Many articles related with photochemical reactions publish in their experimental part very small concentrations (millimolar). What are some advantages about the use of these small concentrations and not greater ones (M)?
Thing here is to maintain absorbance lower, than, say, 1 or so. Otherwise your results wouldn't be reliable. You may calculate, what absorbance will have the sample if concentration was higher.
Low concentrations lower the possibility of multiple interactions between reactants in solution quenching the excited state before a reaction can take place.
mM level of concentration is relatively high for environmental samples. Depending on your environmental water samples, it shoud be choosen for photochemical experiments for standard substances if you would like to do so. For example, it has been shown that in the case of DOM photodegradation, waters with high contents of DOM showed low photodegradation than the waters with low contents of DOM.
To avoid inner-filtering effect. When you do a photopysical experiment (say a fluorescence intensity measurement), having a high concentration of the chromophore may result into reabsorption of the emitted light giving you a wrong result (maybe a decreased quantum yield or a distorted luminecscence band).
Heterogenious photocatalysis is usually an oxidizing process whereby electron /hole pairs are created, the electrons are scavenged by oxygen in air and holes effect the oxidation of the dissolved substrate. If the concentration of TiO2 or other catalysts is too high, all light to create electron/hole pairs is absorbed in a thin film nearest the reactor windows inviting problems with efficient scavenging of electrons by O2. Lower concentration of catalyst allow light to penetrate deeper into the solution phase thereby making scavenging more efficient and, with stirring, ensuring a steady supply of O2 to the catalyst surface to trap electrons thereby preventing recombination and a waste of photons. That's why the concentration is kept low.
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