dependence of convective-diffusion currents in 0.1M HCI 0.4M NaCIO4 Solution: 1.33 MI-, 0 I- = 1/212 + e ; 0 1/212 = I + e. Solution: 0.67 MI2; H I2 = l/2 I2 e; 01/212 = I + e. Solution: 1.33 X M I + ; both waves superimposeon M.
+ +
+
+
+ +
corresponding to the formation of I + is also reversible, provided the disk electrode potential does not become more positive than 0.78 V. At this potential the current is only 10% of the limiting current for this process. At more positive potentials the slope of the logarithmic plot becomes much higher. Presumably the formation of an oxide film on the platinum surface at more positive potentials interferes with the oxidation to I+, making the process irreversible. The shift in as a function of [Cl-] is similar to that found in I + solutions. In acid Is solution containing chloride ion, reduction of Iz to I- is reversible, while the oxidation wave to form I+ had characteristics similar to that observed in I- solutions. Again, the wave analysis was reversible, provided that the potential did not exceed $0.78 V, at which potential the oxidation current was 30% of the limiting current for this process. The shift in E l l 2 as a function of [Cl-] is similar to that found in I+ solutions. The presence of chloride ion has no effect upon the halfwave potential or the reversibility of the 141- system. Disk electrode experiments similar to those shown in Figures 1, 2, and 3 were repeated at different rotation speeds. The limiting currents for various waves are plotted in Figure 5. In all but two cases limiting currents were proportional to the square root of the angular velocity, w , as predicted by the Levich equation for a convective-diffusion limited process. In the case of the limiting currents corresponding to the formation of I+ in I- and in I2solution, smaller currents than predicted by the Levich equation are observed at higher angular velocities. This effect is far more pronounced in solutions containing I- than in solutions containing 12. These deviations from the Levich equation are in agreement with irreversibility arising from fiIrn formation, but no reason 1048
ANALYTICAL CHEMISTRY
(21) J. J. Lingane, “Electroanalytical Chemistry,” 2nd Ed., Interscience, New York, 1958, p 227.
I
-
+
0
+
DISC
100
200
300
Time- minutes Figure 6.
Controlled potential electrolysis
+
Supporting electrolyte - 0.1M HCI 0.4M NaCIOa. 10-3MI - at 0.9 V -e-, at 0.7V -A-, at 0.6 V -C+; 5 X 10-4MIZat 0.9 V -O-. , -A- and -0- upper current scale, -0- and -0lower current scale
electrode is given by the ratio of the slopes of the lines in Figure 6 corresponding to the oxidation of I- to IQand IZ to I +. These lines represent oxidation processes involving one electron and, neglecting any differences in the diffusion coefficients of I- and IZ, should have equal slope. However, the ratio of the slopes is 3.5/1, indicating that the oxidation of I2 to 1' occurs at a much slower rate than the oxidation of I- to 12. Voltammetric Studies of the Oxidation of I- to Io%-. The chronopotentiometric studies of Anson and Lingane established that I- could be oxidized to IO3- in acid medium free of C1- and that this oxidation occurs simultaneously with the oxidation of the platinum electrode. Attempts to study the oxidation of I- to IO3- by means of voltammetry at a platinum disk electrode failed, as it was impossible to obtain a well defined anodic limiting current region for this process. Just before the evolution of oxygen, peak anodic currents could be observed. Similar peak currents have been reported by other workers ( 2 , 4 , 7). The ratio of these anodic current peaks to the limiting current for the oxidation of I- to I2 varied considerably with potential scan rate, verifying that the oxidation of the platinum electrode was inhibiting the oxidation of I- to IO3-. Hence, it was decided to study this process using the ring-disk electrode and thereby avoid the complications introduced by the oxidation of the platinum electrode. We shall treat the general case in which I- is oxidized first to Iz at a disk electrode and then to I+%at more positive potentials. Furthermore, we assume that as the I+n species is transferred by convective diffusion to the ring electrode, it reacts quantitatively with excess I- in solution according to
-
T
\RING
I
0
+ 2.0
1.6
1.2 0.8 DISC Volts
0.4 0.0
Figure 7. Disk current and ring current us. disk potential curve. 10-3MI-in 1MHClOl w = 400 rpm, potential sweep = 10 V/mh Upper current scale = disk current, lower current scale = ring current at a ring electrode potential of 0.0 V. Electrode 2
I+n
+ nI-
= '/z (n
+ 1)Iz
Setting the ring electrode potential such that Iz is reduced to I-, we sweep the disk electrode potential in a positive direction, starting at 0 V, and record the iR-EDand iD-& curves. In the potential region where I- is oxidized to Iz at the disk electrode (0.6 V to 1.1 V) with a current (iD)r-, the current at the ring electrode, (ZR)z2, should equal N(iD)z-,where N is the collection efficiency of the ring-disk electrode (16). In the potential region where I- is oxidized to I+n at the disk electrode with a current ( i R ) z + n , the current at the ring electrode, (iR)z+n, will be (n l)N(iD)z+n. Note that (iD)z+n cannot be measured directly if electro-oxidation of the disk electrode surface and/or evolution of oxygen is occurring simultaneously with the production of I+n,as is the case when IO3- is formed. The ratio, (iR)l+n/(iR)z2= n 1, is diagnostic of the oxidation number the I+%pecies. The above method takes advantage of the reaction between I f n and I- to determine n from currents measured at the ring electrode and completely avoids the problems caused by other simultaneous electrode processes at the disk electrode. It should be noted that this method would still have to be used, even if disk electrode oxidation and evolution of oxygen were not occurring simultaneously. As Anson and Lingane (4) have pointed out, the limiting current for the oxidation of Ito Io3-will be less than six times the limiting current for the oxidation of I- to I?. As Io3- forms it will react with incoming I- to form IZ,some of which will diffuse toward the
+
+
VOL. 40, NO. 7, JUNE 1968
1049
800
600
$00
f
i?
0
2
s
10
Seconds
200
Figure 9. Effect of Iz on the inhibition time of the 103- reduction current. c I o 3 - = 2.00
0.6
0.4 Volts
0.2
10-3~4,cHclol = 1 . 0 ~ 4w,
=
400
0,O
Figure 8. Reduction of 103- in acid chloride medium Rotating platinum disk electrode. A = 0.36 cm*. w = 400 rpm. Voltage scan rate = 0.4 V/min. Solid line = to - scan, dashed line = - to scan (1) O.2M HCl 0.3M NaCl; (2) (1) 2 x 1 0 - 4 ~K I O ~ ;(3) (1) 4 x 10-4.44 ~ 1 0 (4 ~ ; (1) 6 X 10-4M KI08; (5) (1) 8 X lO-4M K I 0 3 ; (6) (1) 1 X 10-3MKIOI
+
+
+
+
x
rPm Curve A-2.5 X 10 -5Madd 12. Potential step from +0.8 V to +0.2 V, Curve B-5 X 10-6M added 12, Curve C-no added In
0
+
+
+
+
electrode and be reduced, while some Iz will diffuse into solution and not contribute toward the Io3-1imiting current. Figure 7 shows the in-ED and the iR-& curves obtained in millimolar I- solution in 1M perchloric acid as supporting electrolyte, if the ring potential is set on the limiting current for the reduction of Iz,0.0 V. The first wave observed on the iR-EDcurve results from the reduction of Iz formed at the disk electrode. The positive shift for the half wave potential of this process on the ring electrode as compared to the disk electrode half wave potential is the result of'the adsorption of Iz on the disk electrode and the rapid voltage scan used in this experiment (10 V/minute). A finite amount of I2 must be produced at the disk electrode before its surface is saturated with adsorbed IZand Izcan escape into solution. The second wave in the 1'R-ED curve shows a maximum at 1.3 V and then becomes constant. The ratio of the two limiting ring currents is 6, demonstrating the formation of Io3- at the disk electrode at potentials more positive than -1.2 V. The ring current maximum a t 1.3 V probably arises from the oxidation of adsorbed Iz at the disk electrode t o IO3-. The ratio of ring currents at 1.6 V and 0.9 V falls below 6 if the speed of rotation of the disk electrode is greater than 400 rpm because the rate of the IOa--I- reaction becomes the controlling factor (22, 23). For example, if the rate of rotation is 8100 rpm, this ratio is approximately 3.3. 1050
5
0
0
ANALYTICAL CHEMISTRY
In a separate experiment, a lO-3M solution of I- in 0.10M perchloric acid was exhaustively electrolyzed using a large area platinum electrode at a potential of +1.15 V. The solution first took on the color of Iz and then became colorless. Addition of I- produced 12. It has been reported that I o 3 can be produced by electrochemical oxidation of I- (7). The Voltammetry of Iodate Ion. Current potential curves obtained for IO3- in acid chloride medium having a total ionic strength (p) of 0.5M is shown in Figure 8. No evidence for the oxidation of IO3- is observed, while the half wave potential for the reduction of IO3- is markedly dependent upon the concentration of Io3- and the direction of the voltage scan. The limiting currents become proportional to concentration when the IO3- concentration exceeds 0.2mM. The behavior exhibited by the current-potential curves in Figure 8 can be interpreted in terms of the catalytic reduction of Io3-uia the following sequence of reactions.
+ 6H+ + 6e -,I- + 6 H 2 0 51- + 103- + 6H+ + 3H20 Iz + 2e -, 21IO3-
slow
+ 12
fast
fast
(1) (2) (3)
As the electrode potential is made more negative in a solution of Io3-,the electrochemical reaction given by Equation 1 begins to proceed in a highly irreversible fashion. As soon as any I- forms at the electrode surface, it reacts extremely rapidly with I- according t o Equation 2, and the I p formed in this reaction is rapidly reduced to I- by the electrochemical process given in Equation 3. The 1.- produced as a result of this process reacts with more Io3-,and the catalytic electroreduction of IO3- occurs. As has been shown elsewhere (23), the rate law governing Reaction 2 in acid chloride media at p = 0.5 is (22) 0. E. Myers and J. W. Kennedy, J . Am. Cliem. SOC.,7 2 , 897 (1950).
(23) P. Beran and S . Bruckenstein, unpublished data, 1968.
dt
=
k[H+]3[I03-] [I-] [Cl-1
where k = 1.7 x 109 mole6liter-5 sec-l. Hence, it is apparent that the catalytic reduction of Io3- increases with 103- concentration. The current-potential curves in Figure 8 were obtained by scanning the disk electrode potential from f0.6 V to 0 V (+ to -)and then from 0 to +0.6 V (- to +). The more positive half wave potentials observed during the - to potential scan as compared to the half wave potential observed during the initial to - potential scan is consistent with the catalytic reduction mechanism. Once the limiting current region for the catalytic iodate reduction has been achieved, the electrochemical reduction of Io3- proceeds through Reactions 2 and 3 , and the surface concentration of I- is independent of Reaction 1. On scanning the potential of the elecReactions 2 and 3 maintain the current trode from - to through the disk electrode until the half wave potential for Reaction 3 is approached. The current ceases when the potential becomes too positive for a significant surface concentration of I- to be produced cia Reaction 3 . Other workers (24-28) have studied the voltammetry and
+
+
+,
~
~~~
(24) I. Shain, ANAL.CHEM.,26, 281 (1954). (25) F. C. Anson,J. Am. Chem. Soc., 81, 1554(1959). (26) Z. B. Rozhdestvenskaya and 0. A. Songina, J . Anal. Chem. USSR English Transl. 15, 155 (1960). (27) J. Badoz-Lambling and C. Guillaume, “Advances in Polarography,” Vol 1, Academic Press, New York, 1960, p 299. (28) P. G. Desideri, J . Electroanal. Chem., 9,218 (1965).
chronopotentiometry of the IO3- reduction in other acid media. The reduction of Io3- cia the reduction catalytically produced Izseems generally accepted (25-28). As a further test of the catalytic mechanism for the reduction of IO3-, an experiment was performed in which the potential of a disk electrode was abruptly switched from +0.8 to +0.2 V in a 2 x 10-3M solution of Io3-in 1 M perchloric acid, in the absence and in the presence of two concentrations of I-. The results of this experiment are shown in Figure 9. In the absence of added I-, the current through the disk electrode remains essentially constant until approximately five seconds after changing the potential, and then abruptly rises to its steady state value. The small amount of over shoot shown in the figure results from the characteristics of the X-Y recorder used. A delay of 2 seconds in reaching the steady state current occurs in a solution containing 10-jM added I-. No delay is observed when the added I- concentration is 5 X 10-jM. These results are in agreement with the catalytic mechanism of the reduction of iodate. Iproduced by Reaction 1 is strongly adsorbed at the platinum disk electrode, and virtually no I- escapes from the electrode surface until it is virtually saturated with I-. At this time the catalytic mechanism begins to function. The presence of added Iz hastens the saturation of the electrode surface with I-. RECEIVED for review January 2, 1968. Accepted March 20, 1968. This work was supported by funds granted to the Space Science Center of the University of Minnesota by NASA.
A Ring-Disk Electrode Study of the Electrochemical Reduction of Copper(l1) in 0.2M Sulfuric Acid on Platinum G . W. Tindall and Stanley Bruckenstein Department of Chemistry, Unifiersityof Minnesota, Minneapolis, Minn. 55455 Current-potential curves of Cu(ll) in 0.2M H,SO, at a platinum disk electrode show three reduction processes during the cathodic potential scan and three oxidation processes during the anodic potential scan. Two of the reduction (and oxidation) processes are associated with the formation (and removal) of a monolayer of zero-valent copper in contact with the platinum surface. At potentials more negative than 0 V, Cu(ll) is reduced to a bulk copper metal deposit with the concurrent production of some Cu(l). Oxidation of the bulk copper deposit produces primarily Cu(ll) and some Cu(l). Bulk copper cannot be deposited until the platinum surface is covered with zerovalent copper.
IN THE COURSE OF AN INVESTIGATION of the anodic stripping analysis of Cu(I1) at a rotating disk electrode, current-potential curves of lO-;M Cu(I1) solutions were determined in 0.2M sulfuric acid. In this supporting electrolyte Cu(I1) is reduced at three distinct potentials. The deposited copper is oxidized from the electrode at three distinct potentials by a linear anodic potential scan. Vassos and Mark ( I ) found that when copper is deposited on pyrolytic graphite from perchloric acid and stripped using (1) B. H. Vassos and H. B. Mark, J . Electroanal. Chem., 62, 1 (1967).
a linear potential scan, copper is removed at three distinct potentials. They made no attempt to study the reduction process, and concluded that the two extra dissolution steps are caused by the removal of a uniform monolayer of copper in contact with two different electrode sites. Breiter ( 2 ) studied the plating and stripping of copper from an oxidized platinum wire electrode. He found that copper strips from such an electrode over three distinct potential regions, and concluded that thick copper patches, thin copper patches, and adsorbed copper metal were the three species being stripped. He reported only one reduction wave during the plating of copper. This paper describes a study of the deposition of Cu(I1) on a reduced platinum electrode from 0.2M sulfuric acid, and the electrochemical oxidation of the deposited copper. EXPERIMENTAL
The instrument design for the simultaneous and independent potentiostatic control of the ring and disk electrodes is described elsewhere (3). The cell, high speed rotator, and (2) M. W. Breiter, J . Electrochem. Sac., 114, 1125 (1967). (3) D. T. Napp, D. C. Johnson, and S. Bruckenstein, ANAL. CHEM., 39, 481 (1967). VOL. 40, NO. 7, JUNE 1968
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