J . Phys. Chem. 1988, 92, 6278-6283
6278
The significantly larger value of K for glucose compared to those for 1-PrOH and ethanol might be attributed to the presence of different cations (K+ and Na') as well as different solution properties of these mixed solutions.
Acknowledgment. I am grateful to the Research and Higher
Degrees Committee of Bayero University for a research grant to purchase a UV-visible spectrophotometer and some research chemicals. Registry No. PS-, 61141-14-8; o-O-C6H4CO2G1u,115651-39-3; 1propanol. 7 1-23-8; D-(+)-glucose,50-99-7.
Product Quantum Yields for the 305-nm Photodecomposition of NO3- in Aqueous Solution Peter Warneck* and Christa Wurzinger Max-Planck-institut fur Chemie, Mainz, FRG (Received: December 7, 1987; In Final Form: April 14, 1988)
The radical scavenging technique with product analysis was employed to determine quantum yields for the two main processes ( 1 ) NO; + hv = NO2- + 0 and (2) NO; + H+ + hu = NO2 + OH in dilute, air-saturated solutions of sodium nitrate. Oxygen atoms were detected by their reaction with cyclopentene and the production of ethene. The quantum yield found The generation of OH radicals was confirmed by their reactions with benzene to give phenol was $(O) = (1.1 & 0.1) x and with 2-propanol to produce acetone. The second reaction was used to establish an OH quantum yield of 4(OH) = (9.2 i 0.4) X Nitrite was formed with a similar yield in the pH region 5-12, whereas at lower pH the yield declined. Formate as a scavenger converts OH radicals toward 0 < / H 0 2 radicals which produce H202. The yield of H202at pH 5.6 was lower than expected whereas that of NOT was higher. Reactions are postulated to explain this observation as well as the pH dependence of nitrite formation
Introduction The ultraviolet absorption spectrum of aqueous nitrate solutions features two bands with maxima at wavelengths near 200 and 300 nm. Photodecomposition of the NO3- ion occurs in both wavelength regions, but only the first has been studied in some detail with regard to photochemical effects (see Wagner et al.' for a review). The 300-nm band, which is weaker, has received much less emphasis. It is this band, however, that can absorb solar radiation reaching down to the Earth's surface (A > 295 nm) so that photolytic products resulting from the interaction may have some bearing upon the chemistry of atmospheric and terrestrial surface waters.* The nature of the products and the associated quantum yields thus are also of interest to the environmental sciences. Daniels et al.,3 who worked with concentrated nitrate solutions, suggested the two decomposition pathways NOj-
+ hv
-
-+
+0
(la)
+ 0-
(lb)
NOT
NO2
where the 0- radical ion subsequently combines with a proton to form an OH radical. Evidence for the Occurrence of the second pathay has been provided by several recent s t u d i e ~ . The ~ ~ ~objective of the present study was to confirm the existence of both reaction channels and to determine the associated primary quantum yields by means of radical scavenger techniques. Accordingly, we searched for suitable scavengers leading to the formation of characteristic products that would allow a quantitative evaluation of the radical yields. For OH radicals we have explored the conversion of benzene to phenol, 2-propanol to acetone, and formate to hydrogen peroxide; for oxygen atoms we have used (1) Wagner, I.; Strehlow, H.; Busse, G. Z . Phys. Chem. (Frankfurt) 1980, 123, 1-33. (2) Zafiriou, 0. C.; True, M. B. Mar. Chem. 1979,8, 33-42. Kotzias, D.; Parlar, H.; Herrmann, M. Natunvissenschaften 1982,69,444-445. Zafiriou, 0 .C.; Joussot-Dubien, J.; Zepp, R. G.; Zika, R. G. Enuiron. Sci. Technol.
their reaction with cyclopentene to produce ethene. Photodecomposition of the NO3- ion at wavelengths outside the 300-nm band will not be discussed.
Experimental Section Photolyses were performed with light from a 150-W xenon short arc lamp. For exploratory experiments the light was collimated onto the entrance window of a cylindrical quartz photolysis cell (1.8-cm i.d., 10-cm length). A cutoff filter was then interposed to eliminate wavelengths below 290 nm (set up no. 1). For quantum yield measurements the arc lamp was combined with a monochromator to produce 305-nm radiation with a spectral resolution of 14 nm at half-width (setup no. 2). The light emerging from the instrument's exit slit was refocused to pass through the photolysis cell and then onto a calibrated thermopile. The decadic absorption coefficient for NO3- at 305 nm is about 7 cm-' M-' so that for nitrate concentrations in the range 1-10 mM the photolyzing radiation filled the entire cell, even though it suffered losses due to absorption. Product quantum yields were calculated as usual from the radiation dose delivered into the cell and the total amount of product formed during the irradiation period. Radiation doses were calculated from the measured light flux. In some runs ferric oxalate actinometry6 was used for comparison; the results were found to agree within experimental error. All experiments were performed at room temperature (22 i 2 "C). Milli-Q-quality water was used throughout. Except where indicated the solutions were air-saturated. Analyses. Nitrite was determined by spectrophotometry at the 540-nm wavelength following the Saltzman diazotization procedure.' In a number of experiments, the procedure was checked M Na,C03 as by means of ion chromatography with 4 X the eluent and detection by UV absorption at 210 nm. Phenol was mainly identified and its concentration estimated by reaction with the diazonium salt of sulfanilic acid in alkaline solution, followed by spectrophotometry at the 430-nm wavelength of the azo compound formed.8 The concentration of acetone was de-
1984, 18, 358A-371A.
(3) Daniels, M.; Meyers, R. V.; Belardo, E. V. J . Phys. Chem. 1968, 7 2 , 389-399. (4) Russi, H.; Kotzias, D.; Korte, F. Chemosphere 1982, 11, 1041-1048. (5) Zepp, R. G.; Hoignt?, J.; Bader, H. Enuiron. Sci. Technol. 1987, 21, 443-450.
0022-3654/88/2092-6278$01.50/0
(6) Hatchard, C. G.; Parker, C. A. Proc. R . Soc. London, A 1956, 235, 518-536. ( 7 ) Saltzman, B. E. Anal. Chem. 1954, 26, 1949-1955. (8) Koppe, P.; Diez, F.; Traud, J. Fresenius 2.Anal. Chem. 1977, 285, 1-19
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6279
Photodecomposition of NO3- in Aqueous Solution TABLE I: Quantum Yields for Ethene and 0-Atoms (Irradiation Time 1-3 h, [C,H,] = 5 X M, pH 5.6, n = Number of Runs) 104 x cor 104 x [NO;], M n b(C2H4) factor b(C2H4hr 103b(0) 0.005 3 2.56 f 0.37 1/0.96 2.66 1.11 i 0.16 0.01 12 2.45 i 0.19 1/0.941 2.60 1.08 i 0.08 3 2.3 f 0.18 1/0.81 2.84 1.18 f 0.09 0.05 a v l . l l i 0.10
termined by its reaction with 2,4-dinitrophenylhydrazine and high-pressure liquid chromatography of the hydrazone p r o d ~ c t . ~ The eluent was a 75:25% mixture of methanol and water, and the wavelength of the absorption detector was set to 360 nm. This procedure separated acetone from formaldehyde and acetaldehyde. Hydrogen peroxide was estimated by means of the 4-aminoantipyrine methodlo which discriminates HzOz from other peroxides. The coupling reaction involves phenol in addition to 4-aminoantipyrine to give, in the presence of horseradish peroxidase, a colored quinone imine compound whose concentration was measured spectrophotometrically at the 505-nm wavelength. The colorimetric and chromatographic procedures were tested for linearity and were calibrated within actual concentration ranges. In the case of H z 0 2the calibration gave a molar absorptivity of 5.1 X lo3 cm-I M-l, which is about 20% lower than that reported by Frew et a1.I0 For the determination of ethene, the contents of an irradiated photolysis cell were expanded into a larger evacuated flask from which various samples were taken to be analyzed by gas chromatography. A packed column served to separate ethene from other volatiles, and a flame ionization detector was used. The system was calibrated by injecting with a gas-tight syringe a known amount of ethene via a septum into the closed-off photolysis cell filled with an aqueous solution of nitrate plus cyclopentene.
Results Cyclopentene was first used by Brown and Hart" as an indicator
I
1
-
I
'
I
'
i
IRRADIATION TIME i hours)
PH
Figure 1. (Left) Evolution of ethene as a function of irradiation time; [NO,-] = 0.01 M, [C,H,] = 0.005 M, pH 5.6. (Right) Relative ethene quantum yield as a function of pH: open circles, present data; closed circles. data from ref 11.
atoms. In the present study the reaction of 0-atoms with nitrite may be ignored in comparison to those with other scavengers because the concentration of NOT did not rise much above 1 X M. Accordingly, the ethene quantum yield should be given by 44C2H4) = O w ( 0 ) k2[C5H81/(k2[C5H81 + k3W03-I + ~ ~ [ O Z I )
= 0.244(0)/f, where
(5) Ratios for the rate coefficients associated with these reactions have been determined by Amichai and TreininI5 and by Brown and Hart." Both used the photolysis of Br0,- as a source of oxygen
The brackets indicate, as usual, molar concentrations. After inserting the relative rate coefficients shown above and the concentrations [ 0 2 ] = 2.6 x IO4, [C5H8]= 5 x 5x < [NO3-] < 5 X one finds that oxygen diverts somewhat less than 2% of the oxygen atoms from reacting with cyclopentene, whereas their reaction with nitrate contributes 2-17% to the total reaction rate depending on the NO< concentration. In calculating the quantum yield for oxygen atoms it is thus necessary to correct for reactions 3 and 5. Results obtained with this correction are entered in the last column of Table I. The 0-atom quantum yield on average. obtained in this manner is (1.1 f 0.1) X The production of ethene was further studied as a function of pH. The results are shown in Figure 1 (right) relative to those obtained at pH 5.6. Brown and Hart" have done similar experiments using the photolysis of Br03- as a source of oxygen atoms, and their data are included in Figure 1 (right). Both sets of data are in good agreement. This substantiates our conclusion that the photodecomposition of nitrate produces oxygen atoms with the yield indicated above. The reaction 0 + OHHOzis thought to be responsible for the falloff of the ethene yield in the pH region beyond pH 11. Benzene. From y-ray and pulse radiolysis experiments it is known that phenol arises as a product from the reaction of OH with benzene in aerated aqueous solutions.'6-22 In addition, a carbonyl compound is formed which has been variously ascribed to mucondialdehyde,'6s20P-hydroxymucondialdehyde,21xz2or an unknown compound of phenolic character.17 Dorfmann et aLi9 showed that the first step in the O H + C6H6 reaction is the formation of the hydroxycyclohexadienyl radical which then adds
(9) Fung, K.; Grosjean, D. Anal. Chem. 1981, 53, 168-171. (10) Frew, J . E.; Jones, P.; Scholes, G. Anal. Chim. Acra 1983, 155, 139-150. (11) Brown, W. G.; Hart, E. J. J . Phys. Chem. 1978, 82, 2539-2542. (12) Treinin, A,; Hayon, E. J . A m . Chem. SOC.1970, 92, 5821-5828. (13) Strehlow, H.; Wagner, I. Z . Phys. Chem. (Frankfurt) 1982, 132, 15 1-1 60. (14) Kilning, U . K.; Sehested, K.; Wolff, T. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 2969-2979. (15) Amichai, 0.;Treinin, A. Chem. Phys. Lerr. 1969, 3, 611-613.
(16) Stein, G.; Weiss, J. J . Chem. SOC.1949, 1949, 3245-3254. (17) Goodman, J.; Steigrnan, J. J . Phys. Chem. 1958, 62, 1020-1022. (18) Baxendale, J. H.; Smithies, D. J . Chem. SOC.1959, 1959, 779-783. (19) Dorfman, L. M.; Taub, I. A.; Biihler, R. E. J . Chem. Phys. 1962,36, 3051-3061. (20) Loeff, I.; Stein, G . J . Chem. SOC.1963, 1963, 2623-2633. (21) Balakrishnan, I . ; Reddy, M. P. J . Phys. Chem. 1970, 74, 850-855. (22) Srinivasan, T. K. K.; Balakrishnan, I.; Reddy, M . P. J . Phys. Chem. 1969, 73, 2071-2073.
for oxygen atoms in aqueous solution. The reaction gives ethene (plus acrolein) with 24% yield. Other radicals, specifically OH, react with cyclopentene as well, but the products are different and ethene is not among them. This was confirmed in the present study. The photolysis of nitrite in aqueous solution served as a convenient source of O H In this system oxygen atoms are not produced. No ethene was found after irradiating a 1 mM solution of NaNOZ in the presence of 1 mM cyclopentene for 14 h. When, however, NaNO, was replaced by NaNO,, significant amounts of ethene evolved after a 1-h irradiation time. Thus, atomic oxygen is indeed a product of NO3- photodecomposition. In aerated sodium nitrate solutions containing 5 mM C5H8the ethene quantum yield at pH 5.6 was independent of irradiation time as shown in Figure 1 (left), but it decreased by about 10% when the concentration of nitrate was raised from 5 to 50 mM, as shown in Table I. Deaerating the solutions caused a slight increase of the quantum yield. The results may be rationalized by several competing reactions: 0 C5H8 products k,/k5 = 2.6 (2) 0 NO3- NOz- + O2 k3/k5 = 0.056 (3)
+
-
+
-+
0 + NOz-+ N03k4/k5 = 0.37 k5 i= 4 X lo9 (ref 14) 0 + 0, O3
-
(4)
-+
Warneck and Wurzinger
6280 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
TABLE 11: Quantum Yields for Acetone, Nitrite, and OH Radicals from 2-Propanol as Scavenger ((NO,-] = 0.01 M, Irradiation Time 1-3 h, R = Number of Runs) 102$(OH)
1 02dA),,
pH
[2-C,H70H1, M
n
102$(Ac)
5.6 5.6 5.6 3.0 4.0 9.0 11.0
0.0013 0.013 0.13 0.13 0.13 0.13 0.13
3 3 12 5 4 4 4
0.77 f 0.03 0.89 f 0.01 1.23 f 0.07 1.33 f 0.10 1.26 f 0.05 1.07 f 0.05 1.18 f 0.10
102&(N02-)
0.78 f 0.04 0.83 f 0.02 1.02 f 0.07 0.72 f 0.09 1.00 f 0.04 1.24 f 0.14 1.22 f 0.07
a
0.76 0.84 1.13 1.23 1.16 0.97 1.08
b 0.76 0.80 1.05 1.15 1.08 0.88 1.00
1029(N02-)caIcd
a
b
C
d
0.88 f 0.03 0.97 f 0.01 1.30 f 0.07 1.42 f 0.01 1.34 f 0.05 1.12 f 0.05 1.25 f 0.10
0.87 f 0.03 0.93 f 0.03 1.21 f 0.07 1.32 f 0.01 1.24 f 0.05 1.02 f 0.05 1.15 f 0.10
0.60 0.59 0.57 0.57 0.57 0.57 0.57
0.55 0.60 0.8 1 0.88 0.83 0.69 0.77
"**Correctionfor acetone from the reaction of 0-atoms with 2-propanol: a, yield = 1; b, yield = 1.86. CAssuming $(OH) = 9.2 X dTaking $(OH) as measured and assuming $(O)/$(OH) = 0.121.
&(O) =
1 . 1 X IO-'.
oxygen to generate H 0 2 and phenol as the major final products. In the present study the photolysis of NOT was first used as a source of O H radicals. Phenol evolved in good yield when a M N a N 0 2 was benzene-saturated, aerated solution of 5 X irradiated for 1 h. Phenol was also found as a product following the irradiation of aerated solutions of nitrate in the presence of benzene. The rate of phenol formation in 0.01 M NaNO, solutions saturated with benzene at pH 5.6 was nearly constant with irradiation time, leading to an average phenol quantum yield of (5.7 f 0.7) X Nitrite was formed as well, and its concentration rose almost linearly with time with an average quantum yield of (7.3 f 0.8) X Both quantum yields were essentially independent of NO3- concentration in the range 0.001-0.01 M. The extent of phenol formation from the reaction of 0-atoms with benzene is not known. In the gas phase, 0-atoms react with benzene less rapidly than O H radicals. The rate coefficients differ by about a factor of 10.23,24If it were allowed to extrapolate the gas-phase data to the liquid phase, oxygen atoms would preferably react with nitrate rather than benzene. However, even if all the oxygen atoms were to react with benzene to form phenol, the 0-atom quantum yield would be insufficient to account for the observed amounts of phenol. Accordingly, much of the phenol must arise from the reaction of benzene with O H radicals. Unfortunately, the yield of phenol from the O H C6H6 reaction is not well established, so that the use of benzene as a scavenger for OH will lead to ambiguities in the determination of the absolute OH quantum yield. We have, therefore, explored other scavengers. 2-Propanol. Pulse radiolysis studies by Asmus et al.25have indicated relative yields for H-atom abstraction from 2-propanol by OH radicals as follows: 85.5% at the a-position, 13.3% at the @-position,and 1.2% at the O H group. Walling and Kato,26who worked with Fenton's reagent as a source of O H radicals, reported similar yields. The radicals produced from 2-propanol by H-atom abstraction at the a-position and at the O H group react with oxygen to form acetone. (CH3I2COH+ O2 H 0 2 + (CH3)2C0 (6)
+
-
The radical obtained by H-atom abstraction at the methyl groups leads to different products. Oxygen atoms are also known to react with 2-propan0l.'~If this reaction occurred by H-atom abstraction, O H radicals would be formed, which subsequently would oxidize additional 2-propanol to acetone. A maximum yield would be obtained, if 0-atoms abstracted hydrogen atoms primarily at the a-position. In this case the total yield of acetone would be 1.86 per oxygen atom. On the other hand, a reviewer has suggested that, since the oxygen atom is generally considered a two-electron oxidant, its reaction with 2-propanol should lead directly to acetone and H 2 0 as products. The process may be visualized to proceed in two consecutive steps, where the initial H-atom abstraction is followed, within the solvent cage, by a fast mutual reaction of the
1
~
2
3
1
IRRADIATION
T I M E I hours)
2
3
Figure 2. Evolution of acetone and nitrite during the irradiation of a 0.01 M aerated solution of NaNO, at pH 5.6, in the presence of 0.13 M
2-propanol. incipient radical products. If this were the major process, the yield of acetone per oxygen atom would be unity. We shall accept this model in the considerations below but note that the true yield may fall between both limits. The irradiation of an aerated 0.01 M solution of N a N 0 , in the presence of 0.13 M 2-propanol produced acetone in accordance with expectation. Nitrite was formed as well, and Figure 2 shows that at pH 5.6 both products evolved at constant rates. The corresponding quantum yields are (1.23 f 0.07) X for acetone and (1.02 f 0.07) X IOw2 for the nitrite ion. Acetone is known to absorb light at the 305-nm wavelength with a decadic extinction coefficient similar to that of NO). The possibility that this absorption leads to losses of acetone by photodecomposition may be discounted, because the probability of the process increases with rising acetone concentration. If such losses were important, they would show up as a nonlinearity in the rise curve for acetone production. This behavior is not observed in the data of Figure 2. Table I1 presents further data to indicate the dependence of the yields of acetone and nitrite on pH and 2-propanol concentration. The pH affects the acetone quantum yield only mildly within the uncertainties of the measurements, but the NO2quantum yield is reduced substantially as the pH is lowered. The concentration of 2-propanol has a stronger effect on the quantum yield for acetone than on that for nitrite. This result must be due in part to the competition between 2-propanol and nitrate in the scavenging of oxygen atoms. The reactions that must be considered in this system in addition to reactions 3-5 are
0 + (CH,),CHOH 0
(23) Atkinson, R.; Pitts, J. N., Jr. J . Phys. Chem. 1974, 78, 1780-1784; Ibid. 1975. 79. 295-297. (24) Atkinson, R.; Darnall, K . R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., J r . Ado. Photochem. 1979, 11, 375-487. ( 2 5 ) Asmus, K. D.; Mockel, H.; Henglein, A . J . Phys. Chem. 1973, 77, 1218-1.221~ - ~ (26) Walling, C.; Kato, S . J . Am. Chem. SOC.1971, 93, 4275-4281.
f
ACETONE
OH
+
H2O
+ (CH,),CHOH
-
+ (CH3)2CHOH
+ (CH,),CO
-
OH
k , / k 5 = 0.045 (7a)
+ (CH,)2C0H
H 2 0 + (CH3)2COH k8 = 2.2
X
(7b) IO9 (8a)
H 2 0 + products kg,/kg = 0.867 (8b)
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6281
Photodecomposition of NO3- in Aqueous Solution (CH3)2COH
+0 2
-
(CH3)2CO
+ HO2
(6)
Here, the ratio of rate coefficients k7/k5 = 0.045 is taken from the work of Amichai and TreininI5 and the value for ks from the compilation of Farhataziz and Ross.27 On the basis of these additional reactions the acetone quantum yield should be given by $(acetone) = 0.867$(0H) + $ ( 0 ) / f 2 k7a >> k7b 0.867$(0H) 1.855$(0)/f, k7b >> k7a
I
'1
I
I
HYDROGEN
I
'
I
1
~
I
I
I
2
3
1
NITRITE
20-
PEROXIDE
10 -
+
where
V
j 2=
+
[NO,-] (2/2)[2-PrOH]
k5 +
k, [2-PrOH]
Here, fi is derived in a manner similar to that for f l for the case of cyclopentene as a scavenger, and [2-PrOH] = [2-propanol]. The expression allows one to correct the acetone quantum yield for the presence of oxygen atoms and, thereby, derive the OHquantum yield. The results are shown in column 8 and 9 of Table 11. It turns out that the uncertainty regarding the yield of acetone from the reaction of 0-atoms with 2-propanol is not critical, because the O H quantum yield is by an order of magnitude greater than that of oxygen atoms. Despite the correction, the O H quantum yields are seen to increase by about 30% as the 2-propanol concentration is raised from 0.0013 to 0.13 M. As discussed previously by Daniels et al.3 in a different context, the effect is probably due to the influence of high concentrations of scavenger on geminate recombination of OH (or 0-)with NO2 inside the solvent cage. Most of the OH radicals resulting from NO3- photodecomposition are expected to undergo this process. Low concentrations of 2-propanol will scavenge only those O H radicals that have escaped in-cage recombination and diffuse freely through the solution. An alternative explanation according to a suggestion by Wagner et a1.l would be an interaction of 2-propanol at high concentrations with the excited NO3-* ion before it disintegrates. In either case, the O H quantum yield for unperturbed conditions is (9.2 f 0.4) X If neither interpretation is correct, one should use an average value for the entire range of 2-propanol concentrations and obtain (1.18 f 0.20) X for the case that the reaction of 0-atoms with 2-propanol produces acetone with a yield of unity. The NO2-. quantum yield varies with 2-propanol concentration in a way similar to that for the O H quantum yield. This behavior provides further evidence for an interference of high scavenger concentrations with incipient recombination (or interaction with excited NO
