Catalytic Kinetic Determinations: Nonenzymatic
Authors: Kawashima Takuji , Teshima Norio , Nakano Shigenori 
Published: September 2006

Although a catalyst does not shift the position of equilibrium but it does markedly change the reaction rate, because the catalyst lowers the activation energy of a reaction providing an alternative favorable route for the formation of the products. The reacted catalyst is regenerated and is repeatedly involved in an indicator reaction. When the rate depends on the concentration of the catalyst, it can be determined by measuring the change in the physical properties of the reactants and/or products in the indicator reaction. The change in a physical property such as absorbance, fluorescence, chemiluminescence (CL) or electrode potential of a reaction system is usually monitored with time. This methodology is called catalytic methods or kinetic–catalytic methods of analysis.

Kinetic–catalytic methods of analysis based on catalyzed reactions have progressed recently and have been applied to trace analyses for various catalysts (element) because of their extremely high sensitivity, low limit of detection and selectivity. Several reactions such as redox, ligand substitution and complexation reactions have been utilized as indicator reactions for the development of kinetic–catalytic methods of trace analyses. Most of the catalysts in redox reactions are metal ions or inorganic anions having different oxidation states such as cobalt, copper, chromium, iron, manganese, vanadium, selenium, iodide, bromide and nitrite. Oxidants capable of regenerating the catalyst are hydrogen peroxide, chlorate, bromate, periodate and dissolved oxygen. Reductants include ascorbic acid, hydroxylamine, hypophosphorous acid, alkali sulfides, dithiothreitol and tin(II).

Certain substances significantly increase the rate of catalyzed reactions. These substances are referred to as activators and have been defined as substances that do not catalyze the indicator reaction but enhance the catalytic effect of the catalyst. Activators provide high sensitivity and selectivity in kinetic–catalytic methods. Such improved specificity can be expected to eliminate the preliminary separation in many instances.

As the measurement includes time as an experimental variable, care is needed to ensure that mixing of reagents takes place at regular time intervals to obtain highly accurate results using a manual procedure. Such disadvantages in the manual system can be overcome by using flow‐injection analysis (FIA), in which the reaction time can easily be controlled by varying the flow rate of the solutions and the length of the reaction coil.

Kinetic–catalytic methods allow the determination of various elements at nanogram and picogram levels (10−11 M) with a simple and reasonable equipment. The detection limits (DLs) of the methods are of the order of picograms per milliliter with good accuracy and precision and are almost equal to those for inductively coupled plasma mass spectrometry (ICPMS).

The application of catalytic effects of metal ions in titrimetry has been recognized for a long time. This methodology is called catalytic titration and involves two consecutive reactions, i.e. titration reaction and indicator reaction. The determinable concentration is almost 10−6 M of the metal ions.