Kinetics of Biocatalytical Synergistic Reactions

Kinetics of biocatalytical synergistic reactions has been analyzed at non-stationary state (NSS) and at quasi steady state (QSS) conditions. The application to the model kinetic constants taken from the first type of the experiments shows that QSS can be established for the enzyme and the mediator at time less than 1 s. Therefore, the analytical solution of the initial rate (IR) may be produced at relevant to an experiment time, and the dependence of the IR on substrates concentration may be analyzed rather easy. The use of kinetic constants from the second type of reactions shows that QSS is formed for the enzyme but not for the mediator. For this reason the modeling of the synergistic process was performed by solving the ordinary differential equations (ODE). For this purpose the novel program KinFitSim (c) was used

Modelling Complex Chemical Processes in Homogeneous Solutions: Automatic Numerical Simulation

Two algorithms for the determination of the necessary limit of local error for the numerical solution of ordinary differential equation (ODE) systems describing homogeneous chemical and biochemical processes, and for the evaluation of their stiffness are developed. The approach for finding the necessary limit of local error of a numerical ODE solver is justified by the proof of the corresponding theorems. The application of the new algorithms implemented in version 2.1 of KinFitSim software to the simulation of real chemical systems is considered on the example of Belousov-Zhabotinsky reaction

An exhaustive search approach for chemical kinetics experimental data fitting, rate constants optimization and confidence interval estimation

Finding reaction rate constants in a complex chemical mechanism is a complicated problem. These complications can be attributed mainly to the nonlinearity of such systems, where the least squares optimization procedure fail when experimental data are being fitted. Here we analyze two cases: analytical and numerical. From analytical case possible issues of optimization are found and methods to cope with them are derived. The complex case shows how an exhaustive search fitting approach works. Our results indicate that certain properties of systems can be found only by running grid search on entire rate constant set and minima found have practical value only if the properties of entire fitting score surface are known

The kinetic and radiolytic aspects of control of the redox speciation of neptunium in solutions of nitric acid

Neptunium, with its rich redox chemistry, has a special position in the chemistry of actinides. With a decades-long history of development of aqueous separation methods for used nuclear fuel (UNF), management of neptunium remains an unresolved issue because of its not clearly defined redox speciation. Neptunium is present in two, pentavalent (V) and hexavalent (VI) oxidation states, both in their dioxocation O=Np=O neptunyl form, which differ greatly in their solvent extraction behavior. While the neptunium(VI) dioxocation is being very well extracted, the dioxocation of pentavalent neptunium is practically non-extractable by an organic solvent. As a result, neptunium is not well separated and remains distributed in both organic and aqueous extraction phases. The aim of this study was to develop or enhance the understanding of several key topics governing the redox behavior of neptunium in nitric acid medium, which are of vital importance for the engineering design of industrial-scale liquid-liquid separation systems. In this work, reactions of neptunium(V) and (VI) with vanadium(V) and acetohydroxamic acid - two redox agents envisioned for adjusting the neptunium oxidation state in aqueous separations – were studied in order to determine their kinetic characteristics, rate laws and rate constants, as a function of temperature and nitric acid concentration. Further were analyzed the interactions of neptunium(V) and (VI) with nitrous acid, which is formed as a product of radiolytic degradation of nitric acid caused by high levels of radioactivity present in such systems. Once HNO₃ is distributed between both the aqueous solutions and organic solvent, nitrous acid is also formed in both phases and has a key influence on redox speciation of neptunium; therefore, the effects of gamma-radiation on the redox speciation of neptunium were investigated. The work also includes the results of examination of scavenging of nitrous acid by hydrogen peroxide, which is generated along with nitrous acid during radiolysis of aqueous solutions of nitric acid, and also by chemical reactions with added scavenging agents (methylurea, acetohydroxamic acid)

’KINFITSIM’–a software to fit kinetic data to a user selected mechanism.

A new software package, ’KINFITSIM’, for fitting and simulating kinetic data is presented. The main goals of the KINFITSIM package are to obtain the best-fit parameters-rate constants, amplitudes and others-to a user specified chemical mechanism, plots of the calculated and experimental absorbance versus time, and a report to the user with the results. The KINFITSIM package can be used in either chemical research or for educational purposes