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Research Center. The acquisition of the mass spectrometer was made possible by grants from the Materials Research Center, the U. S. Atomic Energy Commission, and Northwestern University. P. M. K. thanks the Public Health Service for a predoctoral fellowship from the National Institute of General Medical Sciences.
The Disproportionation of Hypoiodite
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by 0. Haimovich and A. Treinin Department of Physical Chemistry, Hebrew University, Jerusalem, Israel (Received December 6 , 1966)
The disproportionation of IO-, as that of other hypohalites, is a second-order process - - d(I0-) =
Experimental Section The systems investigated were prepared by rapidly dissolving 1 2 in solutions containing 4 M NaOH and various concentrations of halates. The decay of IOwith time at 25" was followed spectrophotometrically as described elsewhere.' The spectrum of IO- above 300 mp was found to be hardly affected by the halates. Most results were obtained at 365 mp, where IOhas its absorption peak; no variation of rate constant with X was noticed in the range 300-400 mp. The sum (IO-) (IOr-) was measured by titration: a 2-ml sample was introduced into excess of N arsenite and 4 g of NaHCOa. Then 4 N H2S04was added with vigorous shaking until pH 8.3 was reached. Excess of K I (-0.2 M ) was added and the mixture left in the dark for 1 hr. Under these conditions 1 0 4 - was found to be quantitatively reduced by I- to IO3-. The excess of arsenite was titrated with 3 X N 12 solution.
IC, (IO -) 2
Results and Discussion The disproportionation is retarded by 1 0 3 - and M-I sec-l at 25" in 4 M alkali with k, = 4.0 X C103- (Figure 1). The effect becomes apparent only solution. The hitherto accepted mechanism2involves when the halate concentration exceeds some threshold, the rate-determining step above which the change induced is rather abrupt: the kt rate constant of the reaction, which is still second order, IOIO- --t 102I(1) M-I sec-I. Further falls down to (2.6 f 0.2) X increase of (XO3-) has no effect. The threshold rises where kl is defined by the relation -d(IO-)/dt = with an increase of (IO-). Thus it happens that the 2kl(IO-)2, followed by the fast reaction disproportionation starts unretarded, but as the ratio mechanism I : 1 0 2 IO- --+ 1 0 3 I(2') (XO3-)/(1o-) increases there is a sudden drop of rate constant. This is clearly shown in curve a where 1 0 3 However, an alternative fast reaction should be conis supplied only by the reaction. sidered The following results prove that the influence of the kr halates involves their conversion to perhalates. (a) mechanism 11: 101- 1 0 2 - ----+ 1 0 3 IO- (2) A solution containing IO- and excess of 1 0 3 - was left where k2 is defined by the relation d(I0-)/dt = kz. until effectively all the IO- was consumed. Some (102-)2. An analogous reaction was proposed for precipitate was formed. It was analyzed by X-ray BrO- as a result of studying its radiation chemi~try.~ diffraction and found to consist mainly of NaI04. Both mechanisms account for the stoichiometry and (I(&-) was found to remain constant The sum (IO-) kinetics of the decomposition. Hence the study of during the reaction (curve d). (b) A similar experiIO- alone cannot distinguish between them. The ment was carried out with KC103 and KOH instead of ratio kl/k, derived by assuming a steady state for IOZ KIO3 and NaOH, respectively. At the end of the refor mechanisms I and 11, respectively. is '/3 and action KC104 was precipitated by acidifying the soluHere we report how the value of kl and thus the tion to pH 8 and adding ethanol. It was identified by mechanism of the disproportionation can be detergravimetric6 and colorimetricB methods. KC104 (1 mined. For this purpose the systems IO--IOa- and mole) was produced for each mole of IO- consumed. IO--C103- were investigated. Some previous results concerninn these svstems4 are not reliable. since the (1) 0.Haimovich and A. Treinin, Nature, 207, 185 (1965). solutions were relatively of low alkalinity and high (2) C.H. Li and C. F. White, J . Am. Chem. SOC.,65, 335 (1943). This could be (I-) and so were chemically ill-defined. (3) C. H. Cheek and V. J. Linnenbom, J . Phys. C h m . , 67, 1856 shown by measuring the spectra of the solutions. (1963). Moreover, the formation of perhalate was overlooked. (4) A. Rashid and E. Ali, Anal. Chem., 36, 1379 (1964). dt
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occurs, which is second and zero order with respect to IO- and XO3-, respectively. This indicates that it is rather a complex process which involves reaction 1 as its primary and slowest step. The subsequent step appears to involve the halate acting as efficient scavenger for IO2-
xo3- + 1 0 2 -
kd --f
xo4-
+ 10-
(4)
Reactions 1 and 4 constitute a full mechanism for reaction 3 with the rate law (5)
where k,’ = kl. As further support to this mechanism we found that the activation energy of reaction 3 with C103- is nearly the same as that of the disproportionation-10 i l kcal/mole. Now, the observed value of k,’ = 2.6 X is nearly 2/3ka, which proves that the disproportionation proceeds by mechanism 11. The effect of c103-and 1 0 3 - was also tested a t 6 M OH-, where k, is lower, and then remains nearly constant up to 10 M Na0H.l Again it was found that the halates could reduce the rate constant to 2/3k,. The effect of (OH-) is not an ionic strength effect, since the addition of 2 M NaCl to 4 M NaOH does not affect the kinetics. It may be due to some H I 0 still present in solution at (OH-) 2 4 M . The effect M and (I-) of I- was also examined: a t (IO-) 8X M no effect could be detected. This result is a t variance with the kinetic law proposed by Li and White.2 On further raising (I-), k a increased but the spectrum showed changes indicating the formation of
time, min
Figure 1. The effect of 1 0 3 - and ClOs- on the disproportionation of IO-. Curves a and b, no halate initially added; curve c, 4.8 X IO-* M ( 0 )and M IO$-; 8.6 x 10-2 M (+) Ios-; curve d, 4.8 X curves e and f, 1.2 X IOd2 M and 2.4 X 10+ M clos-, respectively; curve g, 4.2 X lo-* M (0) and 6.1 X 10-2 M (A) ClOa-, 2.0 X M CIOa1.2 x 10-9 M Clod- ( 0 ) .(Continuous and dashed curves are second-order plots for IO- and IOIOd-, respectively.)
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> k4-1(8k~k~)’/8. may be involved which is a very efficient scavenger for 102-. While the proposed mechanism accounts for the effect Finally, it is of interest to note that Br03- has no of the halate being a function of (XOs-)/(IO-), it retarding effect on the disproportionation. This is does not explain the nature of this function. In parin accord with the nonexistence of the perbromate ticular, it appears that as soon as the halate starts to ion. have a distinct effect, it acquires its full effect. Some kind of autocatalysis is probably involved. And indeed we observed that little IO4- or c104- largely (6) F. P. Treadwell, “Analytical Chemistry,” John Wiley and Sons, Inc., New York, N. Y., 1951,p 392. enhances the effect of the corresponding halate. This (6) F. Feigl, “Spot Tests in Inorganic Andysis,” Elsevier Publishing is shown for c104--c103- in Figure 1 (C104- alone Co., Amsterdam, The Netherlands, p 300; L. Ben-Dor and E. has no effect on the reaction). Moreover, following Jungreis, Mikrochina. Acta, 1, 100 (1964).
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The Journal of Physical Chemistry
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Acknowledgment. We are indebted to Dr. I. Mayer from the Department of Inorganic and Analytical Chemistry for carrying out the X-ray analysis.
Small-Angle X-Ray Scattering from a Macroreticular Sulfonic Acid Cation-Exchange Resin-Amberlyst
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by B. ChuS and D. M. Tan Creti Chemistry Depart&, The U n i v e r d y of Kanma, Lawrence, Kansas 6604 (Received September SO, 1986)
Small-angle scattering of X-rays has been a rather powerful technique for characterizing the structure of noncrystalline substances. As part of a preliminary program of investigating the inhomogeneities in biological membranes which exhibit pore structures of appropriate dimensions suitable for studies using the small-angle scattering of X-rays, we have examined the angular distribution of the scattered intensity of a resin matrix which has an estimated specific surface area of tens of square meters per cubic centimeter.4 It should be noted that small-angle scattering of Xrays, being an interference technique, is the only known method capable of directly examining the amorphous structure of substances immersed in a fluid. Such a technique should become a useful tool for studying the pore structures of some of the biological membranes in their natural environment.
Experimental Methods The Amberlyst 15,4 supplied in a toluenesaturated form, was converted to the water-saturated form, dried, and sieved. The fraction of resin beads from 0.35 to 0.85 mm in diameter was taken for the experiments. Deionized distilled water and reagent grade solvents were used. The X-ray sample cell, 0.318 cm thick, had windows of 0.0025-cm thick mica held in place with Dow Corning (Q2-0046) aerospace sealant. A vial containing a small amount of the dry resin was filled with solvent in vacuo. Care was taken to avoid possible resin damage due to shock swelling. The solvent-saturated resin and excess solvent were then transferred to the cell with a syringe and needle. X-Ray measurements were performed with a Kratky camerab using Cu K a radiation. The details have been described elsewhere.6 The observed scattered intensity was corrected for background scattering by subtracting from the meas-
ured intensity the product of the scattering from an empty cell and the sample transmission. The solventcorrected points refer to those which have taken into account the scattering due to immersion solvents. Infinite slit-length collimation corrections were accomplished with the aid of an IBM 7040 computer.’
Results and Discussion The scattering results have been summarized in Figure 1. Contributions made by scattering from the immersion fluid were estimated to be small except at relatively large angles, as shown by the solventcorrected points in Figure 1 for Amberlyst 15 immersed in water and in methanol. The small-angle X-ray scattering technique has been applied to catalysts,* porous glassesl9 and synthetic zeolites.’O For characterizing ionic species within the resin matrix, measurements in the angular range beyond 0.03 radian are important. There one may consider the ionic species as frozen-in suspensions of strong scatterers within the resin phase and the particle scattering theory of Guiniel.8 becomes appropriate. This part of the scattering curve has been deemphasized in the present investigation. Instead, we are concerned with the scattering at very small angles. As we have limited the macroscopic size of those spherical resin beads to approximately 0.35-0.85 mm in diameter, both the degree of compaction and the macroscopic size of the beads become unimportant. Only inhomogeneities in the resin matrix contribute substantially to the scattering within the angular range of our investigation. Here, the scattered intensity, I , for a random two-phase system may be represented by the relationg
where V is the scattering volume, k is 2r/X with X the X-ray wavelength, s is 2 sin (0/2) with 0 the scattering angle between the direction of propagation of the (1) Courtesy of Rohm and Haas Co., Philadelphia, Pa. (2) We wish to make acknowledgment to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. (3) Alfred P. Sloan Research Fellow. (4) R. Kunin, E. F. Meitzner, J. A. Oline, 5. A. Fisher, and N. Frisch, I d . Eng. Chem. Prod. Res. Develop., 1, 140 (1962). (5) 0. Kratky and Z. Skala, 2. Elektrochem., 62, 73 (1958). (6) B. Chu, J . Chem. Phys., 42, 426 (1965). (7) B. Chu and D. M. Tan Creti, Acta Cryst., 18, 1083 (1965). (8) A. Guinier and G. Fournet, “Small-Angle Scattering of X-Rays,” John Wiley and Sons, Inc., New York, N. Y., 1955. (9) P. Debye and H. Brumberger, J . Phy8. Chem., 61, 1623 (1957). (10) P. A. Howell, ibid., 64, 364 (1960).
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