302
BARAT,GILLES,RICKEL, AND LESIGNE
Pulsed RadioIysis and Flash Photolysis of Iodates in Aqueous Solution by F. Barat, L. Gilles,* B. Hickel, and B. Lesigne Dhpartement de Recherche et Analyse, Centre #Etudes Nuclcjaires de ,Saclay, 91-Gif sur Yvette, France (Received January $6, 1071) Publication costs assisted by the French Atomic Energy Commission
The pulsed radiolysis and flash photolysis of aqueous solutions of the iodate ion have been investigated. In both cases, at neutral pH, an absorption band attributed to 1 0 2 is observed with a maximum at 480490 nm: erso(I02) = 800 i 100 M-l cm-l. This transition disappears according a second-order process with 2k = (3.6 f 0.8) X lo9 M-' sec-l. Another absorption is also observed that grows in at wavelengths less than 320 nm (band C); in the pulsed radiolysis of 0.1 M 1 0 3 - it decays with second-order kinetics and 2k/ezso = 5.7 X lo6 cm sec-'. Furthermore, pulsed radiolysis gives rise in neutral media to a second absorption band (band R) centered around 360 nm, formed by the reaction of OH radicals, which is assigned to 1 0 3 . A t the same wavelength in alkaline media we have observed another transient species, tentatively attributed to 1042-, whose molar extinction coefficient would be about 2600 M-1 cm-1.
Introduction The results of the numerous studies of the radiolysis of crystalline halates have been discussed in a recent paper by Boyd and Brom7n.l Investigations of the radiolysis and photolysis of aqueous solutions of the same ions are less advanced,2-8 and with regard to the 1 0 3 - ion in paiticular, a disagreement in the attribution of one absorption band has been noted between Amichai and Treinin's work6ngand a preliminary flash photolytic study.' The aim of this paper is to point out the differences which exist between our results and those of previous studies and which lead to different conclusions. I n addition, some new results relative to alkaline media are presented. Experimental Section 1. Pulsed Radiolysis, The irradiations were performed with electrons whose energy spectrum showed a maximum at 1.8 MeV. The electrons were delivered by an electron gun, Febetron 707, in pulses of halfwidth 27 nsec with a peak intensity of 4000 A using a magnetic field of 4000 G. The 1 cm deep cells used are made of high-purity silica (Spectrosil). The optical path through the cell is 2.2 cm long. The energy deposition in the solution varies with depth in the cell, which results in a concentration gradient of the transient species formed: the effects of a given additive described later were always carried out under identical experimental conditions. To vary the dose and the effective duration of the electronic pulse in the irradiated solution (measured by observing the Cerenkov emission) two procedures have been simultaneously used: aluminum plates, 5 rnm thick, with holes of different diameters were placed in front of the electron exit window; the magnetic focusing field was varied from 0.2 to 0.4 Wb/m2. The Journal of Physical Chemistry, Vol. 76,No. 5, 1978
The mean dose per pulse, measured at half-depth of the irradiation cells, ranged from 6 to 150 krads. Doses were measured using 0.1 M ferrocyanide aqueous solution saturated with N20 taking E440 nm(Fe(CN)eS-S600 M-I cm-' and GOH G,,,- = 5.5. When the monitoring light source (Osram Xe lamp, 450 W) was intensified during 1 msec,1° an MQ4 Zeiss Monochromator coupled with 150 CVP (Radiotechnique) and 1 P28 (RCA) photomultipliers was employed. When this source was used without pulsing, the analyzing light was modulated by a rotating disk (100 Hz) to prevent photomultiplier saturation; the illumination time of the photocathode was about 100 psec/cycle. For the studies in the infrared region a grating monochromator (Huet, Type M25) or interference filters (Balzers) were used. The results described were obtained with deaerated solutions and are concerned with single-pulse effects only. 2. Flash Photolysis. The flash photolysis setup has already been described elsewhere. The transient species are formed at micromolar concentrations under a flash of 1300 J with a 4-psec half-width.
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(1) G. E. Boyd and L. C. Brown, J . Phys. Chem,, 74, 3490 (1970). (2) N. K. Bridge and M. 8. Matheson, ibid., 64, 1280 (1960). (3) 0. Amichai and A. Treinin, Chem. Phys. Lett., 3 , 611 (1969). (4) 0.Amichai, G. Czapski, and A. Treinin, Isr. J . Chem., 7, 351 (1969). (5) 0. Amichai and A. Treinin, J . Phys. Chem., 74, 3670 (1970). (6) 0 . Amichai and A. Treinin, ibid., 74, 830 (1970). (7) F. Barat, L. Gilles, B. Hickel, and J. Sutton, Chem. Commun., 1485 (1969). (8) F. Barat, L. Gilles, B. Hickel, and E. Lesigne, ibid., 847 (1971). (9) 0. Amichai and A. Treinin, J . Chem. Phys., 53, 444 (1970). (10) F. Barat, L. Gilles, B. Hickel, and E. Lesigne, Report CEA-N1438. (11) F . Barat, L. Gilles, B. Hickel, and J. Sutton, J . Chem. SOC.A , 1982 (1970).
PULSED
RADIOLYSIS AND FLASH PHOTOLYSIS
OF
303
IODATES
1
3. Pyoducts. The solutions of potassium iodate (Prolabo RP ; Analar, RIerck) were prepared with triply distilled water and then deaerated by argon (N-55, Air Liquide) or nit rous oxide (N-40, Air Liquide) bubbling. The alcohols-ethanol, 2-propanol, and teyt-butyl nlcohol-and the sodium hydroxide were RIerck chemicals used without further purification.
Results I . Pulsed Radiolysis. A . Results Obtained in N e u tral Media. FOTa dose of 6' loads the absorption spectrum between 280 and GOO nm of a lo-' Ai deaerated iodate solution is presented in Figure 1. Three absorption bands are observed: the first (A) centered around 480 nm is formed immediately (100 nsec) after the pulse and remains approximately constant during 1 psec, the second (B) centered around 300 nni reaches its maximum intensity after 1.3 psec, and the third (C) observed at wavelengths less than 320 nm increases continuously in the uv region and is maximal after 200 nsec. At [IO3-] = 10+ A4 the same shape of the spectrum is observed (Figure 2), and if the solution is saturated with nitrous oxide, the intensity of band A decreases by about SO% whereas the intensity of band B notably grows in. For 2 X [A [IOa-] 10-2 A4 an intense absorption was observed during and immediately after the pulse in the 500-760-nm wavelength region. This band had the characteristic shape of the solvated electron absorption and disappeared rapidly, between 200
