Abstract
The photochromic properties of the system CuCl(s)H 2O(l) in relation to the composition of the aqueous solution were studied using diffuse reflectance spectroscopy. The photochemical disproportionation of Cu + into Cu 2+ and Cu 0 proved to be essentially independent of pH, ionic strength and the concentrations of Cu 2+ and Cl − in the solution. The reverse reaction, i.e. thermal fading, takes place in two steps, one initial exponential step followed by a second step of zero order, i.e. of constant reaction rate. For both steps the reaction rate is independent of the concentration of metallic copper. The rate of the initial step is proportional to the product of the Cu 2+ and Cl − concentrations of the solution. The rate of the second step is independent of the Cu 2+ concentration and proportional to the Cl − concentration. The results are consistent with the following mechanism for the bleaching reaction: (I) diffusion of Cu 2+Cl − complexes to reaction sites on the surface; (II) fast chemical reaction between Cu 2+Cl − and Cu 0 at the reaction sites yielding CuCl; (III) desorption of the product CuCl by complex formation with Cl − ions.
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