Kinetics of hydrolysis of iodine monochloride measured by the pulsed-accelerated-flow method

Abstract

First-order rate constants from 75 000 to 150000 s™1 are measured by the pulsed-accelerated-flow technique for the rate of loss of IC12™ in neutral and basic solutions. The rate varies inversely with the Cl™ concentration because ICl(aq) is the reactive form. The hydrolysis of ICl(aq) to give HOI and Cl™ is extremely rapid (A:H;0 = 2.4 X 106 s'1 at 25.0 °C, µ = 0.50). Hydroxide ion accelerates the rate of hydrolysis. The second-order rate constant for the reaction of OH' with ICl(aq) is very large (& = 4.5 X 109 M™1 s'1). Reactions of ICl(aq) with H2P04™/HP042™ and with HC03"/C032™ buffers are also very rapid and are first order in the total buffer concentration, with resulting second-order rate constants, (1.3 ± 0.2) X 109 M'1 s™1, that do not depend on the basicity of the buffer or the ratio of buffer species. Two ICl(aq) species are proposed to be in rapid equilibria: H2OICl (82%) and IC1 (18%). Kinetic data indicate that IC1 (the less hydrated species) is so reactive that it is unselective and reacts with H2P04™, HP042', HC03™, C032™, and OH™ at the diffusion limit (7 X 109 M'1 s'1). The more hydrated species, H20IC1, also reacts rapidly with OH™ (3.9 X 109 M™1 s™1) by proton transfer to form H20 and HOIC1™, which rapidly loses Cl™ to give HOI. Spectrophotometric measurements of the equilibrium constants at 25.0 °C give ([lCl(aq)][Cl™])/[ICl2™] = 0.013 (µ = 1.0 and 0.5) and ([HOI][Cl™]2[H+])/[ICl2™] = 1.06 X 10™6 3 (µ = 0.5).Kinetics of the hydrolysis of Cl2, Br2, and I2 have been de-

The Heterogeneous Kinetics of the Reactions ClONO2 + HX/ice (X = Br, I), BrONO2 + HI/ice and the Reactivity of the Interhalogens BrCl, ICl and IBr with HX/ice (X = Cl, Br, I) in the Temperature Range 180 to 205 K

“…4, and for BrCl whose γ value is larger for HBr than for HCl/ice. These observed trends compare rather well with the results obtained by Wang [44] and Troy [37] in aqueous solutions from a qualitative point of view. These authors investigated the interaction of ICl and IBr with I Ϫ and proposed the mechanism displayed in reactions (15) and (16) with (X ϭ Cl, Br) which accounts for the products we detected upon desorption for reactions (IH7) and (IH11) listed in Table 3.…”

“…The experimental equilibrium constant obtained for reaction (20) in solution is almost an order of magnitude lower than that measured for reaction (17) between ICl and Cl Ϫ with X ϭ Cl Ϫ [42,44], indicating that BrCl 2 Ϫ is prone to redissociation compared to ICl 2 Ϫ , hence the lower uptake coefficients measured in this study for reaction (IH2). One has to remember that the uptake measurement refers to the net effect of combined adsorption and desorption rate.…”

A modeling study of iodine chemistry in the marine boundary layer

“…A lower limit to • can be estimated by setting the C1-and Br-concentrations at 10 times their seawater values (simulating equilibration of seawater droplets with ambient humidity) and the pH to 5.5 (representing aerosol that has been acidified by internal mixing of sea-salt and sulphate aerosol, or by the scavenging of }-I2SO4, HNO3, or SO2 followed by aqueous oxidation of the sulphur (IV) to sulphur (VI)), compared with fresh sea-salt aerosol with a pH close to 9.5. The lifetime of HOI(aq) with respect to reaction with Cl-and Br' [Wang et al 1989;Troy et al 1991] is then about 2 [rs. However, the average time for diffusion across an aerosol particle of the volumetric mode radius is about 0.8 las, compared with lifetimes for IC1 and IBr to dissociate back to HOI of 0.4 and 1.3 laS, respectively [Wang et al, 1989;Troy et al, 1991].…”

“…The lifetime of HOI(aq) with respect to reaction with Cl-and Br' [Wang et al 1989;Troy et al 1991] is then about 2 [rs. However, the average time for diffusion across an aerosol particle of the volumetric mode radius is about 0.8 las, compared with lifetimes for IC1 and IBr to dissociate back to HOI of 0.4 and 1.3 laS, respectively [Wang et al, 1989;Troy et al, 1991]. This implies that if IX is assumed insoluble, • is about 3 las, and hence there will be no HOI accumulation at these halide ion concentrations.…”

Active chlorine release from marine aerosols: Roles for reactive iodine and nitrogen species

“…The possibility that HOI chemistry is analogous to that of HOBr has been proposed by Vogt et al [1999], such that the third‐order aqueous reaction of HOI described by reaction , and calculated from the equilibrium constant and back‐reaction rate reported by Wang et al [1989], leads to the production of relatively insoluble ICl which preferentially partitions into the gas phase. Thus reaction of HOI with acidified sea‐salt aerosol could lead to chlorine released as ICl, which could rapidly photolyze.…”