J . Phys. Chem. 1990, 94, 2191-2198
2191
Photooxidation of SO,?- in Aqueous Solution Ursula Deister and Peter Warneck* Max-Planck-Institut fur Chemie (Otto-Hahn-Institut), Mainz, FRG (Received: May 5, 1989; I n Final Form: August 15. 1989)
Aqueous solutions of sodium sulfite, with or without additives, were exposed to 254-nm radiation, and the yields of products were determined. In the absence of oxygen the major products were dithionate and sulfate with a molar ratio of 0.5 0.08. From their total yield the quantum yield for the primary products, SO3- and e,, was determined as 0.85 0.04. In the presence of oxygen, dithionate was not observed and the formation of sulfate was extremely rapid, occurring with a quantum yield of about 500. The addition of benzene as a radical scavenger led to the formation of phenol. The further addition of tert-butyl alcohol, which reacts rapidly with OH but slowly with SOL, did not cause a reduction in the yield of phenol, indicating that the SO4- radical is the main chain carrier. When benzene was replaced by either ethanol or 2-propano1, the products were acetaldehyde and acetone, respectively, in addition to sulfate. From the product ratios [S042-]/ [carbonyl compound] as a function of the concentration ratio [S032-]/[alcohol],the rate constant for reaction 16, SO4- + SO3 SO3+ S042-,was derived by competition kinetics: kl6 = (5.5 A 1.5) X lo8 M-' s-l. The data also provided the branching ratio for the two channels of reaction 3: (a) SO; + S032- SO3- + SOS2-and (b) SO5-+ S032- SO4- SO:-. The value obtained was k 3 a / k 3 b = 0.41 0.25 or k3b/k3= 0.72 f 0.1 1. The reaction mechanism discussed involves also the reaction of peroxymonosulfate with sulfite. At pH 7-8 a rate constant of 350 A 90 M-I s-l was obtained for this reaction.
*
*
Introduction Sixty years has passed since Backstrom' first demonstrated the Occurrence of a chain reaction in the photooxidation of sulfite in aqueous solution. Although subsequent studies based on various photolysis"' and pulse radioly~is"'~techniques have greatly added to our knowledge of the process, the full details of the reaction mechanism have not yet been elucidated. The ultraviolet absorption spectrum of the SOj2-ion extends from about 265 nm toward shorter ~ a v e l e n g t h s . ~ In J ~this spectral region the primary photochemical process consists of charge transfer to which leads to the formation of a sulfite radical, SO3-, and a hydrated electron. In the presence of oxygen, the sulfite radical is usually a s ~ u m e dto ~ ,associate ~ with O 2 to form the peroxymonosulfate radical, SO5-, whereas the hydrated electron is well-known to form the 02-radical anion.16 The existence of the SO< radical is well-established by its ultraviolet absorption5v6 and electron spin resonance ~ p e c t r a . ~ Recent J~ pulse radiolysis studies also have better defined its redox potential and reactivity.l23I3 The evidence for the existence of the SO< radical is weaker. The absorption spectrum assigned to it in the 250-300-nm (1) BackstrBm, H. L. J. J . Am. Chem. SOC.1927, 49, 1460-1472. (2) Alyea, H. N.; Backstrom. H. L. J. J. Am. Chem. SOC.1929, 51, 90-109. (3) Backstrom, H. L. J. Z . Phys. Chem. 1934, 258, 122-138. (4) Haber, F.; Wansbrough-Jones, 0. H. Z. Phys. Chem. 1932, 188, 103-123. _.
Dogliotti, L.; Hayon, E. J . Phys. Chem. 1968, 72, 1800-1807. Hayon, E.; Treinin, A.; Wilf, J. J . Phys. Chem. 1972, 94, 47-57. Chawla, 0. P.; Arthur, N. L.; Fessenden, R. W. J . Phys. Chem. 1973, 77,-772-716. (8) Kontges, H. Untersuchungen iiber die Radiolyse des Sulfits und des Selenits in wassriger Losung. Ph.D. Thesis, Technische Universitat, Berlin, 1971. .. (9) Zagorski, Z. P.; Shested, K.; Nielsen, S. 0. J . Phys. Chem. 1971, 75, 35 10-35 17. (10) Eriksen, T. E. J . Chem. SOC.,Faraday Trans. 1 1974, 70,208-215. (1 1) Sadat-Shafai, T.; Pucheault, J.; Ferradini, C. Radiat. Phys. Chem. 1980, 17, 283-288. (12) Huie, R. E.; Neta, P. J . Phys. Chem. 1984, 88, 5665-5669. (13) Huie, R. E.; Neta, P. Chem.-Bio[. Interact. 1985, 53, 233-238. (14) Huie, R. E.; Neta, P. Atmos. Enuiron. 1987, 21, 1743-1747. (15) Deister, U.; Neeb, R.; Helas, G.; Warneck, P. J . Phys. Chem. 1986, 90, 3213-3217. (16) Anbar, M.; Bambenek, M.; Ross, A. B. Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution: 1 . Hydrated Electron; 2. Hydrogen Afom; Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 43 and 51; U.S.Government Printing Office: Washington, D.C., 1973 and 1975. (17) Norman, R.0. C.; Storey, P. M. J. Chem. SOC.81971, 1009-1013.
0022-3654/90/2094-2191$02.50/0
-
-
*
-
+
TABLE I: Some Conceivable Reactions of the Peroxymonosulfate Radical" reaction no. reaction ref 1 SO3- + O2 SO5-* SO53, 4, 6, 12 2a SO
