J . Am. Chem. SOC.1982, 104, 5911-5918 the doublet is due to two (slightly inequivalent) ortho protons in the latter two complexes, whereas it stems from a single ortho proton in V02+-His. The line-width change is in agreement with the proposed structures of the complexes. Future theoretical work may provide a better understanding of the factors that affect the magnitudes of the hyperfine and quadrupole splitting components. Then the measured parameters could serve to provide us with a more detailed picture of the geometric structure of V02+-Im complexes. However, even in ihe absence of a more rigorous quanfitative analysis, the data presented here make it evident that ENDOR can be-of great value
591 1
in giving a semiquantitative insight into V 0 2 + binding site structure.
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Financial support by the National Science Foundation (PRM-8 100525, PCM-7903440) and Department of Energy (DE-AC02-81ER1091 l ) for the purchase of equipment is gratefully acknowledged. Registry No. VO(Im)t*, 82871-04-3; VO(H,O)?+, 15391-95-4; Lhistidine, 71-00-1; L-carnosine, 305-84-0; thiazole, 288-47-1.
Chlorite Oscillators: New Experimental Examples, Tristability, and Preliminary Classification1 Miklbs Orbin,**Christopher Dateo, Patrick De Kepper,zband Irving R. Epstein* Contribution from the Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254. Received March 16, 1982 Abstract: Sustained oscillations are reported in the absorbance at 460 nm and in the potential of a Pt redox or iodide-sensitive electrode in a stirred-tank reactor (CSTR) containing solutions of chlorite, iodide, and an oxidizing substrate (IO3-, Cr2072-, MnO,-, Br03-) or chlorite, iodine, and a reducing substrate (Fe(CN)6", S032-,S2032-).In addition to oscillations, a system containing chlorite, iodide, iodate, and arsenite can exhibit three different steady states, depending on initial conditions, for the same set of input flows, residence time, and temperature in the CSTR. Until a detailed mechanism for chlorite oscillators is developed, it seems reasonable to divide these systems into four classes: (A) the fundamental Cloy-I- (or C102--I--I03-) oscillator, (b) ClO,--I--oxidant systems, (C) C10,--I03- or (or I,)-reductant systems, (D) iodine-free systems, e.g., CIO~-S,032-. A corresponding classification of bromate oscillators is discussed.
Oxyhalogen ions-bromate, iodate, or chlorite-are indispensable constituents of all known homogeneous isothermal chemical oscillators in solution. Although the first oscillating reaction discovered, the Bray-Liebhafsky r e a ~ t i o nis , ~an iodate system, the overwhelming majority of oscillators discovered to date contain either bromate or chlorite. The only other iodate oscillators developed since Bray's discovery in 1921 have been the related Briggs-Rauscher system^.^.^ Bromate oscillators have been by far the most thoroughly studied and characterized, notably the prototype Belousov-Zhabotinskii (BZ) system, which was discovered over 20 years ago.6 Extensive bodies of experimental and theoretical work are now in remarkably good agreement' for the BZ reaction. Noyes* has recently formulated a generalized mechanism for bromate oscillators and has used this scheme to divide the known bromate oscillators into five distinct categories. While recent discoveries in this laboratory' have extended the range of bromate-driven oscillation beyond those categories treated by Noyes, the fundamental mechanism and notion of classification remain sound. The study of chlorite oscillators is at a more primitive stage, since the first of these systems9 was discovered only a year ago. However, progress has been remarkably swift, and the systematic classification and mechanistic description of chlorite oscillators (1) Part 11 in the series Systematic Design of Chemical Oscillators. Part 10: Orbin, M.; De Kepper, P.; Epstein, I. R. J . Am. Chem. SOC.1982,104, 2657-2658. (2) Permanent addresses: (a) Institute of Inorganic and Analytical Chemistry, L. Edtv6s University, H-1443, Budapest, Hungary. (b) Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence, France. (3) (a) Bray, W. C. J. Am. Chem. Soc. 1921, 43, 1262-1267. (b) Liebhafsky, H. A. Ibid. 1931, 53, 896-91 1. (4) Briggs, T. C.; Rauscher, W. C. J . Chem. Ed. 1973, 50, 496. (5) Cooke, D. 0. Inr. J . Chem. Kinet. 1980, 12, 683-698. (6) Belousov, B. P. Ref. Radiat. Med. 1959, 1958, 145-147. (7) Field, R. J.; Noyes, R. M. Acc. Chem. Res. 1977, IO, 214-221. Noyes, R. M.: Field. R. J. Ibid. 1977. IO. 273-280. (8)'Noyes, R. M. J. A m . Chem. Soc- 1980, 102, 4644-4649. (9) De Kepper, P.; Epstein, I. R.; Kustin, K. J. Am. Chem. Soc. 1981, 103, 2133-2134 (Part 2).
0002-7863/82/1504-5911%01.25/0
Table I. Chlorite Oscillators in a CSTR
no.
system
1
c102--1-
2
c10,--1--10,-
3 4
c102--1-malonic acid clo2--I--cr20,~-
5
C102~-1~-Mn0;
6
ClO,--I--BrO,-
7
CIO,~-IO,~-H,AsO,
bistability between stationary and oscillating states batch oscillation, spatial wave patterns
bistability between stationary and oscillating states first chlorite oscillator discovered
8a C102'-I0,~-Fe(CN),4~ 8b C10, --I -Fe(CN) 6 4 -
,
9a C102~-I03--S03z9b C10,'-12-S0,210a C 1 0 2 - - I 0 3 ~ - S 2 0 3 z ~ batchoscillation 1Ob ClO,~-I,-S, 0,'-
11
12 13 14 15
c102--103-CH,O.HSO, c102--103-ascorbic acid C102~-I03~-I~malonic acid C102~-I~-I03~H,As03 CIO,~-S,O,z~
ref
special features
10 this work 11 this work this work this work 9 13 this work 13 this work 11 this work 13
13 batch oscillation
11
tristability
this work 12
first iodine-free chlorite oscillator
now seem within reach. In this paper we report the discovery of several new chlorite-based oscillating reactions, summarize the 0 1982 American Chemical Society
5912 J. Am. Chem. SOC.,Vof. 104, No. 22, 1982
Orbdn el al.
known chlorite oscillators, and make a start toward a phenomenological classification of these systems according to their behavior relative to the prototype chloriteiodide system.l0 A more detailed classification must await a mechanistic description of chloritebased oscillation, which we are in the process of developing. In Table I, we list the homogeneous oscillatory reactions discovered to date in which chlorite is a constituent. Most of these reactions undergo oscillation only in a stirred-tank reactor (CSTR) with input and output flows of reactants and products, though some as indicated show batch oscillation as well." Experimental data on several of these systems will be reported for the first time in this paper. We note that with the single exception of the C102--S2032- reaction (no. 15), which is discussed in detail elsewhere,I2all of these systems contain one or more iodine species (IO3-, 12, I-) in addition to chlorite. Since the C102--I- system (no. 1) alone generates oscillation, it is reasonable to ask whether systems containing chlorite, iodide, and an oxidizing agent (no. 2-6) are in fact different oscillators; Le., do the additional oxidants promote oscillation, or are they merely perturbations of the chlorite-iodide oscillator. We shall seek an answer to this question by using the same type of analysis that led to the initial discovery of these oscillators. That is, we shall examine the phase diagram of the system and scrutinize how the regions of monostability, bistability, and oscillation shift and distort as the input flows of oxidizing species are varied. In a similar vein, one may ask what role the reducing agent plays in the C102--I03- (or 12)-reducing agent oscillators (no. 7-12).13 While the failure to oscillate of the C102--I03- (Iz) reaction alone shows that the reducing agent is an essential part of the system, one may justifiably enquire whether the reducing agent does anything other than provide a source of 1- by its reaction with IO3- (I2). If this is its only role, then these oscillators are little more than variants of the more fundamental CI02--I- and C102--I--I03- systems.
I
lo-'
I
I
10-8
10-4
[ClO;]o
I
10-
10-8
fM1
Experimental Section The flow experiments were carried out in two thermally regulated stirred-tank glass (Pyrex) flow reactors. Each reactor contains a freeliquid-air interface. The first of these (reactor A) has been described in detail e1~ewhere.l~Reactor B differs from reactor A primarily in that reactor B is built into the sample chamber of a Beckmann 25 recording spectrophotometer. In either reactor, simultaneous measurements could be made of the absorbance of the solution and of the potentials of a platinum redox electrode (vs. HglHg2S04JK2S04 reference) and/or an Orion iodide-specific electrode. Absorbances were generally measured at 460 nm, the wavelength of maximum I2 absorbance. The path lengths were 4.05 and 3.4 cm in reactors A and B, respectively. All experiments were carried out at 25.0 k 0.1 OC. The constraints ) that were varied were the residence time 7 (or flow rate ko = 1 / ~ and the concentrations [A,], that the input species Ai would reach in the reactor if no reaction were to occur. All solutions were prepared with doubly distilled water from high-purity commercially available grades of NaC102, KI, KI03, I*, and other reagents. Chlorite solutions were stabilized by the addition of 0.001 M sodium hydroxide. Acetate or sulfate buffers of the appropriate pH were generated in situ as described earlier.'" Phase diagrams were determined by either of two methods. In the first approach, the potentiometric or spectrophotometric response of the system was followed as a function of flow rate at a fixed composition. The composition was then changed and the flow rate varied again. After a number of compositions had been investigated, we selected an appropriate flow rate at which to construct the constraint-constraint ([XIo-[Y],) plot. A more rapid, though less thorough, approach employed in some cases consisted of choosing a fixed flow rate and then changing the reservoir compositions, allowing the system to reach a stationary or os(10) Dateo, C. E.; Orbin, M.; De Kepper, P.; Epstein, I. R. J . Am. Chem. SOC.1982, 104, 504-509 (Part 5).
(1 1) De Kepper, P.; Epstein, I. R.; Kustin, K.; OrbPn, M. J . Phys. Chem. 1982.86, 170-171 (Part 8).
(12) OrbPn, M.; De Kepper, P.; Epstein, I. R. J . Phys. Chem. 1982, 86, 431-433 (Part 7). (13) Orbin, M.; De Keppcr, P.; Epstein, I. R.; Kustin, K. Nature (London) 1981, 292, 816-818 (Part 4). (14) De Kepper, P.; Epstein, I. R.; Kustin, K. J . Am. Chem. SOC.1981, 103, 6121-6127 (Part 3).
1
10-0
1
I
1
IO"
10-4
I
1 10-1
[CIOi]o(M)
Figure 1. Two sections in the [C10~],-[1-], plane of the phase diagram for the chlorite-iodide system. Solid lines divide the regions of monostability (SS I, V; SS 11, A), bistability (0), and oscillation (0). Symbols show experimental points. Dashed line indicates region inaccessible experimentally because of iodine precipitation. P and P' are cross points or critical points where the bistable region disappears as SS I and SS I1 coalesce. Constraints: (a) p H 3.35, k, = 5.4 X lo-) s-I, T = 25 OC; (b) pH 2.04, ko = 1.1 X lo-' SKI,T = 25 OC. cillating state before each reservoir change was made.
Phase Diagrams Much of our analysis of these systems is based on the study of two types of graphs: the constraint-response plot and the phase diagram, which is a constraint-constraint plot. The constraintresponse plot shows long-term behavior of a system (e.g., a stationary-state concentration or range of sustained concentration oscillations) as a function of a constraint ( k , or an [ A i ] , ) . An example may be found in Figure 4 below. The phase diagram divides the constraint space into different regions according to the type of long-term behavior or phase (e.g., monostability, bistability, oscillation, chaos) shown by the system in response to the constraints. Two sections of the phase diagram for the chlorite-iodide system are shown in Figure 1. The regions of monostability of the two stationary steady states, SSI, characterized by low I- and (when an oxidizing substrate is present) high
J . Am. Chem. SOC.,Vol. 104, No. 22, 1982 5913
Chlorite Oscillators
-> -.-z E
230200-
e C
' O - L 0
2 170*
I
'10-4
io-'
10-1
10-2
i'lO;],
2
(M)
Figure 2. Two [I-]o-[IO M. Arrows at a and b indicate [I-], M or [H3AsO3l0> 4 X values along which constraint-response plots in Figure 9 are made.
k/g
I
”
10-0
”
’
IO“
L
IO*)
[I-], (MI Figure 7. Constraint-response plots of steady-state [I-] vs. [I-], for three values of [H3As0310on the phase diagram of Figure 6. Symbols: (‘I) SS I, (A)SS 11, (A)S S 111. Fixed constraints as in Figure 6. [H3As03]: M (a), 8 X M (b), 3.2 X M (c).
The full system exhibits an extraordinary richness of behavior, even in the relatively small portion of the constraint space that we have been able to explore thus far. We first consider a section of the phase diagram (Figure 6 ) with all constraints except [I-], and [H3As03], fixed at the same values as in Figure 2b and [IO,-], = 0.025 M. In the absence of arsenite, the system shows a narrow region of bistability as a function of [I-],, with hysteresis in the transitions between a low [I-],, steady state (SSI) and a high [I-],, steady state (SSII), as in Figure 2b. As [H,As0310 is increased, the stability boundaries of both states shift to lower [I-],, and the relative width of the bistable region increases. At [I-], < 5 X M, with the introduction of an arsenite flux of about 2 X M, a new steady state (SSIII) appears, which has an intermediate value of [I-], and a low [Iz],. In various regions of the phase diagram we now see three different types of bistability (SSI/SSII, SSI/SSIII, SSII/SSIII), as well as a region in which any of the three states can exist, depending upon the initial conditions: the system is tristable. The various types of multistability are illustrated in a different fashion in the constraint-response plot of Figure 7, where [I-],, is plotted as a function [I-], for three different values of [H3AsO3], as indicated on the phase diagram of Figure 6 . Note that only for the intermediate arsenite flux (Figure 7b) do we encounter the region of tristability. While bistability is now a fairly well documented phenomenon, tristability is a rather novel Occurrence in chemical systems. Only one other example has been reported.18 It is an exceedingly difficult experimental task to identify and to characterize three or more different steady states of a single system. It is of critical importance to be able to measure as many different responses as possible; in this case we had three (redox potential, iodide-sensitive electrode potential, absorbance at 460 nm). Another useful clue in distinguishing the different states lies in their dynamic behavior. For example, when a jump perturbation is made in the flow rate, SSII shows excitability (large departure of concentrations from steady-state values before returning) while SSIII does not.
In Figure 8 we show a section of the phase diagram with all constraints as in Figure 6 except that the pH is now 2.0 instead of 3.35. Here, as in several other chlorite systems, it is found that lowering the pH narrows the region of bistability, shifting the system toward an oscillatory domain. We note the absence of SSIII and the occurrence of relatively large regions of oscillation and of subcritical bifurcation, in which the initial conditions determine whether the system oscillates or resides in SSI. As in the previous example, the stability boundaries of both stationary states shift to lower [I-], as [H3As03], is increased. Constraint-response plots of [H,As0310 vs. [I-], for two values of [I-], are shown in Figure 9. The data reported here represent the most complex phase diagram yet observed for a homogeneous liquid-phase reaction. We have, however, only begun to probe this and related systems. The existence of additional stationary and/or oscillatory states would not be surprising. Furthermore, although we primarily observed periodic oscillations, some aperiodic phenomena were also noted, though no systematic search for or analysis of this latter behavior was carried out. In this respect, the nonlinear dynamics of this system would seem to be at least as complex as those of the Belousov-Zhabotinskii reaction for which chemical chaos has been reported.19
Discussion While we are still far from being able to present a full mechanistic treatment of chlorite oscillators, it may be helpful to consider, in Noyes’ terminology,* the major “component stoichiometric processes” involved in chlorite-iodine oscillators. This consideration will enable us both to derive some understanding of the role of the various oxidizing and reducing substrates and to formulate a preliminary classification of chlorite oscillators. Also, each of the component processes has been studied individually, so constructing a mechanism for the oscillators may be largely a matter of “fitting together the pieces”. The major overall processes taking place in systems containing chlorite and iodide, iodine, and/or iodate appear to be2, I(V)
+ Cl(II1) 4I(O) + SCl(II1) 41-
-
+ 51-
6I(O)
+ C14I(V) + 5C1-
4I(O)
(MI) (M2) 043)
where roman numerals in parentheses refer to oxidation states of iodine and chlorine, such as I(0) for 0.512and Cl(II1) for Cloy. All three processes are strongly favored thermodynamically. (19) Schmitz, R. A,; Graziani, K. R.; Hudson, J. L. J . Chem. Phys. 1977, 67, 3040-3044.
(18) De Kepper, P.; Pacault, A. C. R. Hebd. Seances Acad. Sei., Ser. C 1978, 286, 437-441.
(20) At least at pH’s above about 1, where decomposition of chlorite to C102is not significant (Gordon, G.; Kieffer, R. G.; Rosenblatt, D. H. Prog. Inorg. Chem. 1972, 15, 201-286).
5916 J . Am. Chem. Soc., Vol. 104, No. 22, 1982
Orbiin et al.
34-
5-
PI-
678-
9-
I 0
10
5
[H,AsO,],
x
5[ 6
91
I
Oscillatory state ~
4
2
Tim. fminl
IO3 (MI
4t PI-
I
40
20
Figure 10. Redox potential (Pt electrode vs. SHE) vs. time for several oxyanion (Ox)-iodide batch reactions at pH 1 (a) and pH 2 (b). [I-] = 6.6 X IO4 M, [Ox] = 3.3 X M. Arrows indicate start of the reaction, when acidic Ox solution was added to neutral I- solution. Curves: (1) MnO;, (2) BrOC, (3) Cr2072-,(4) Io
