J. Chem. Research (S), 1997, 54–55† Flash-photolysis Study of Potassium Hydroxide Solutions† M�onica C. Gonzalez* and Daniel O. M�artire Instituto de Investigaciones Fisicoqu�ýmicas Te�oricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina Flash-photolysis experiments of KOH solutions ([KOH]a0.03 M) are carried out in hydrogen- and air-saturated solutions; the formation of hydrated electrons is observed in hydrogen-saturated solutions, but the formation of O2 .µ and O3 .µ is observed in the presence of molecular oxygen and a reaction mechanism supported by kinetic computer simulations of the results is proposed.The dissociation threshold energy of water, yielding H. atoms and HO. radicals, lies in the region 6.41–6.71 eV.1 Accordingly, photodissociation of water [eqn. (1a)]2 has been reported to occur with quantum efficiencies of the order of 0.3 at 184.9 nm, 0.7 at 147.9 nm, and approaching 1 at 123.6 nm.In addition to eqn. (1a), the photolytic formation of eµ aq from eqn. (1b) has already been demonstrated in flash-photolysis experiments for photons of 6.54 eV.6 Experiments using the formation of fluoride ions from SF6 as a specific monitor for eµ aq indicate that photolysis of water in the wavelength range 175–200 nm produces hydrated electrons with a constant quantum yield of 0.05�0.01 at pH 4–9,3,4 increasing slowly with the irradiation energy.5 The observed independence of the quantum yields on pH was suggested as an indication that water, but not HOµ, is a source of eµ aq.An increase in the quantum yield above pH 9 was reported to be due to the reaction between H. and HOµ yielding eµ aq [eqn. (10)]. However, photoionization of OHµ, eqn. (2), has been proposed as the main reaction in H2-saturated KOH solutions.3 Photolysis of alkaline H2-saturated solutions has been reported as a nearly ideal source of eµ aq, since the reactions [eqns.(15) and (10)] add to the yields of hydrated electron produced from eqns. (1) and (2). However, no studies have been reported on the photolysis kinetics in oxygen- or airsaturated KOH solutions. Here we report the results of our flash-photolysis studies in the presence of dissolved oxygen, which turned out to be a rather clean method of ozonide radical generation. Experimental KOH (Mallinckrodt) was used without further purification.Distilled water was passed through a Millipore system and deaerated in order to avoid dissolution of carbon dioxide. Flash-photolysis experiments were carried out in a conventional apparatus (Xenon Co. model 720C) with modified optics and electronics.7 For observation wavelengths higher than 600 mm, a 500 nm cut-off filter was placed in front of the monochromator in order to eliminate detection of harmonics. In those experiments where detection above 650 nm was necessary, high analysing lamp intensities were required due to the low sensitivity of the photomultiplier (PMT) at these wavelengths.Under these conditions, detection at lower wavelengths (higher PMT sensitivity) required amplification values almost below the limit of the recommended range of linear responsivity of the PMT. This may be the cause of the discrepancy between our spectrum and that reported, both of which are shown in Fig. 1. In order to detect the formation of hydrated electrons, aqueous solutions of KOH (pHa12.5) degassed by three freeze-pump-thaw cycles were saturated with almost 1 atm of H2 and the cell was sealed.The solutions were irradiated for 10–20 min with a continuous mercury lamp, prior to the flash-photolysis experiments, in order to eliminate traces of oxygen and/or organic material.3 Photolysis studies of KOH in the presence of dissolved oxygen were performed in synthetic air-saturated solutions in order to avoid CO2 dissolution.The presence of CO3 2µ in the solutions was checked by flash irradiation experiments at 600 nm (lmax of CO3 .µ absorption8). Results and Discussion The absorption traces observed immediately after pulsed irradiation of H2-saturated KOH solutions showed a decay lifetime of the order of 0.5 ms and an absorption spectrum with a maximum around 715 nm, Fig. 1. Both the absorption spectrum and the decay lifetime are in agreement with reported values for the hydrated electron,3,9 thus indicating hydrated electron formation under our experimental conditions.On the other hand, the absorption traces observed after flash irradiation of KOH solutions saturated with synthetic air show a complex dependence on wavelength in the range 260–500 nm, indicating the absorption of more than one species. Fig. 2 shows the traces observed at 440 and 270 nm (inset), respectively. In the presence of molecular oxygen, solvated electrons and H. atoms are known to be efficiently scavenged by molecular oxygen, yielding O2 .µ/HO2 ., eqns.(4) and (11), respectively. Moreover, in strongly alkaline solutions, OH. radicals formed in the primary photochemical steps efficiently react with OHµ ions yielding O.µ, eqn. (13), which in the presence of molecular oxygen yields O3 .µ radical ions in a reversible reaction [eqns. (20) and (24)]. In fact, the absorption spectrum observed for irradiation wavelengths a300 nm agrees with that reported for O3 .µ10–15 and the build-up signal observed at 270 nm can be assigned to the absorption of O2 .µ, which is known to be relatively highly stable in alkaline solutions (free of catalytic amounts of metal impurities which may catalyse its decomposition).16 The participation of reactive HO., H.and eµ aq during flash photolysis of KOH solutions resembles the radiation chemistry of aqueous solutions and so the extensive information reported on these systems can be used. A set of well established reactions reported in the literature and involving the species present in the irradiated system are listed in Table 1.17–19 Those reactions whose participation was considered to 54 J.CHEM. RESEARCH (S), 1996 *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Fig. 1 Absorption spectrum of eµ aq.Data taken from ref. 3 represented by ——— and experimental data obtained in this work represented by s (uncorrected spectrum, refer to text for details)be negligible under our experimental conditions were omitted for simplicity. A detailed discussion on some of these reactions and on the establishment of HO./O.µ and O3 .µ/O.µ equilibria conditions can be found elsewhere.20 An ab initio computer program based on the numerical resolution of the differential equations system by the Runge Kutta method was used in order to simulate the reaction kinetics.20 The program considers the flash emission as a delta function, producing HO.radicals, hydrogen atoms and hydrated electrons. The initial concentrations of these transient species present after the pulse of light were handled as input parameters. According to eqns. (1a,b) and (2), the stoichiometry condition [HO.]0=[H.]0+[eµ aq]0 was considered. The simulation concentration profiles of O2 .µ and O3 .µ are not sensitive to the [eµ aq]0:[H.]0 ratio used as input parameter, as suggested by the participation of the efficient reaction [eqn.(10)] readily converting H. atoms into hydrated electrons. Those [HO.]0 values which best fitted the experimental concentrations of O3 .µ were used for the simulations. The simulated O2 .µ and O3 .µ concentration profiles obtained at 270 and 440 nm, respectively, are shown in Fig. 2. An acceptable agreement between simulated and experimental profiles supports the proposed mechanism.Any low-intensity emission of the flash lamp below 200 nm is efficiently absorbed by the water contained in the 0.8 cm pathlength thermostat jacket. Consequently, H2O photodissociation [eand 1(b)] is expected to be negligible and photoionization of HOµ ions [eqn. (2)] should be the main photochemical reaction in the present system. M. C. G. and D. O. M. are research members of CONICET and CICPBA (Argentina), respectively.This research was partially supported by the grants number A-13218/1-000062 and A-13359/1-000084 of Fundaci�on Antorchas (Argentina). Received, 12th September 1996; Accepted, 21st October 1996 Paper E/6/06288A References 1 D. N. Nikogosyan and H. G�orner, J. Photochem. Photobiol. B: Biol., 1992, 13, 219. 2 J. Barret and J. H. Baxendale, Trans. Faraday Soc., 1960, 56, 37; U. Sokolov and G. Stein, J. Chem. Phys., 1965, 44, 2189, 3329; Radiation Chemistry in Aqueous Systems, ed.G. Stein, Weizmann Science Press, Jerusalem, 1968, pp. 83–89, and references cited therein. 3 E. J. Hart and M. Anbar, in The Hydrated Electron, Wiley–Interscience, New York, 1970; D. Phillips, in Photochemistry, The Chemical Society, London, 1973, vol. 4, part II, p. 366 and references cited therein. 4 K.-D. Asmus and J. H. Fendler, J. Phys. Chem., 1969, 73, 1583. 5 G. O. Schenck and N. Getoff, Proc. Int. Conf. Photochem., 1967, 2, 720. 6 J. W. Boyle, J. A. Ghormley, C.J. Hochanadel and J. F. Riley, J. Phys. Chem., 1969, 73, 2886. 7 E. San Rom�an, P. F. Aramend�ýa and H. J. Schumacher, An. Asoc. Qu�ým. Argent., 1980, 70, 887. 8 G. L. Hug, Nat. Stand. Ref. Data Ser. (U.S. Nat. Bur. Stand.), 1981, 69, 30 and references cited therein. 9 K. Schmidt and E. J. Hart, ACS Adv. Chem., 1968, 81, 267 and references cited therein. 10 K. Sehested, J. Holcman, E. Bjerbakke and E. J. Hart, J. Phys. Chem., 1982, 86, 2066. 11 R. E. B�uhler, J. Staehelin and J.Hoign�e, J. Phys. Chem., 1984, 88, 2560. 12 G. Czapski and L. M. Dorfman, J. Phys. Chem., 1964, 68, 1169. 13 G. E. Adams, J.W. Boag and B. D. Michael, Nature (London), 1965, 205, 898. 14 G.E. Adams, J.W. Boag and B.D. Michael, Proc. R. Soc. London, A, Math. Phys. Sci., 1966, 289, 321. 15 L. J. Heidt and B. R. Landl, J. Chem. Phys., 1964, 41, 176. 16 B. H. J. Bielski, Methods Enzymol., 1984, 105, 81; B. H. J. Bielski and R. L. Araudi, Anal. Biochem., 1983, 133, 170. 17 P. Neta, R.E. Nuie and A. B. Ross, J. Phys. Chem. Ref. Data, 1988, 17, 1027. 18 Fahataziz and A. B. Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solutions. Part II. Hydroxyl Radical and Perhydroxyl Radical and their Radical Ions. NSRDSNBS 59, National Bureau of Standards, Washington DC, 1977. 19 G. V. Buxton, C. L. Greenstock, W. P. Helman and A. B. Ross, J. Phys. Chem. Ref. Data 1988, 17, 513. 20 M. C. Gonzalez and D. O. M�artire, Int. J. Chem. Kin., in the press.J. CHEM. RESEARCH (S), 1996 55 Fig. 2 Experimental (---) and simulated (———) absorbance traces for absorption curves as obtained for O3 .µ decay (440 nm) and O2 .µ build-up at 270 nm (inset) in 1 M KOH containing 2.5Å10µ4 M O2. Optical path length 20 cm, e(O3 .µ)440 nm=1900 dm3 molµ1 cmµ18 and e(O2 .µ)270 nm=1360 dm3 molµ1 cmµ1 10 were used in the calculations Table 1 Important reactions taking place during UV irradiation of alkaline water Reaction Rate constant k/dm3 molµ1 sµ1 H2O+hvhHO.+H. (1a) H2O+hvhHO.+H++eµ aq (1b) HOµ+hvhHO.+eµ aq (2) eµ aq+eµ aq (+H2O)hH2+2HOµ (3) 5.5Å109 eµ aq+O2hO2 .µ (4) 1.9Å1010 eµ aq+H.hH2+HOµ (5) 2.5Å1010 eµ aq+HO.hHOµ (6) 3.0Å1010 eµ aq+O.µ (+H2O)h2HOµ (7) 2.2Å1010 H.+H.hH2 (8) 7.7Å109 H.+HO.hH2O (9) 7.0Å109 H.+OHµhH2O+eµ aq (10) 2.2Å107 H.+O2hHO2 . (11) 2.1Å1010 HO.+HO.hH2O2hH++HO2 µ (12) 5.5Å109 HO.+HOµhH2O+O.µ (13) 1.3Å1010 HO.+O.hHO2 µ (14) R2Å1010 HO.+H2hH2O+H. (15) 4.2Å107 HO.+HO2 µhHOµ+HO2 . (16) 7.5Å109 HO.+O2 .µhO2+HOµ (17) 8.0Å109 O.µ+H2OhHOµ+HO. (18) 1.8Å106 O.µ+O.µ (+H2O)hHO2 µ+HOµ (19) 8.0Å109 O.µ+O2hO3 .µ (20) 3.6Å109 O.µ+H2hHOµ+H. (21) 8.0Å107 O.µ+HO2 µhHOµ+O2 .µ (22) 4.0Å108 O.µ+O2 .µ+H2OhO2+2HOµ (23) 6.0Å108 O3 .µhO.µ+O2 (24) 3.6–6.0Å103a O.µ+O3 .µhO2 .µ+O2 .µ (25) 8.0Å108 HO.+O3 .µhH++2O2 .µ (26) 1.1Å1010 O2 .µ+O2 .µhO2+HOµ+HO2 µ (27) s102 O3 .µ+O2 .µ+H2Oh2HOµ+2O2 (28) 5.0Å104 aU