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J. Phys. Chem. 1986, 90, 2802-2804
water, even in surfactants containing saturated paraffin chains; this tendency would be expected to be enhanced substantially when the CH3 group is replaced by unsaturated or polarizable groups. Some words of caution are in order. We have concentrated our attention on the properties of I, 11, and 111 when they are incorporated in host micelles. It is however possible that the average conformations of these molecules in host micelles could be different from those in the self-aggregates or intrinsic micelles of the pure molecules themselves. It is not clear whether this is why the cmcs of the I, 11, and 111 are only modestly larger than that of undecanoate and are not comparable to the cmc of say hexanoate or octanoate, which might be expected if every molecule is self-coiled and packed in that fashion in their intrinsic micelles. Also, the spectral results on I, 11, and 111 in host micelles cannot rigorously distinguish between the interaction of the terminal group
of a molecule with its own polar head group or with those of the neighboring molecules.26 In this connection, a detailed characterization of the intrinsic self-aggregated micelles of I, 11, and 111 would be of value, a project that is currently under way in our laboratory.
Acknowledgment. We are grateful to Dr. Krishnamurti and Prof. Ranganathan for their kind gifts of the test acids and to Mr. V . Srinivas with help in tensiometry. We thank the Bangalore N M R Facility and the University of Hyderabad for time in their instruments. Financial assistance from DST and from CSIR is gratefully acknowledged. (26) Haide and OConnell (Haide, J. M.; OConnell, J. P. J. Phys. Chem. 1984, 88, 6363) have reported this idea in their simulation studies.
New Bromate Oscillator: The Bromate-Thiourea Reaction in a CSTR Reuben H. Simoyi Department of Chemistry, University of Zimbabwe, Mount Pleasant, Zimbabwe (Received: January 21, 1986; In Final Form: March 14, 1986)
The reaction between bromate and thiourea in acidic medium (HCIO,) has been studied in a closed system and in a continuously stirred tank reactor (CSTR). The stoichiometry of the reaction at pH less than 2 has been deduced to be 8Br0< + 5CS(NH2)2 + 8H+ + 6 H 2 0 4Br2 + 5(NH4)2S04+ 5c02.In closed systems the reaction is characterized by an induction period whose length is inversely proportional to the initial concentrations of both acid and bromate. The induction period is followed by production of molecular bromine. In the CSTR the reaction displays sustained simple periodic oscillations in the bromine concentrations and the redox potential. No bistability was observed in the system.
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Introduction Bromate-driven oscillators are the most thoroughly studied of all known homogeneous oscillating chemical systems.' Among the earliest known chemical oscillators is the Belousov-Zhabotinskii reaction in which bromate oxidizes an organic substrate in the presence of an acid and a metal ion catalyst.2 Until quite recently two-component, uncatalyzed bromate oscillators were ~ n k n o w n . We ~ report here another such chemical oscillator which involves an organic substrate, thiourea. The chlorite-thiourea system is known to show simple periodic and complex periodic oscillations in a continuously stirred tank reactor (CSTR)., In batch conditions the reaction displays oligoosciliatory behavior, giving several C102 concentration maxima before reaching equilibrium. The oxidation of thiourea by iodate has also been found to exhibit oligooscillatory b e h a v i ~ r . ~ In general, reactions involving thiourea have been found to be quite complex, and in most cases, the stoichiometries of the reactions can never be fully established, being dependent often on the ratio of the concentrations of thiourea to the ~ x i d a n t . ~This , ~ results in the existence of certain concentration regions in which a number of stoichiometries apply to varying degrees. The stoichiometry of the bromate-thiourea reaction had already been established as' (1) Noyes, R. M. J. Am. Chem. SOC.1980, 102,4644-4649. (2) (a) Belousov, B. P. Sb. Ref. Radiat. Med. 1958 1959, 145. (b) Zhabotinskii, A. M. Dokl. Akad. Nauk SSSR 1959, 157, 392. (3) Alamgir, M.; De Kepper, P.; Orban, M.; Epstein, I. R. J. Am. Chem. SOC.1983, 105, 2641-2643. (4) Alamgir, M . ; Epstein, I. R. Int. J. Chem. Kine?. 1985, 17, 429-439. ( 5 ) Rabai, G.; Beck, M. T. J. Chem. Soc.,Dalton Tram. 1985, 1669-1672. ( 6 ) Nurakhmetov, N. N.; Beremzhanov, B. A,; Utina, Z. E. J. Gen. Chem. USSR (Engl. Transl.) 1977, 47, 1832.
0022-3654/86/2090-2802$01.50/0
+
3CS(NH2)2 4HBr03
+
-
3H20 3CO(NH2)2+ 3H2SO4 + 4HBr ( R l )
Experimental Section The following analytical grade chemicals were used without further purification: potassium bromate (British Drug Houses), sodium perchlorate (Aldrich), perchloric acid (70-72% PAL Chemicals), and thiourea (Fisher). The 5 M stock sodium perchlorate solutions were first filtered before use. In batch environments the reactions were performed in a 150-mL reactor with a thermostating jacket which maintained a temperature of 25 0.1 OC. The ionic strength was maintained at 0.2 M (NaC104). Potentiometric measurements were done by using a platinum electrode with a double junction calomel reference with 10% ammonium nitrate as the contact electrolyte. Spectrophotometric measurements were done on a Unicam SP1750 UV spectrophotometer. The CSTR experiments were performed in a Pyrex glass thermally regulated flow reactor of volume 23.5 mL.8 The reactor temperature was maintained at 30 0.2 The stirring rate was maintained constant throughout this study. To minimize stirring problems, only two input streams were used since it is much easier to homogenize reactants coming into the reactor via two input streams than three; local effects are minimized. One feed line contained thiourea, and the other contained a mixture of
*
*
(7) Szebeledy, L.; Madis, W.; Fres, W. Z. Anal. Chem. 1938, 114, 253. (8) The same reactor design has been used before on the chlorite-bromide system. See: Alamgir, M.; Epstein, I. R. J. Phys. Chem. 1985, 89, 361 1-3614. (9) A reactor temperature above room temperature was used because of the slow evolution of the oscillations. The higher temperature speeded up the reaction, thereby reducing the period of oscillations.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2803
Letters
I
o
A
I
-003MH+ 0 04M H'
----
20
LO
TIME Ininuterl
Figure 1. Redox potential traces for the batch reaction between bromate and thiourea at different initial acid concentrations. The rapid rise in
potential corresponds to the commencement of molecular bromine production (see Figure 2). [BrO> [CS(NH,),]), show no change in pH with time. It appears that we are now beginning to unravel a whole new subclass of chemical oscillators whose "drive" reaction is the oxidation of thiourea. A full understanding of the mechanism of these oscillators is only possible after the mechanism of thiourea oxidation is better understood.
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Acknowledgment. I thank Professors I. R. Epstein and Kenneth Kustin and all the members of the Brandeis University Chemical Instabilities Group for invaluable assistance. I also acknowledge many helpful discussions with Professor Sheppard. This work was supported by research Grant No. RB 49/83:2543 from the University of Zimbabwe Research Board. Registry No. Br03', 15541-45-4; CS(NHJ2, 62-56-6. (12) Good examples are the CI02--I- system in which C102- can oxidize 1- to give I,, while C102can also oxidize iodine to give iodate and the BrO,--Isystem in which the iodine produced by the bromate-iodide reaction can also react with bromate to give iodate. See: Dateo et al. J . Am. Chem. SOC.1982, 104, 504-509; Simoyi, R. H . J . Phys. Chem., submitted for publication. (13) This was extraoolated from ref 5 in which iodine oxidizes thiourea according to the followhg stoichiometry: 41, + CS(NH2) + 7 H 2 0 (NH4)2S04 + 81- CO,*- + lOH+. (14) Simoyi, R. H., unpublished work. (1 5 ) Vogel, A. I. Quantitative Inorganic Analysis; Longmans: New York, 1971.
+
-.
