In the Laboratory
Chemical Actinometry: Using o-Nitrobenzaldehyde to Measure Light Intensity in Photochemical Experiments Kristine L. Willett and Ronald A. Hites* Department of Chemistry and School of Public and Environmental Affairs, Indiana University, Bloomington, IN 47405; *
[email protected] Rationale
sion can be derived from the Beer–Lambert law
Photochemical degradation is a significant process by which contaminants are removed from the environment. Many laboratory studies have investigated the kinetics by which aqueous and gas-phase pollutants are photochemically degraded; these compounds include polycyclic aromatic hydrocarbons (1, 2) and polychlorinated dibenzo-p-dioxins (3). In order to compare these laboratory studies to ambient conditions, it is necessary to know the intensity of the laboratory light source, at least over a specific wavelength range. One interesting way of making this intensity determination is chemical actinometry, which uses a reaction of a compound with light to measure the intensity of the light source over a specified wavelength range. The extent of the reaction as a function of time can be measured with common analytical techniques, and the resulting rate of reactant loss can be converted into light intensity. An easy-to-perform experiment that could be used in undergraduate analytical or physical chemistry courses or in graduate environmental chemistry courses is described. The rate of the photochemical reaction of o-nitrobenzaldehyde (NBA), irradiated with a medium-pressure mercury vapor lamp, is used to calculate the light intensity over the wavelength range of 300–410 nm. The experiment has analytical, photochemical, and kinetic aspects, and it introduces some unusual unit conversion concepts, such as the relationship between einsteins per liter-second and watts per square meter. Concepts Chemical actinometry has been used by photochemists since at least the 1920s (4 ), and the concepts have been well developed (5). The rate of loss of an actinometric compound (Act) is the product of the incident light intensity (I0), the quantum yield (φ), and the fraction of light absorbed by the compound (f ) (5):
d Act = I 0φf dt
(1)
The quantum yield is the number of molecules decomposed divided by the number of quanta or photons absorbed, and the concentration of the actinometric compound is given in mol L1. Therefore, since φ and f are unitless, the units of I0 are einstein L1s1, where an “einstein” is defined as Avogadro’s number, or a mole, of photons. The fraction of light absorbed (f ) is given by
I –I f= 0 =1– I I0 I0
(2)
where I is the light intensity after passing through the sample. The ratio of the incident to transmitted light in this expres900
I0 = εb Act I
α = log
(3)
where α is absorbance, ε is the molar absorptivity in L mol1 cm1, and b is the path length in cm of the light. From eqs 2 and 3, we derive f = 1 – 10εb[Act]
(4)
Therefore, from eq 1
I0 =
d Act dt
1 φ
1 εb Act 1 – 10
(5)
One carries out the experiment in a way that the concentration of the actinometric compound and the path length of the exposure cell are sufficiently high to make the kinetics of this reaction approximately zero order. In this way, the rate of the reaction does not depend on the concentration of the reactant as it would if one assumed first-order kinetics. With zero-order kinetics, the negative differential term in eq 5 is replaced by a zero-order rate constant, k0, with units of mol L1 s1. At high concentrations and path lengths, the rightmost term in eq 5 is very close to unity and is often omitted. With these substitutions, the incident light intensity is
I0 =
k0 φ
(6)
Remember that the units of I0 are einstein L1 s1. These are not intuitively obvious units, but they can be thought of as the rate at which the photon concentration is delivered to the solution. Experimentally, one measures the concentration of the actinometric compound as a function of time and fits a straight line to the data. The negative of the slope is k0. Sometimes the data do not form a straight line because of light absorption due to the photoproduct (2). In these cases, one uses the slope of the curve at time = 0 as the zero-order rate constant. We will demonstrate an approach to this complication. The goal of this experiment is to measure the intensity of a light source over a specified wavelength range in units of watts per square meter (W m2). This is a light flux that we will define as F0, and it is given by (2)
F0 =
I 0V NA E λ A
(7)
where V is the volume (in L) and A is the area (in m2) of the exposed actinometric solution, NA is Avogadro’s number, and E λ is the energy of a photon of wavelength λ (in J photon1).
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
Notice that the product of I0 and V is the flow of photons to the actinometric solution (in einstein s1) and that the ratio of V to A is the path length of the solution. We also note that a watt is a joule per second (1 watt = 1 J s1). The energy of the photon is easily calculated from Planck’s law
Eλ = hν = hc λ
(8)
where h is Planck’s constant (6.63 × 1034 J s photon1), c is the speed of light (3 × 108 m s1), and λ is the average wavelength over which the actinometer is used. The best chemical actinometers have quantum yields that (within certain wavelengths) are high and constant. A particularly convenient compound for actinometry measurements is o-nitrobenzaldehyde (NBA), which is photochemically converted into o-nitrosobenzoic acid: H
O
300
200
100
0
0
5
10
15
20
25
Time / min Figure 1. Concentration of NBA as a function of time during photolysis. Each data point represents 4 replicate measurements. The best-fit second-order polynomial is shown (see eq 9).
OH
O
C
NBA Conc. / (ng /µL)
In the Laboratory
C NO2
hν
NO
The quantum yield is 0.5 over wavelengths of 300–410 nm, the range in which this compound absorbs light (4–6 ). Using this information and measurements of NBA concentrations as a function of time of irradiation, the light intensity in einstein L1 s1 and the light flux in W m2 in the spectral region 300–410 nm can be calculated for a light source. Equipment and Chemicals A high-intensity lamp (available from Ace Glass, Vineland, NJ, or from other sources). A gas chromatograph or gas chromatographic mass spectrometer. o-Nitrobenzaldehyde (CAS # 552-89-6).
Experimental Procedure In these experiments, we used a 450-W, medium-pressure mercury vapor lamp as the light source, but any intense lamp would do. In our case, a water-cooled Pyrex jacket eliminated light with wavelengths less than 290 nm and dissipated heat from the lamp. The lamp and jacket were inside a foil-lined box so that UV light leakage was controlled. • SAFETY PRECAUTIONS: Protective glasses should be worn, and the lamp should not be viewed directly.
A cylindrical quartz reaction chamber is hung 18 cm from the lamp in such a way as to facilitate periodic sampling of the irradiated solution. A ∼2.5 × 103 M solution of NBA is prepared in dichloromethane and stored in the dark. The NBA solution (25 mL) is added to the quartz reaction chamber. With our chamber, this volume corresponded to an irradiation area of 5.0 by 1.9 cm or 9.5 × 104 m2, as measured with calipers. Before irradiation starts, a 10-µL sample is collected from the chamber representing t = 0 samples. After the lamp has been turned on, 10-µL samples are collected at 5, 10, 15, 20, and 25 min. If time is available, samples can be taken in duplicate (or more) and the results averaged.
The 10-µL samples are diluted with 1000 µL of dichloromethane. For quantitative analysis, 100 µL of the diluted sample is mixed with 200.0 ng of phenanthrene as an internal standard and with 900 µL of dichloromethane. We analyzed the samples on a Hewlett Packard 5973 gas chromatographic mass spectrometer system operating in the electron impact mode and fitted with a DB-5 GC column. However, the analyte mixture could also be measured on a gas chromatograph equipped with a flame ionization detector or by liquid chromatography. Results and Discussion The average concentration of NBA in our experiment is plotted as a function of time in Figure 1. The best-fit straight line (not shown) had a slope of 7.82 ± 1.01 ng µL1 min1. The correlation coefficient (r 2) of .937 indicated that this was not a good fit, and a quick inspection of the data suggested considerable curvature. In this case, we need the initial slope of the curve. One way to get this value is to fit a secondorder polynomial to the data, take the derivative of this polynomial, and evaluate it at t = 0. With the data shown in Figure 1, the polynomial is [Act] = 0.271t 2 – 14.60t + 325.5
(9)
and the derivative at t = 0 is 14.60 ± 0.90 ng µL min1. The quality of this fit is much better than the straight line (see Fig. 1), giving an r 2 of .997. Incidentally, the molar absorptivity of this solution at 355 nm (the average wavelength range over which this particular actinometric compound absorbs light) was 260 L mol1 cm1 (6 ), the path length was 2.7 cm, and the concentration was 2.2 × 103 mol L1. Thus the right-most term in eq 5 is 1.03, which can easily be assumed to be unity. Using the negative of the slope at t = 0 for the value of k0, we can now calculate the value of I0 from eq 6 as follows: 1
14.6 ng µL⋅min
mol 151 g
g 9 10 ng
106 µL L 6
1 einstein min 1 = mol 60 s 0.5 1
3.2 × 10 einstein L s1
JChemEd.chem.wisc.edu • Vol. 77 No. 7 July 2000 • Journal of Chemical Education
(10) 901
In the Laboratory
Next, we need the energy of a photon in the wavelength range of 300–410 nm. For simplicity, we have used the average wavelength of 355 nm in eq 8: 34
Eλ =
6.63 × 10 J⋅s photon
3 × 10 m s 8
1 = 9 355 × 10 m (11) 19 1 5.6 × 10 J photon
Finally, we use eq 7 to determine the light flux, F0: 6
3.2× 10 einstein L⋅ s
0.025L 4 9.5× 10 m2
5.6× 1019 J mol photon 1einstein
23
6.02× 10 photons × mol 1W⋅ s = 28 W m2 J
(12)
The error analysis is straightforward. The only variables that have any experimental error are k0 and the ratio V /A (from eq 7), which is the light path length. The relative error in k0 is 0.90/14.60 = 6.2%, and in the path length it is
902
0.1/2.7 = 3.7%. The square root of the sum of the squares of these relative errors is 7.2%. Thus, from this experiment, we determine that a 450-W medium-pressure mercury vapor lamp has a light flux of 28 ± 2 W m2 over the wavelength range of 300–410 nm. It would be interesting to repeat this experiment with sunlight as the light source. Literature Cited 1. Behymer, T. D.; Hites, R. A. Environ. Sci. Technol. 1988, 22, 1311–1319. 2. Daisey, J. M.; Lewandowski, C. G.; Zorz, M. Environ. Sci. Technol. 1982, 16, 857–861. 3. Koester, C. A.; Hites, R. A. Environ. Sci. Technol. 1992, 26, 502–507. 4. Bowen, E. J.; Harley, H.; Scott, W. D.; Watts, H. S. J. Chem. Soc. 1924, 125, 1218–1221. 5. Calvert, J. G.; Pitts, J. N. Jr. Photochemistry; Wiley: New York, 1967; pp 781 ff. 6. Pitts, J. N. Jr.; Vernon, J. M.; Wan, J. K. S. Int. J. Air Water Pollut. 1965, 9, 595–600.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
