L. DOGLIOTTI AND E. HAYON
1800
Flash Photolysis Study of Sulfite, Thiocyanate, and Thiosulfate Ions in Solution by L. Dogliotti and E. Hayon Pioneering Research Laboratory, (Receized December 14, 1967)
U.S . A r m y n’atick Laboratories, ,Vatick, -?!fassachusetts
01 760
The primary photolytic processes, as observed by the technique of flash photolysis, were studied for the sulfur polyvalent anions SOS2-, CNS-, and S20a2- in aqueous solution. Sulfite ions were found to produce two hv
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eaq-. X shorttransient species, resulting from the primary photolytic process: S 0 3 2 - . H 2 0+ SO3lived species apparently formed by the addition of eaq- to sulfite ions and tentatively assigned to HS032-,,A,( 720 m,u) decays by first-order process, k = (1.9 i. 0.3) X lo4sec-l, and a longer lived species assigned to 803(A, -275 mp) decays by second-order process, 2k/e = 1.5 X 106 a t 275 mp. Thiocyanate ions decompose
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on photolysis by a t least two processes: CNS-aHzO CNS eaq- and CNSCNS. A transient spectrum, due to (CKS),- radical anions, was found absorbing a ,t, ,A 485 mp, with species decaying by a firstorder process in air-free solutions and by a second-order process in the presence of oxygen. Thiosulfate ions hv
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decompose on photolysis by two processes: SzOa2-.H2O -+ S203- eaq- and SzOa2-.Hi0-+ S202OH OH-. The OH radicals produced react with S20a2- to form S203-radicals. A transient optical spectrum with 380 mp assigned to Sz03- decays by a second-order process in the absence of O2 and by a first-order a A,, 280 mp and decays by a second-order process in the presence process in the presence of 02. SzOz- has a A,, or absence of oxygen. The CTTS absorption bands of the three sulfur anions have been determined. The esr spectra of the radicals produced on uv irradiation of SOS2-, CNS-, and Szos2- ices a t 77°K are given.
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The ultraviolet absorption spectra of the sulfur polyvalent anions SO?-, CSS-, and Sz032-are known, but, except for thiosulfate anions, their absorption spectra have not been characterized and their photochemistry has not been studied for many years. Albu and Goldfingerl examined the absorption spectrum of the sulfite ion on the basis of the FranckHaber theory of the electron-affinity spectrum of ions and showed that the S032- spectrum has a long wavelength limit of 2660 and corresponds to the process S032-.H20 -% SO3H OH-. Haber and Wansbrough-Jones2 studied the photochemistry of sulfite ions in the absence of oxygen and postulated the reactions H H + H, and SO3SO3- --t 5 ~ 0 6 ~ taking place under their experimental conditions. However, they found the quantum yield of dithionate to be 0.07 at pH 7.0 and suggested the possibility of the H + HSO3- occurring. “back reaction’’ SO3Backstrom3 studied the effects of inhibitors, particularly alcohols, on the photochemical oxidation of sulfite ions in solution. Thiocyanate ions were found4 to decompose with light in the wavelength range 1800-3000 Lk with the formation of sulfur. The reaction was postulated to be reversed in the dark. Matheson, et u Z . , ~ found evidence for the formation of solvated electrons in the flash photolysis of aqueous solutions of CNS- ions, and Gusarsky and Treinine interpreted the absorption band of thiocyanate ions a t -220 mp as due to a chargetransfer-to-solvent (CTTS) spectra.
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The Journal of Physical Chemistry
The spectrum of S203*-in solution has an intense absorption peak at about 215 mp78* and has been assigned to a CTTS type of The limiting (maximum) quantum yield of decomposition of the CTTS band was foundlo to be 2250 A, and no transient species were produced on flash photolysis of M sodium bisulfite or M sodium sulfite at pH 1.1-2.0, in air or Yz-saturated solutions. Instead of using H + ions as an electron scavenger, nitrate ions were added to a M so32-solution. I n Wz-saturated solution containing 2 X M E(N03, no transient was observed at 720 mp. However, using lower concentrations of N03- ions, it was possible to form some HS032- and observe its decay. At 720 mp, the transient was found to decay by a pseudo-first-order process dependent on [?TO,-], from which a k(HS03'NO3-) = 109 M-1 sec-1 was calculated. I n addition, from the competition between reaction 2 and Ic(e,,Nos-), it was estimated that k(e,,503'-) 6 2 X lo6M-l sec-'. Pulse radiolysis of aqueous solutions of sulfite ions was examined by Adams, et u Z . ' ~ J ~ In a first pub-
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(13) M. Anbar and P. Neta, I n t . J . A p p l . Radiat. Isotopes, 18, 493 (1967). (14)G. E. Adams, J. W. Boag, and B. D. Michael, Proc. Chem SoC., 411 (1964).
1803 l i ~ a t i o n 'a~transient spectrum with A, ~ 3 5 mp 0 was reported on irradiation of neutral aqueous solutions in the presence of oxygen. This spectrum was suggested to 503'SOabe formed from the reaction OH OH- and was assigned to the SOa- radical. I n a later work, Adams, et al.,15 found no transient on pulse radiolysis of 0.5 M sulfite ions in presence of 2 X M N20 gas. In the presence of oxygen, they now observed a trmsient with an absorption maximum at 750 mp but with no transient peaking at -350 mp, indicating that the transient has a reducing species (eaq- or 0,-) as a precursor. Their later results'6 are in accord with the results presented above for the species absorbing at 720 mp in the flash photolysis of SO3'- ions. The difference in A,, is thought to be due to the use16 of flash spectrography in deriving the absorption spectrum of the transient. Since k(e0, -+ 02-) = 1.9 X 1O1O M-l sec-l and k(eS032-) was estimated as 2 X J4-l sec-l, all the electrons produced in reaction 1 must react with oxlygen under the experimental conditions described above. The formation of the transient on photolysis of air-saturated solutions is then probably the result of the reaction 0 2 803'- 2 HS03'- 3.
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