Revisiting the Kinetics and Mechanism of Bromate-Bromide Reaction

Abstract

A reação bromato-brometo, em meio de ácido perclórico, foi observada numa faixa de acidez até então não estudada. A reação foi acompanhada através da medida da absorvância no ponto isosbéstico para as espécies Br 2 e Br 3 -(λ = 446 nm). Observou-se um comportamento de primeira ordem para o bromato e para o brometo e um comportamento de segunda ordem para o H + , levando à lei de velocidade ν = k[BrO 3 -][Br -][H + ] 2 . Esta lei de velocidade sugere um mecanismo envolvendo duas protonações sucessivas do íon bromato, formando o H 2 BrO 3 + , que então reage com o íon brometo.Estes resultados discordam de outros estudos que verificaram um comportamento de segunda ordem para o íon brometo, bem como um comportamento de primeira ordem para o H + , que levaram a propor a existência de intermediários tais como H 2 Br 2 O 3 e HBr 2 O 3 -. O comportamento de segunda ordem observado para o H + na faixa de concentração 0,005 ≤ [H + ] ≤ 2,77 mol L -1 permite afirmar que o pK a do ácido brômico, HBrO 3 , deve ser menor do que -0,5 a 25 °C, diferentemente de todas as propostas existentes até agora na literatura para o valor deste pK a .The bromate-bromide reaction was investigated in an acidity range not studied yet. The reaction was followed at the Br 2 /Br 3 isosbestic point (λ = 446 nm). It was observed a first-order behavior for bromate and bromide ions and a second-order behavior for H + ion that results in the rate law ν = k[BrO 3 -][Br -][H + ] 2 . This rate law suggests a mechanism involving two successive protonation of bromate followed by the interaction of the intermediate species H 2 BrO 3 + with bromide. These results disagree with the obtained by other authors who observed a second-order behavior for the bromide and first-order for H + , and have proposed intermediate species like H 2 Br 2 O 3 and HBr 2 O 3 -. The second-order for [H + ] observed in the range 0.005 ≤ [H + ] ≤ 2.77 mol L -1 sets down that the pK a of bromic acid, HBrO 3 , must be lower than -0.5 (T = 25 °C), different from all other values for this pK a proposed in the literature.

Kinetics of the bromate–bromide reaction at high bromide concentrations

“…The extinction coefficient was measured in the stopped-flow cell at the end of experiments giving a total conversion of bromate, and the obtained value was ε = 120 L mol −1 cm −1 . Cortes and Faria [14] had obtained ε = 111 L mol −1 cm −1 at λ = 446 nm, and Rabai et al [13,15] had obtained ε = 83 L mol −1 cm −1 at λ = 455 nm (there is a misprint in [13]). This low ε value can be explained by the location of the isobestic point at a slightly too high wavelength.…”

“…[14] called this work in question, reinvestigated the kinetics of reaction (1) and con- cluded that it follows the rate law (2) without additional terms. However, most of their experiments were performed with bromide concentrations lower than 0.1 M, too low to see the term proportional to [Br − ] 2 [3,4,13], and we have decided to study once more this venerable reaction using higher bromide concentrations.…”

“…Several authors have studied the kinetics of reaction (1) [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”

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“…where C bromate is bromate concentration in bulk solution (activity for concentrated solutions), D bromide , diffusion coefficient of bromide, and where the comproportionation reaction constant, k com , depends strongly on pH [52], and -The ratio of the limiting diffusion currents for bromine and bromate from bulk solution toward the electrode surface (in the absence of the comproportionation step):…”

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“…A review of k Br values along eighty years can be found in the literature: 10 at 25 °C and ionic strength between 0.1 and 1.05, k Br ranged between 1.1 and 3.6 mol -3 L 3 s -1 in different electrolytes (mainly perchlorates). Taking into account the values of temperature (27 ºC) and ionic strength (0.14-0.34) used in the present work, the value of k Br in equation 6 is in good agreement with those precedents.…”