7652
J. Phys. Chem. A 2010, 114, 7652
Comment on “New Experimental Data and Mechanistic Studies on the Bromate-Dual Substrate-Dual Catalyst Batch Oscillator” Cristina M. P. Santos and Roberto B. Faria* Instituto de Quı´mica, UniVersidade Federal do Rio de Janeiro, AV. Athos da SilVeira Ramos, 149, Centro de Tecnologia, Bloco A, 21941-909 Rio de Janeiro, RJ, Brazil ReceiVed: January 1, 2010; ReVised Manuscript ReceiVed: April 22, 2010 Experimental results obtained by Szalai et al.1 on the dual substrates dual catalyst bromate-hypophosphite-acetone-Mn(II)/Mn(III)[Ru(bpy)3]2+/[Ru(bpy)3]3+ batch oscillator and on some of its subsystems allow one to get a good insight on the complex chemistry of this system, especially on its inorganic part. However, the proposed model (see Tables 1 and 2 in ref 1) used to explain the nonlinear behavior of this system contains some cross effects between its species that prevent it from being considered as a realistic model. Initially, it is worthwhile to mention that we were able to reproduce Figures 7 and 9 of their article. To reproduce Figure 7, we used k11 ) 2 s-1 and [Mn(II)]0 ) 0.003 mol dm-3, and to reproduce Figure 9, we used [Ru(III)]0 ) 5 × 10-5 mol dm-3. All other initial concentrations were the same as indicated in the captions of Figures 7 and 9. All numerical integration has been done using a program written by Dr. Istva´n Lengyel in Turbo Pascal 6.0 and adapted to Free Pascal 2.0.4 to solve autonomous ordinary differential equation systems by a semi-implicit Runge-Kutta method.2 Initial concentrations formally considered as zero were indeed 1 × 10-10 mol dm-3 background concentrations.3 However, calculating Figure 9 for the bromate-acetone-Ru(II) subsystem, we have found that [Mn(III)] oscillates in a concentration range, approximately 100 times higher than [Ru(III)], even when [Mn(II)]0 is equal to zero, as can be seen in Figure 1. The reason for this is that (R3) is a kind of global reaction that produces Mn(III) even when no Mn(II) is present. As the rate law for this reaction does not contain [Mn(II)], the program code for numerical integration does not prevent the buildup of Mn(III) concentration, which is independent of [Mn(II)]0. The artificially formed Mn(III) reacts with Ru(II), producing Ru(III) by (R14).
BrO3- + HBrO2 + 3H+ + 2Mn(II) f 2HBrO2 + 2Mn(III) + H2O (R3) Mn(III) + Ru(II) f Mn(II) + Ru(III)
(R14)
When reproducing Figure 7 of ref 1, we found another problem. Because [Mn(II)]0 was not given in this figure caption, * Corresponding author. E-mail:
[email protected].
Figure 1. Simulation results for the bromate-acetone-Ru(II) subsystem showing the synchronized Ru(III) and Mn(III) unrealistic oscillations. [BrO3-]0 ) 0.02 mol dm-3, [Br-]0 ) 1 × 10-5 mol dm-3, [Ru(III)]0 ) 5 × 10-5 mol dm-3, [Ru(II)]0 ) [Mn(II)]0 ) [Mn(III)]0 ) [H3PO2]0 ) 0 mol dm-3, f ) 1, k10 ) 0.5 s-1, k11 ) 2 s-1. Compare with Figure 9 in ref 1.
we used several different values for [Mn(II)]0 and observed that the results did not change, all being equal to that shown in Figure 7. The explanation is the same as above: as [Mn(II)] does not appear in the (R3) rate law, it does not matter which initial concentration value is used; Mn(III) will be formed at a speed that depends only on bromate and HBrO2 concentrations. In conclusion, the simulated oscillations on the Ru(III) concentration shown in Figure 9 of ref 1, at zero initial Mn(II) concentration, are pumped by the oscillation on the Mn(III) concentration, which, obviously, should not happen in the absence of manganese ions. Therefore, a model for the bromatehypophosphite-acetone-Mn(II)-Ru(bpy)32+ batch oscillator is still in need of development. Acknowledgment. We thank Dr. Istva´n Lengyel for the use of his Turbo Pascal code and CNPq and FAPERJ for financial support. References and Notes (1) Szalai, I.; Kurin-Cso¨rgel, K.; Horva´th, V.; Orba´n, M. J. Phys. Chem. A 2006, 110, 6067. (2) Kaps, P.; Rentrop, P. Num. Math. 1979, 33, 55. (3) Pereira, J. A. M.; Faria, R. B. J. Phys. Chem. A 1997, 101, 5605.
JP100011B
10.1021/jp100011b 2010 American Chemical Society Published on Web 06/29/2010
