EQUILIBRIUM-CONSTANT OF THE GAS REACTION CL2O + H2O = 2HOCL AND THE ULTRAVIOLET-SPECTRUM OF HOCL

The gas-phase system Cl2O + H2O ⇄ 2HOCl (1, -1) has been investigated by UV photometric measurements from 200 to 500 nm at 333 K. The initial concentrations of Cl2O varied from about 0.4 to 10 × 10-4 mol L-1, those of H2O being always in excess, from 8 to 59 × 10-4 mol L-1. All runs were carried out in about 600 torr of N2. Very clean Cl2O/H2O/HOCl mixtures proved to be remarkably stable. Thermal decomposition led to intermediate ClO2 and main Cl2 production: (d[ClO2]/dt)/[Cl2O] ≈ 1.8 × 10-8 s-1, (d[Cl2]/dt)/[Cl2O] ≈10-7 s-1. Four isosbestic points were found at λ/nm = 214, 233, 335, and 380. The equilibrium constant at 333 K, K = 0.132 ± 0.008, and HOCl cross sections were derived from absorbance measurements of the mixtures at equilibrium by means of a least-squares analysis. The resulting HOCl spectrum shows two absorption bands with peaks at 240 nm and at about 310 nm (εHOCI/L mol-1 cm-1 = 58.4 and 16.3, respectively). The equilibration reaction is known to be strongly influenced by the wall at least at 298 K. The rate constant k1 ≈ 2.7 × 10-2 L mol-1 s-1 at 333 K, obtained from the analysis of Cl2O and HOCl concentration-time profiles, may be regarded as an upper limit of the rate constant of the homogeneous reaction (1). Complete description of the measured absorbances of all mixtures over the whole spectral range and in dependence of time was possible on the basis of measured concentrations and known (Cl2O, ClO2, Cl2) and derived (HOCl) ε values. The absorption cross sections of Cl2O, ClO2, and Cl2 used for the computer multicomponent analysis were determined separately at 333 K and those of Cl2O were determined at 298 K. © 1979 American Chemical Society.