Modeling ozone depletion observed in the Arctic troposphere requires mass transport of halogen species between the aqueous and the gas phases, which is proportional to its Henry's Law constant (H X). UV measurements are made in both the aqueous and gas phases to determine the solubility (H BrCl = [BrCl(aq)]/[BrCl(g)]RT) of BrCl (279−299 K). The gas-phase absorbance spectrum (330−450 nm, 1 nm intervals) is fit for [BrCl(g)], [Br2(g)], and [Cl2(g)], while the absorbance value at 343 nm is used to determine [BrCl(aq)] (and [BrC ]). Dissociation (K aq = [Br2(aq)][Cl2(aq)]/[BrCl(aq)]2) data are obtained from K aq = H Cl2 H Br2 K g/H BrCl 2 where H Cl2, H Br2, and K g have analogous definitions to H BrCl and K aq. Comparison of Henry's Law constants in Table reveals that the order of solubility at 298.2 K is BrCl > Br2 ≫ Cl2. At temperatures relevant to the Arctic, BrCl is much more soluble than Br2 because of differences in ΔH°. Consequently, the relative flux of BrCl from the aqueous to the gas phase should not be as large as that of Br2. Comparison of dissociation constants in Table reveals the relatively high degree of association of BrCl with H2O molecules in the aqueous phase.

A pulsed-accelerated-flow (PAF) spectrometer (model V) capable of non-ambient temperature studies of fast reaction kinetics is described. The PAF method uses accelerated flow mixing of reactants during short time periods to enable the resolution of mixing and reaction rate constants. A new mixing/observation cell and cell supports are designed to permit measurement of reaction kinetics from 40 ЊC to below 0 ЊC. The cell consists of two machined PEEK [(-OC 6 H 4 OC 6 H 4 COC 6 H 4 -) n ] pieces joined together to give an internal solution distribution system, which greatly reduces the number of connections needed compared to previous instruments to bring the reactants together. The reaction between W(CN) 8 4Ϫ and IrCl 6 2Ϫ in 0.50 M H 2 SO 4 is studied at 0.0, 25.0, and 40.0 ЊC. Second-order rate constants of 0.650 × 10 8 M Ϫ1 s Ϫ1 , 1.05 × 10 8 M Ϫ1 s Ϫ1 , and 1.29 × 10 8 M Ϫ1 s Ϫ1 are obtained, respectively. These data give activation parameters of ∆H ‡ = 10.0 ± 0.8 kJ mol Ϫ1 and ∆S ‡ = Ϫ58 ± 3 J mol Ϫ1 K Ϫ1 . Activation parameters for reverse bromine hydrolysis (HOBr ϩ Br Ϫ ϩ H ϩ Br 2 ϩ H 2 O) were determined from rate constants measured from 0.0 to 40.0 ЊC. These were used to calculate the activation parameters for the forward bromine hydrolysis (∆H ‡ = 66 ± 1 kJ mol Ϫ1 and ∆S ‡ = 10 ± 20 J mol Ϫ1 K Ϫ1 ). The temperature dependence of the extremely rapid BrCl hydrolysis reaction (in equilibrium with BrCl 2 Ϫ) is determined as well. For reactions at temperatures of 25.0 ЊC, 10.0 ЊC, and 0.0 ЊC the values are 3.3 × 10 6 s Ϫ1 , 2.06 × 10 6 s Ϫ1 , and 1.75 × 10 6 s Ϫ1 , respectively. These values correspond to activation parameters of ∆H ‡ = 15 ± 7 kJ mol Ϫ1 and ∆S ± = Ϫ71 ± 24 J mol Ϫ1 K Ϫ1 for BrCl hydrolysis.