Spectrophotometric study of the rate of the aqueous iodate-iodide reaction

Abstract

The initial rate of the reaction of I03-with Iin acetate buffer has been measured at fixed ionic strength and acetate concentration with varying concentrations of I03-, I-, and H+ at temperatures between 293 and 308°K. The method involves spectrophotometic determination of the product I3_ at a wavelength of 353 nm. In contrast to some previous results, the reaction is now found to be first order in I03" and second order in H+; the apparent order with respect to Iincreases with temperature in the concentration ranges considered. Agreement with some earlier amperometric results is found. The applicability of previously proposed rate laws and mechanisms is considered, and upper limits are estimated for the enthalpies of possible reactive intermediates.

The detailed mechanism of the Dushman reaction explored by computer

“…For [7], in which HAc-AE a t total ionic strength of 0.2 was used, F A according to [2] Then rate law 1 gives R1os = 9.O(1Op7), while rate law 5b gives 7.8(1OP7); the agreement is good. Now, for [I-] = other concentrations being the same, rate law 1 gives 9.0(10-9), and rate law 5b gives 21.9(10-9); agreement is no longer good.…”

“…(e) The most extensive and the most precise constant-rate results available are those from the spectrophotometric iodine determinations of [7]. The corresponding specific rates in [7, table I, column 81 are 180-fold too high.…”

“…For [7], in which HAc-AE a t total ionic strength of 0.2 was used, F A according to [2] other concentrations being the same, rate law 1 gives 9.0(10-9), and rate law 5b gives 21.9(10-9); agreement is no longer good. The possible reason is that the exponent 1.55 for [I-] in rate law 5b is more nearly satisfactory at lop2 than a t (The reader will remember Dushman's value of 1.9 for this exponent.)…”

“…(e) The most extensive and the most precise constant-rate results available are those from the spectrophotometric iodine determinations of [7]. The corresponding specific rates in [7, Rate laws 5a, 5b, and 5c are alternative representations of extensive spectrophotometric results; rate law 5b represents them best.…”

Reactions of (CO2)2+ and (CO)2+ association ions

“…Many of the measurements were deliberately taken in the presence of water collisionally desorbed within the reaction chamber, while the majority were taken under essentially dry conditions in order to minimize hydrated species in the reactant ion spectrum. With the exception of those experiments involving 0 2 as an additive, the contribution of 0 2 + formed in C02 by the reaction [7,[12][13][14][15] (5) o+ + co2 -0 2 + + co2 as well as its solvated analogs (C0202)+ AND [(C02)202]+, are not included in the figures since O+ only comprises about 4% of the primary ionization in CO2 at 74 nm.…”