**4. Kinetic modeling**

Given the proposed scheme (**Figure 6**), tartaric acid speciation, and iron speciation [12, 13], the experimental Fe(III), oxygen, and hydrogen peroxide simultaneous measurements were fitted using kinetic modeling software, Kintecus [25]. The fitting curves for pH 2.5, 3.5, and 4.5 are shown in **Figure 5** and the resulting kinetic constants are shown in **Table 1**. The modeling provided a reasonable fit for all conditions, especially the highly complex pH 2.5 and 265 μM Fe(II) case where Fe(III) decomposes with a similar timing to hydrogen peroxide.

The ability of the model to fit experimental data across pH provides directional confidence. It should be recognized that the k values are not constant across pH and *Recent Advances in Chemical Kinetics*


**Table 1.**

*Estimated kinetic constants for 265 μM initial Fe(II) in air-saturated 26.7 mM tartaric acid at pH 2.5.*

further examination of species and pH dependent reactions is required, however the robustness of the model and k values within a single pH can be evaluated by making predictions and examining the resulting fit. **Figure 7** shows two predicted curves against experimental measurements where the initial condition of hydrogen peroxide concentration is changed by adding 2.65 μM and 26.5 μM. The predicted and actual measurements are nearly identical, which speaks to the power of the modeling and mechanism.

#### **Figure 7.**

*Oxygen consumption dependence on addition of H2O2 and predictions. Time traces with 0 μM ( ), 2.65 μM ( ), and 26.5 μM ( ) hydrogen peroxide added at initiation (t = 0); reaction of 265 μM Fe(II) in air-saturated 26.7 mM tartaric acid at (A) pH 2.5. Fitted trace with 0 μM ( ) added hydrogen peroxide. Predicted traces with 2.65 μM ( ), and 26.5 μM ( ) added hydrogen peroxide. Reproduced from [10], with the permission of AIP Publishing.*
