Experimental Methods

A kinetic study generally proceeds after the reactants, products and stoichiometry of the reaction have been satisfactorily characterized. The more one knows about the chemistry of the reaction, the better the conclusions that one can draw from a kinetic study. The discussion here describes techniques often used in inorganic studies, emphasizes their time range and general area of applicability and gives some examples of their use. Further details can be found in other sources. Any experimental kinetic method must somehow monitor change of concentration with time. Many studies are done under pseudo-first-order conditions, and then one must monitor the deficient reactant or product(s) because the other species undergo small changes in concentration. The kinetic method(s) of choice often will be dictated by the time scale of the reaction. The detection method(s) will be determined by the spectroscopic properties of the species to be monitored. The efficient use of materials can be a significant factor in the choice of method because a kinetic study generally involves a number of runs at different concentrations and temperatures, and conservation of difficult to prepare or expensive reagents may be a critical factor. The detection method should be as species specific as possible, and ideally one would like to measure both reactant disappearance and product formation. The method must not be subject to interference from other reactants and should be applicable under a wide range of concentration conditions so that the rate law can be fully explored. Often there is a practical trade-off between specificity, sensitivity and reaction time. For example, NMR is quite specific but rather slow and has relatively low sensitivity, unless the system allows time for signal accumulation. Spectrophotometry in the UV and visible range often has good sensitivity and speed, but the specificity may be poor because absorbance bands are broad and intermediates may have chromophoric properties similar to those of the reactant and/or product. Vibrational Spectrophotometry can be better if the IR bands are sharp, as in the case of metal carbonyls, but the solvent must be chosen to provide an appropriate spectral window.