J. Phys. Chem. 1995, 99, 5214-5221
5214
Photodecomposition of Iron(II1) Hydroxo and Sulfato Complexes in Aqueous Solution: Wavelength Dependence of OH and sod- Quantum Yields Heinz-Jiirgen Benkelberg and Peter Warneck" Max-Planck-Institut fur Chemie, 55020 Mainz, Germany Received: September I , 1994; In Final Form: January 16, 1995@
Effective quantum yields for the production of OH radicals from iron(II1) hydroxo species in aqueous solution were measured in the wavelength range 280-370 nm at pH 2 and 3 under aerated conditions and at pH 3 in argon-saturated solutions of femc perchlorate. 2-Propanol was used as radical scavenger; the yield of product acetone was measured. Spectra of femc perchlorate solutions were taken and evaluated to determine absorption coefficients at various pH values and to calculate K4, the first hydrolysis constant of Fe(III), at 46 wavelengths in an attempt to obtain evidence for the presence of Fe(OH)z+ in addition to FeOH2+ in the spectra. Whereas the influence of the dimer Fe2(0H)z4+ was clearly evident, no indication for Fe(OH),+ was found. Values obtained for the first hydrolysis constant K4, corrected to 25 OC,were (2.76 f 0.19) x M and (3.63 f 0.15) x M at ionic strengths of 0.1 and 0.05, respectively, in excellent agreement with literature values. These data served to derive absolute OH quantum yields for FeOH2+(H20)5, which were found to rise from 0.07 at 370 nm to 0.31 at 280 nm. At wavelengths below 300 nm the OH production from Fe3+(Hz0)6 contributed markedly with an estimated quantum yield of ~ 0 . 0 5 .Quantum yields of S 0 4 - from the FeS04+ complex were determined in solutions of femc perchlorate, to which of sodium sulfate was added at pH 2. Benzene was used as scavenger, and sufficient 2-methyl-2-propanol was added to quench OH radicals resulting from iron(II1) hydroxo species. Absolute S04- quantum yields were found to rise from 1.6 x at 350 nm toward 7.9 x at 280 nm.
Introduction In examining the effects of transition metals on the chemistry of continental liquid water clouds, Graedel et al.1-3 called attention to the potential role of iron(II1) complexes as a photolytic source of radicals in aqueous solution. Iron is abundant in common soils, where it occurs mainly in the form of insoluble oxides and hydroxides? Its presence in cloud water results from the incorporation of soil-derived particles. In the more acidic environment of atmospheric droplets, femc oxides and hydroxides are partly soluble, and in the pH range below 4, one expects trivalent iron to be present largely in ionic form in solution. For rain and fog waters this expectation has been confirmed in several recent The survey of Weschler et aL2 indicates that under these conditions FeOH2+and FeSO4+ are major ionic Fe(II1) species. Both complexes feature wellknown ultraviolet charge transfer ~ p e c t r a overlapping ~.~ with solar radiation in the 290-400 nm wavelength region. If the naked Fe3+ ion is included, the associated photochemical processes are
+ hv - FeZf(H,O), + OH + H+ FeOH2'(H,0), + hv - Fe2+(H,0)5 + OH FeSO,+(H,O), + hv - Fe2+(H20),+ SO4-
Fe3+(H20),
(Rl) (R2) (R3)
We have measured quantum yields for processes R2 and R3 as a function of wavelength using radical scavenger techniques. Whereas process R3 appears to have not been studied so far, the generation of radicals by process R2 has been amply demonstrated (for reviews see refs 10 and 11). The majority of studies involved radiation at 313 nm from mercury lamps; at this wavelength quantum yields in the range 0.14-0.19 have @
Abstract published in Advance ACS Absrrucrs, March 1, 1995.
0022-365419512099-5214$09.00/0
been reported."-15 David and David16 have used radiation of 340 nm wavelength and obtained a quantum yield of 0.105. Faust and Hoignt" have also measured the quantum yield at 360 nm and found it to be 0.017. In addition they determined the photolysis rate in full June sunlight as 6.3 x s-l, in the presence of radical scavenger. Most of these data represent effective quantum yields that do not distinguish between FeOH2+ and other light-absorbing femc ion species. In the pH range 5 4 at least four different Fe(II1) ions capable of photochemical OH production coexist in aqueous solution: Fe3+, FeOHZf, Fe(OH),+, and the dimer Fe2(0H)z4+. Their concentrations are governed by the equilibria
+
Fe3+ H,O = FeOH2+
+ H+
log,,K4 = -2.19 (R4)
+
Fe3+ 2H,O = Fe(0H);
+ 2H+
log,& = -5.70 (R5)
2Fe3+
+ 2H,O = Fe,(OH)? + 2H'
log,,K6 = -2.90
(R6)
Here, the water molecules in the hydration spheres of the ions are not shown. The ionic distribution established by the equilibria primarily depends on the pH of the solution but in the case of the dimer also on total iron concentration. Values for the equilibrium constants, shown for infinite dilution, were taken from the compilation of Smith and Martell.I7 From the beginning of this work there was some doubt about the validity of the value for K5, because there was little evidence for Fe(OH)2+ in the published absorption ~ p e ~ t r a in~ -the~ ~ ~ ~ - ~ ~ wavelength range 270-350 nm, at pH 5 3, and there was no reason to assume that this species should be a weaker absorber than FeOH2+. Weschler et al., have used the data given by 0 1995 American Chemical Society
Wavelength Dependence of OH and SO4- Quantum Yields Smith and Martell in an attempt to derive absorption coefficients for Fe(OH)2+ by subtracting from measured absorption spectra for iron(II1) solutions at pH 3 and 4 the contributions due to Fe3+ and FeOH2+, whose absorption strengths were essentially known. They obtained Fe(OH)2+ absorption coefficients rather similar to those for FeOH2+. Their results would be questionable, however, if the true value for K5 differed from that given by Smith and Martel1.l’ Faust and HoignC” have pointed out that the free energy data contained in the widely used NBS Thermochemical Tables21 lead to K5 = 7.7 x lo-*, a value about 25 times smaller than that shown above. In order to deal with this problem we have also analyzed absorption spectra taken at various hydrogen ion concentrations in a search for evidence of the presence of Fe(OH)2+.
J. Phys. Chem., Vol. 99, No. 14, I995 5215
................ -.-.-
t
--
1@ [H’] 26.70 12.60 6.18 3.10 1.55
-
Experimental Section Absorption spectra of solutions containing iron(II1) salts were obtained with a double beam spectrophotometer against a blank of pure water. The spectral resolution in this case was 2 nm at half-width, and matched quartz cells with an optical path length of 1 cm were used. The photolysis setup for the measurement of product quantum yields combined a 150 W xenon arc lamp with an j72 grating monochromator, whose slits were adjusted so that the spectral resolution was 4 nm at full peak half-width. The light emerging from the monochromator was refocused by means of a quartz lens to pass first through the photolysis cell 200 250 300 350 400 and then further onto a calibrated thermopile. The photon fluxes calculated from the thermopile output were approximately 1015 Wavelength I nm photonsk The photolysis cell was a 2 cm diameter quartz Figure 1. Spectra of aqueous iron(1II) perchlorate solutions, I = 0.05, cylinder of 1 cm depth sealed with flat quartz windows. Its at different hydrogen ion concentrations calculated from the amounts volume including stoppered filling port was 2.95 cm3. Solutions of acid used. Note two isosbestic points at 224 and 272 nm. Also were prepared in volumetric flasks and transferred to the cell, shown is a spectrum of FeOH*+ derived from these data. and the filling port was then capped to prevent evaporative losses. of a solution did not change. Figure 1 shows a set of absorption Chemicals were used as received from commercial suppliers, spectra for different pH values (13) at 22 “C and a constant and aqueous solutions were made up with deionized (Milli-Q, ionic strength I = 0.05. The existence of two isosbestic points “organic free”) water. Most solutions contained about 2 x (unchanging E) at a wavelength of 224 and 272 nm should be M of total iron(II1). They were prepared from femc perchlorate, noted. The second of these has been observed p r e v i ~ u s l y ~ ~ J ~ ~ ~ ~ and perchloric acid was employed to adjust the pH to a fixed at the same wavelength (272-274 nm). Milburn and Vosvalue in the range 1-3. The pH was measured with a glass have pointed out that this isosbestic point disappears at electrode-voltmeter combination calibrated with available higher Fe(III) concentrations due to the formation of polynuclear standards. In the experiments designed to determine equilibrium species. The total iron(1II) concentration used here was 4.6 x constants sodium perchlorate was added to achieve a predeterM, which was somewhat higher than in the other mined constant ionic strength. For the determination of OH experiments. The total absorbance at a given wavelength is quantum yields 2 x M of 2-propanol was added as a due to four species radical scavenger and the production of acetone monitored. The quantum yield of sulfate radicals was determined by the addition of 2 x 1OP3 M benzene as radical scavenger and the observation e[Fe(III)],, = e,[Fe3+] E,[F~OH*+] of phenol as product. In this case it was necessary to discriminate against the simultaneous production of OH radicals by the addition of excess 2-methyl-2-propan01, which reacts with where the absorption coefficient €1 refers to Fe3+,€2 to FeOH2+, OH much more rapidly than with sulfate radicals.22 and so forth. Under the present experimental conditions, the Product analyses were carried out by means of highmainly arises from the first two terms, whereas performance liquid chromatography as previously d e s ~ r i b e d . ~ ~ . ~absorption ~ contributions from Fe(OH)2+ and [Fe2(0H)z4+1are small and When the product was acetone, it was reacted with 2,4may be neglected to a first approximation. Any influence of dinitrophenylhydrazine to form the corresponding hydrazone, the last species would then show up as a trend in the K4 values which was then subjected to chromatographic separation and as a function of wavelength. With this assumption and K4 = detected by optical absorption at 360 nm wavelength. Phenol [Hf][FeOH2+]/[Fe3+]one obtains was directly detected by optical absorption at 270 nm wavelength after separation from benzene on the chromatographic column.
+
Results Spectrophotometry. Fe(II1) solutions at pH 5 3 were stable in that spectra taken 12 h, 3 days, or 3 months after preparation
+
K4 was determined by two methods. One procedure, which is due to Richards and Sykes,26made use of the following equation derived from eq 2
5216 J. Phys. Chem., Vol. 99, No. 14, 1995
Benkelberg and Wameck
([H+lo - [H+I>/(E- €0) = (K4
+ [H+l&l + [H+]/&)/(E2 - €1)
(3) O
where [H+]o is a fixed hydrogen ion concentration, [H+] is varied, and EO is the effective absorption coefficient when [H+] 3.0 . = [H+]o. The right-hand side of eq 3 linearly depends on hydrogen ion concentration, so that a plot of ([H+Io - [H+])/(c 2.8 - €0) versus [H+] should yield a straight line yielding K4 as the ratio of intercept to slope. This method was applied to the wavelength region 280-370 nm at 2 nm intervals. At wavelengths above 330 nm, where 61
