PREMYSL BERANAND STANLEY BRUCKENSTEIN
3630
A Rotating Disk Electrode Study of the Catalytic Wave Produced by the Reduction of Iodine in the Presence of Iodate by Premysl Beranla and Stanley Bruckensteinlb School of Chemistry, University of Minnesota, Minneapolis, Minnesota 65466 (Received April 89, 1968)
The catalytic reduction of iodate at a rotating platinum disk electrode has been employed to study the kinetics of the reaction between iodate and iodide. In acid perchlorate media, trace iodine was added to the iodate solution, and the electroreduction of iodine to iodide initiated the catalytic reduction of iodate. A closedform solution for the catalytic current is obtained by using the method of moments. In acid chloride media, iodine monochloride was added to initiate the catalytic process. In perchlorate media, dCI,/dt = (4 X 106)CEt2CIOsCI- (5.1 X 109)CHtzCIQs-CI-2, while in chloride media dC,/dt = (1.7 X 108)C~+aCIos-C,-C,,-.
+
Conventional chemical studies of reaction 2 have Introduction been made7-15 and a radiochemical study of the exOne of the potentially important applications of change rate of Iz and IO3- which bears on the 1--103voltammetry is the study of catalytic reactions. The reaction was reported by Myers and Kennedy. l6 dropping mercury electrode has been used for a conThe various rate laws and mechanisms proposed by siderable number of catalytic studies,2 but no applicathese workers have been discussed by Morgan, Peard, tion of the rotating disk electrode has been reported. and Cullisjl’ who proposed their own mechanism. In A theoretical treatment of one type of catalytic wave the absence of C1-, the rate of reaction of I- with IOahas been given by Kouteckg and Levichj3 but only obeys the rate law qualitative studies are reported in the l i t e r a t ~ r e ~ - ~ at platinum electrodes. I n general, only electrode configurations for which mathematical statements of the chemical and convecwhere kl = 400 L 3 mol-3 sec-l and k2 = 1.3 X loQla4 tive-diffusion problem can be made are subject to quantitative interpretation. We have employed a rotating platinum disk electrode to study the kinetics of the reaction of I- and 1 0 3 - in acid perchlorate and (1) (a) Department of Analytical Chemistry, Charles University, Prague, Czechoslovakia. (b) Address all inquiries concerning this acid chloride medium. Under these conditions, 1 0 3 work to this author at the Department of Chemistry, State Univeris not reduced until potentials very close to hydrogen sity of New York at Buffalo, Buffalo, N. Y. 14214. evolution are reached, provided the electrode speed ex(2) Summary of these studies: J. Heyrovskjr and J. Kuta, “Principles of Polarography,” Publishing House of the Czechoslovak ceeds -900 rpm. Academy of Sciences, Prague, 1965, pp 381-392. I n acid perchlorate solution, we add a small amount (3) (a) J. Koutecki. and V. G. Levich, Zh. Fiz. Chem., 32,1565 (1958). of IQto IO3- solution. Since 1, is reversibly reduced, (b) J. Koutecki. and V. G. Levich, Dokl. Akad. Nauk SSSR, 117, 441 (1957). reaction 1 occurs at the electrode surface (4) J. Badoz-Lambling and C. Guillaume, Advan. Polarogr., Proc. Int. Congr. grid, Cambridge, Engl. 1969, 1 , 299 (1960). Iz 2e -+-21(1)
+
and the resultant I- reacts with 1 0 3 - in the hydrodynamic diffusion layer via reaction 2. The 1 2 produced by this reaction sequence 511036H+ +31% 3Hz0 (2)
+
+
+
gives rise to a catalytic current, which can be related to the kinetics of reaction 2. I n acid chloride medium we add a small amount of IC1 to the IOa- solution. The IC1 is reduced in two reversible steps, first by 21+ 2e -+ 1 2 and second by reaction 1, The half-wave potential of the catalytic IO3- wave observed in the presence of IC1 corresponds to that for reaction 1, showing that the over-all reaction scheme in acid chloride media is still described by reactions 1 and 2.
+
The Journal of Physical Chemistry
(5) Z. B. Rozhdestvenskaya and 0. A. Songina, J . Anal. Chem. USSR, 15, 155 (1960). (6) P. G. Desideri, J . Electroanal. Chem., 9, 218 (1965). (7) A. Skrabal, Z. Elektrochem., 30, 109 (1924). (8) A. Skrabal, ibid., 40, 232 (1934). (9) W. C. Bray, J . Amer. Chem. SOC.,52, 3580 (1930). (10) E. Abel and K. Hilferding, Z. Phus. Chem. (Leipzig), A135, 186 (1928). (11) E. Abel and F. Stadler, ibid., A122,49 (1926). (12) S . Dushman, J . Phus. Chem., 8, 453 (1904). (13) J. Sigalla, J . Chim. Phys., 55, 758 (1958). (14) hf. Wronska and B. Banas, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 13, 5 (1965). (15) M .Wronska and B. Banas, Rocz. Chem., 39, 1745 (1965). (16) 0 . E. Myers and J. W. Kennedy, J. Amer. Chem. SOC.,72, 897 (1950). (17) K. J. Morgan, M. G. Peard, and C. F. Cullis, J. Chem. Soo., 1865 (1951).
A ROTATING DISK ELECTRODE STUDY
3631
sec-' a t 25018and C represents the concentration of the species indicated by the subscript. Morgan, et al., propose a mechanism starting with the basic dissociation of iodic acid, according to the equilibrium
H+
+ 103-
IOz+
IOz+ reacts according to IOz+
+ I--
+ OH-
-
I02+** * I I
(4)
(5)
and the intermediate produced may react according to
2IO+ + IO-
I02+*' *I-
(6)
Theoretical Section
(7)
Consider a solution which contains H+, IOa-, and small amounts of 1 2 . On applying a potential sufficient to reduce I, to I-, we induce the reduction of 1 0 3 - via the scheme given by eq 1 and 2. The convective-diffusion equations for a rotating disk electrode at hydrodynamic equilibrium in a solution of I-, IO3-, and H+ containing no C1-, after a steady-state concentration distribution has been obtained, is given by eq 13-15, where vu, the velocity of
or according to I02+**
-1-
+ II+ klI +
IOz+
+ 210-
which drove the spindle holding the electrode via a pulley arrangement. The three-electrode polarograph employed in this work makes use of the disk electrode circuit of a fourelectrode polarograph described elsewhere.z o , 2 1 All voltages are reported us. the saturated calomel electrode. All experiments were performed at 25". Catalytic currents were measured at 0.2 V for angular velocities ranging from 400 to 8100 rpm at various conM centrations of IO3- in the presence of to 12, or I + if C1- was present. The ionic strength was maintained constant at 0.5 M using sodium perchlorate.
II1 The other reactions are
IO+ IO-
+ I - ka_ I+ + IO+ H+ A I + + OH-
+ I- kie 1 2 H+ + OH- 5HzO I+
dCIz dzCIz = DI, dY dY klCH tzCIOs-CI- f +
0 -,
(8)
(9) (10)
-UV
Experimental Section Chemicals. Reagent grade chemicals and triply distilled water were used to prepare all solutions. I + solutions were prepared by mixing equivalent amounts of KI03 and K I in 4 M HC1. Apparatus. The rotating disk electrode was fabricated by silver-soldering 0.060-in. platinum sheet to a stainless steel shaft, turning to size, and press fitting into a predrilled Teflon cylinder, as described elsewhere.lg The surface of the electrode was polished using conventional metallographic techniques, finishing with 0.05-p alumina. The rotator was powered by a synchronous motor,
dzCIOsdCIo,=D I O a - ydY dY 1/3klCH t~C1os-CI - - /3kZCH +zcIOs-cI
-' (14)
(11)
Morgan, et al., obtain an expression of the same form as the observed rate law. No systematic study has been reported for the reaction of I- with IO3- in acid chloride medium. Wronska and Banas14 reported that C1- produces a positive salt effect. A.s shown below, the effect of C1- is to change the rate law to the form
-' (13)
k 2 C H t2CIOa-CI
vu-
dC1- - D1--d2CI- dY dYa 5/3klCHtzCIOa-CI-
- 5/3k2CH+zCIOs-CI-'
(15)
the supporting electrolyte normal to the rotating disk electrode, is given by =
0.51~'/2~-~'2 Y2
(16)
where o is the angular velocity of rotation, D represents the diffusion coefficient of the subscripted species, and v is the kinematic viscosity of the solution. In the limiting-current region, the surface concentration of iodine is zero CSI*= 0
(y = 0)
(17)
Points in the solution distant from the electrode will have the bulk concentration, Cb,of the various species, i.e (eq 18-20) (18) Myers and Kennedy16 have collected and interpreted earlier results. (19)'W. J. Albery and S . Bruckenstein. Trans. Faradav SOC.,62, 1920 (1966). (20) D. T. Napp, D. C. Johnson, and 8. Bruckenstein, Anal. Chem., 39, 481 (1967). (21) P. Beran and S . Bruckenstein, ibid., 40, 1044 (1968).
Volume 78, Number 10 October 1968
PREMYSL BERANAND STANLEY BRUCKENSTEIN
3632
CIz(Q),t) = CbIz
CIOa-(m, t )
CbIOac I- = 0
=
C,-(Q), t ) =
b
(18) (19) (20)
Hydrogen ion is present in large excess and is assumed to be constant at all points in the solution. The exact solution to this problem is difficult and would be extremely complex. However, a solution corresponding to experimentally realizable conditions can be obtained using the method of moments.22 The details of this solution are given in the Appendix and the solution applies to experimental situations where the catalytic current is a relatively small fraction of the convective-diffusion current for the reduction of IOa-. The final result is of the form
R3
+ AR2 - 1 = 0
(21)
I
where
Electrode Potential
R = i,/i
(22)
In eq 22 i represents the convective-diffusion current for the reduction of I2 (added as catalyst) in the absence of IO3-, and i, represents the experimental catalytic current, which is defined as the total current actually observed on the limiting-current region in the presence of both 1 2 and Io3-. Note that this definition of the catalytic current is somewhat different from that customary in polarography in which any contributions arising as a result from the reduction of 1 2 would be subtracted from the total current. The constant A has the value
A =
(DI~/DI -ICE
t2CbIOa-
0.92w(D1,/?)"a
X
Equation 23 may be rearranged to give
A'
=
0.307w(D1/' 2 7 - )l'aA (DIz/DI - ) C H +'CbIOa-
in which A' is predicted to be a linear function of C b ~atz a constant speed of rotation. Using eq 21 and 22, we calculated the value of A from the experimental values of i, and i for the particular values of CH+and Cbloaand w . A' was then calculated and plotted to obtain kl and kz. I n chloride media the rate law is given by eq 12 and i.e., k2 = 0. EXcontains no term involving cbIZ2, amination of the details given in the Appendix leads immediately to the result that the solution is of the form R3 A"R2 - 1 = 0, where
+
Figure 1. Catalytic iodide wave ( m = 400 rpm; scan rate, 0.4 V/min; scan to - ; platinum disk electrode area, 0.39 cmz): (1) 0.1 M HC1, 0.4 M NaClO4, 1.8 X M IC1; (2) 0.1 M HC1, 0.4 M NaC104, 1.8 X M ICl, 2 X 10-3 M KIOa; (3) 0.1 M HClO4, 0.4 M NaClO4, 6.6 X 10-4 M 1%;(4)0.1 iM HC104, 0.4 M NaC104, 6.6 X 10-4 M Iz,6 X M Moa. Initial and final potentials are indicated.
+
and i in eq 22 is the current difference between the two limiting-current regions observed in the reduction of IC1, i.e., only that portion of the current which corresponds to eq 1.
Results and Discussion The current-potential curves for the catalytic reduction of IO3- in the presence of I2 or IC1 are given in Figure 1 along with the current-potential curves for I2 and IC1 in the absence of IO3-. It is apparent that a well-defined limiting current is obtained at potentials corresponding to the reduction of iodine. Catalytic currents were measured at 0.2 V. Initially, in order to verify the reported rate laws for Table 1: Reaction Orders from Logarithmic Plots of Catalytic Current ( f i = 0.5, NaC104 added) Species concn varied
1% (or I + ) 103-
H+
c1-
----SlopeaN o C1-
1.48 1.14 1.96
c1-
0.98 0.87 2.95 0.92
a Concentrations of other species not, varied were Cat = 0.1 M , CbIoa- = 10-3 M , CbI, = 10-4 M , Cbr+ = 5 X M, and CbCI- = 0.05 M .
(22) S. Bruckenstein and 5. Prager, Ana2. Chem., 39, 1161 (1967).
The Journal of Physical Chemistry
3633
A ROTATING DISKELECTRODE STUDY Table I1 : Summary of Catalytic Studies in Chloride Media Angular velocity range, rpm
900-4900 400-4900 400-3600 400-4900
0,100 0.100 0.0500 0.100
1.00 1.00 1.00 1.00
Figure 2. The effect of CH+on catalytic current (W = 2500 rpm, rotating platinum disk electrode area, 0.36 cmz): 0,2.2 x M KIOs; 3.3 X M 12; p = 0.5 by addition of NaC104; slope, 1.96. 0, 10-3 M KIOs; 7.5 X 10-6 M ICI; 0.05 M NaC1; p = 0.5 by addition of NaC104; slope, 2.95.
the reaction of iodate with iodide ion in the absence and presence of chloride, plots of log io vs. log C, where C is the concentration of hydrogen, iodate, iodide, or chloride ion, were made holding the concentration of the other species constant. The slopes of such lines represent the order of the chemical reaction with respect to the ion whose concentration is being varied. Two such plots are shown in Figure 2, where the hydrogen ion concentration is varied. The slope is 2 when no C1- is present and 3 when 0.1 M C1- is present. Table I summarizes the slopes obtained with such plots for solutions of the specified concentrations. The reaction orders obtained in Cl--free solution are in agreement with the literature. The nonintegral value for I- in the absence of C1- arises because of the two parallel mechanisms involving CI- to the first power in one case and in the other case to the second power. Only one previous study of this reaction in C1media has been reported. Wronska and Banas16found the same orders as we do for H+ and IO3- in the pH
0.050 0.100 0 0100 0.0250 I
0.75-3.8 0.75-7.5 1.50-15.0 1 50-15.0 I
1.1zk 0.3 1 . 5 -I: 0 . 3 2 . 6 -I: 1.5 1.7 -I: 0.5
range 4-6. They also found that C1- increased the reaction rate but ascribed this behavior to a primary salt effect. They reported that the rate depends upon CI-2. As is shown in Table I and also in Table 11, our data are fit by a first-order dependence on CI-. Some of the results obtained in C1--containing media are summarized in Table 11, which gives values of k for , Cbl+, and Cb1o8-. It different values of C b ~ +Cbcl-, was found that IC was constant, provided the assumption that Cbloa- - CS1oa- 0 ) (28)
Figure 3. Plot of A'
VS.
where 6 and 6, represent the thickness of this layer in the presence and absence of I03-. The thickness of this layer is the same for I 2 and I- in the presence of z if no I2 is produced IO3-, since CIzcan equal C b ~only by reaction 2. Hence CI- must become zero a t the same point in space that CI, = C b ~ z . Integrating eq 13 with respect to y over the thickness 6, yields
Cbl,(CH+ = 0.200M ,
M).
C b ~ o a -= ~ 4.00 X
-
~
+
Table 111 : Summary of Catalytic Studies in the Absence of Chloride
CH+ C I O , - ~ * ' ( ~-I C I~ Z C-2)I dy
(29)
From eq 27 we obtain
Angular velocity
range,
CHI,
1OaCbIOa-,
rpm
M
M
400-1600 400-2500 400-2500 400-2500 400-3600 400-8100 900-2500
0.100 0.100 0.100 0.200 0.200 0.200 0.200
2.00 4.00 6.00 4.00 4.00 4.00 4.00
1o4CbI9, M
io-skl
10-9k~
1.00-6.00 1.00-6.00 1.00-6.00 1.00-6.00 1.00-6.00 1.00-6.00 0.100-0.600
9 4 3 4 5 6 1.5
5.4 7.6 9.6 7.2 8.2 5.9 5.1
of I1 (eq 7), where one of the I- is replaced by C104-, which then decomposes to form C104- plus IO+ IO-.
+
and are left with the problem of expressing CI-in terms of CbIzJy, and 6, before eq 29 can be integrated. Neglecting the diffusion of loa-, we can write
Differentiating eq 28 with respect to y to obtain dC1-l dy, and substituting dCI-/dy and dCI,/dy from eq 30 into eq 31 yields
Conclusion The method of moments permits the quantitative treatment of catalytic processes a t the rotating disk electrode, as has been verified above by studying the catalytic iodide wave observed in the presence of iodate. The experimental and mathematical approach is general and can be used to study catalytic processes obeying different rate laws. The rotating disk electrode should have wide application in the study of very rapid reactions for which catalytic currents can be observed. Acknowledgment. This work was supported by the Space Science Center of the University of Minnesota using funds supplied by NASA.
Appendix The convective-diffusion problem described by eq 13-20 can be considerably simplified if catalytic currents are measured when CbIoa- >> CbIOa- - C810a-* The Journal of Physical Chemistry
C81- =
2acbI,
(32)
where a = DI,/DI-. Hence, substituting eq 32 into 28 yields CI- = 2acb1,(l
-
);
(33)
Substituting eq 16, 30, and 33 into 29 and integrating yields -0.51
( y z * , 2
3
- --DI,
+
6, 6caCH +2CbIOa-CbI,(kI-I- 4/3QCb1zk2) (34)
Now, as has been shown elsewhere,22the method of moments predicts that the value of the thin layer, 6, adjacent to the electrode, when only convective-diffusion is important (in the absence of 10s-) is given by
ESRSPECTROSCOPY OF THE X A N T ~ FREE L RADICALS
=
(0.51(w3/lv)"' 3D=2 Ya
,
6,3
6,2
63
62
a =
(35)
Hence, substituting eq 35 into 34 yields
-+A--l=O
3635
(36)
which is the form of eq 21, where R = 6,/6 and A is given by eq 23. According t o the Nernst diffusion-layer approach, the current through the electrode is given by
nFADCb 6*
where 6* is the thin layer across which mass transport occurs. It is assumed t h a t C* = 0. Hence, we see that
i- --_ 6, = R
i,
6
where i is the current in the absence of 1 0 3 - and i, is the catalytic current in the presence of IO3-.
Electron Spin Resonance Spectroscopy of the Xanthyl Free Radicals.
I.
Xanthyl Radical: a Planar Diphenylmethyll
by Michael D. Sevilla2 and Gershon Vincow* Department of Chemistry, University of Washington, Seattle, Washington 98106
(Received M a y 1, 1968)
The xanthyl radical (Figure 1, R = H) is investigated because of its structural similarity to diphenylmethyl. This radical is produced by homolytic thermal cleavage of dixanthyl in n-tridecane solvent in the temperature range 180-260". The esr spectrum has been reduced to hyperfine-coupling constants. The splittings are l a p = 3.425 =t0.008 G, lazHl = 0.988 0.005 G, IaaHl = 4.047 =t0.010 G, la& = 0.894 =t 0.005 G, and lagHl = 12.729 =t 0.008 G. Esr results obtained for 3-deuterioxanthyl and 2-chloroxanthyl radicals and theoretical calculations of the spin density distribution of xanthyl radical are employed in the assignment of the above splittings to ring positions. The proton splittings of xanthyl radical have been calculated by means of the McConnell relation and Huckel M O and McLachlan SCF-MO computations of spin densities. Xanthyl is assumed to be planar in these calculations. The McLachlan theory spin densities lead to fairly good agreement with experiment. It is shown that the effect of the oxygen heteroatom on the spin density distribution is small and, consequently, the xanthyl radical can be considered as essentially a planar diphenyL methyl.
Introduction This is the first of a series of three papers on the esr spectroscopy of a number of neutral xanthyl free radicals in solution. These radicals have been generated by the homolytic thermal cleavage of their dimers, the dixanthyls (see Figure 1). The dixanthyls were first synthesized 40 years ago by Gomberg4 and by Conant and his coworkers6 who also presented strong evidence for their dissociation into free radicals. The results of the above-mentioned investigators have been substantiated on spectroscopic grounds in this work. This paper presents the results of a detailed investigation of the unsubstituted xanthyl radical (Figure 1, R = H). The paper which follows reports on the esr of the 9-phenylxanthyl radical (Figure 1, R = CeH6).6 An approximate determination of the configuration of
the 9-phenyl ring is made by means of a comparison of the experimental hyperfine splittings due t o protons on the 9-phenyl substitutent with calculated values. The third paper in this series describes a n investigation of a number of 9-alkylxanthyl radicals (Figure 1, R = CH3, (1) Part of this research has been reported a t the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (2) Weyerhaeuser Fellow, 1965-1966; National Science Foundation Graduate Teaching Assistant Summer Fellow, 1965, 1966. (3) Alfred P. Sloan Research Fellow. (4) M. Gomberg, J . Amer. Chem. Soc., 39, 1652 (1917). (5) (a) J. B. Conant and A. W. Sloan, ibid., 47, 572 (1926); (b) J. B. Conant and L. F. Small, ibid., 47,3068 (1925); (c) J. B. Conant, L. F. Small, and A. W. Sloan, ibid., 48, 1743 (1926) ; (d) J. B. Conant and B. S. Garvey, Jr., ibid., 49, 2080 (1927); (e) J. B. Conant and M. 'CV. Evans, ibid., 51, 1925 (1929); (f) J. B. Conant and A. W. Sloan, ibid., 45, 2466 (1923); (g) J. B. Conant, ibid., 48, 1023 (1926). (6) M. D. Sevilla and G. Vincow, J. Phys. Chem., 72, 3641 (1968). Volume 72, Number 10 October 1968
