Reaction quantum yield

Kari FG, S Hilger, S Canonica (1995) Determination of the reaction quantum yield for the photochemical degradation of Fe(III)—EDTA implications for the environmental fate of EDTA in surface waters. 

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

While yields greater than unity provide evidence for chain reactions, yields less than unity do not indicate the absence of a chain reaction. Quantum yields as high as 106 have been observed in the photochemical reaction between H2 and Cl2. 

To calculate Ar//(I0.10), it is necessary to know n, the amount of substance of Cr(CO)6 consumed during period t. This is best done by analyzing the final reaction mixture, but as mentioned, it can also rely on the reaction quantum yield, [Pg.150]

The data obtained by Nakashima and Adamson for reaction 10.10 were / = 0.0174 and

derived from the analytical determination of n(n = Pliterature value for the quantum yield. The uncertainty recommended by the authors includes the experimental errors in P and F values. 

Other error sources discussed for the isoperibol instrument are not a problem in Teixeira and Wadso s microcalorimeter. For instance, as shown by equations 10.15 and 10.16, the radiation wavelength does not influence the precision or the accuracy of the final A rH result. However, the precision is still affected when the reaction quantum yield is low, because the experimental error will be divided by a small value of n. On the other hand, problems like side reactions or secondary photolysis, already mentioned, that are not related to the instrumental design may also lead to large errors. 

If the reaction quantum yield is known, its enthalpy (A rH) can be determined after the amount of heat dissipated in solution (A0bSH) is obtained from the photoacoustic experiment by equation 13.8,... 

One may be somewhat disappointed by the error bars in A0bs7/ and Ar/7, which are larger then most obtained by classical calorimetric methods, such as reaction-solution or combustion calorimetry. However, the comparison is unfair because PAC deals with species that have lifetimes smaller than a microsecond, not amenable to those classical methods. Although the quality of the photoacoustic measurements is rather good, as shown by the correlations obtained (see figure 13.7), a realistic error in the ratio of the slopes (0bs) is 1-2%, which implies an uncertainty in A0bsH of 4 to 8 kJ mol-1. This error bar is particularly serious when the reaction quantum yield is low (recall equation 13.15). 

From an environmental chemist s point of view, it is often not necessary to determine all the individual quantum yields for each reaction pathway (which is, in general, a very difficult and time-consuming task). Rather we derive a lumped quantum yield which encompasses all reactions that alter the structure of the component. This lumped parameter is commonly referred to as reaction quantum yield and is denoted as Oir(A) ... 

Unfortunately, there are no simple mles to predict reaction quantum yields from chemical structure, and, therefore, Ojr(A) values have to be determined experimentally. We will address such experimental approaches in Section 15.4, and confine ourselves here to a few general remarks. First, we should note that, in principle, reaction quantum yields may exceed unity in cases in which the absorption of a photon by a given compound causes a chain reaction to occur that consumes additional compound... 

Before we turn to discussing reaction quantum yields, we want to introduce an approximation for calculating a(A) values of a given compound in a well-mixed... 

In Section 15.2 we defined a reaction quantum yield, Oir(A), describing the total number of compound molecules (e.g., moles compound) transformed by a chemical reaction per total number of photons (e.g., einsteins) absorbed by a given system resulting from the presence of the compound (Eq. 15-9). In Eq. 15-18, we denoted the rate of light absorption of wavelength A by the pollutant per unit volume (e.g., einstein per liter per second) as 7a(A). It is now easy to see that the product of these two entities describes the number of compound molecules transformed per unit volume per time [e.g., (mol compound i) per liter per second]. This is also equal to the concentration change per unit time in a given system, or the rate of transformation of the pollutant ... 

As indicated by Eq. 15-36, to estimate the rate of direct photolysis of a pollutant in a given system, one needs to know the k3 value as well as the reaction quantum yield for the compound considered. As we have extensively discussed, k3 values may be estimated with the help of spreadsheet calculations or computer programs. However,... 

Table 15.7 Direct Photolysis Reaction Quantum Yields of Some Selected Organic Pollutants in Aqueous Solution...

Compound Wavelength a (nm) Solvent Other Than Water b (pH) Reaction Quantum Yield (< >, ) f Ref ... 

In Table 15.7 the reaction quantum yields are given for some selected organic pollutants. As can be seen, reaction quantum yields vary over many orders of magnitude, with some compounds exhibiting very small Oir values. However, since the reaction rate is dependent on both ka and Oir (Eq. 15-34), a low reaction quantum yield does not necessarily mean that direct photolysis is not important for that compound. For example, the near-surface direct photolytic half-life of 4-nitrophenolate (Oir = 8.1 x 10 6) at 40°N latitude is estimated to be in the order of only a few hours, similar to the half-life of the neutral 4-nitrophenol, which exhibits a Oir more than 10 times larger (Lemaire et al., 1985). The reason for the similar half-lives is the much higher rate of light absorption of 4-nitrophenolate as compared to the neutral species, 4-nitrophenol (compare uv/vis spectra in Fig. 15.5 and Illustrative Example 15.3). As a second example, comparison of the near-surface photolytic half-lives (summer, 40°N... 

Substitution of Eq. 15-43 into Eq. 15-40 then yields the reaction quantum yield of the compound of interest at wavelength A ... 

What does the reaction quantum yield, ,r(A), exactly describe In aqueous solution one usually assumes that Oir is independent of A. Is this assumption always correct Can you give an example where reaction quantum yields may have to be determined at different wavelengths ... 

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