Kinetic modeling and parameter optimization of iodine hydrolysis reactions

Modeling of iodine hydrolysis is discussed, and rate constants for two particular formulations are obtained from a mathematical optimization procedure. The two models include six and ten principal reaction steps, and axe derived to be relevant in situations where radiolysis is also occurring. The resulting kinetic rate equations form a nonlinear system of ordinary differential equations whose solution is straightforward, given values for the rate coefficients and initial species inventories. Since the former are not known a priori, they are treated as parameters whose optimal values are determined by fitting the calculated system solution to data. To accomplish this, a methodology is developed which utilizes a quasi-Newton method to minimize a least squares residual. In addition, an experimental effort has resulted in over 130 data points taken in a range of pH between 2.8 and 9.0, and at temperatures 25, 50, and 90°C. The optimization procedure is applied to yield rate constants at 25°C for both models and activation energies for the ten-reaction model. Using the optimized values, the rate equations are solved to obtain time-dependent iodine species inventories. These calculated results compare favorably with actual data, indicating that the models are useful for simulating both forward hydrolysis and the reverse (Dushman) reaction.