Kinetic Methods of Analysis
Asystem under thermodynamic control is in a state of equilibrium, and its signal has a constant, or steady-state value (Figure 13.1a). When a system is under kinetic control, however, its signal changes with time (Figure 13.1b) until equilibrium is established. Thus far, the techniques we have considered have involved measurements made when the system is at equilibrium.
By changing the time at which measurements are made, an analysis can be carried out under either thermodynamic control or kinetic control. For example, one method for determining the concentration of NO3– in groundwater involves the diazotization reaction shown in Figure 13.2. The final product, which is a reddish-purple azo dye, absorbs visible light at a wavelength of 543 nm. Since the concentration of dye is determined by the amount of NO3– in the original sample, the solution’s absorbance can be used to determine the concentration of NO3–. The reaction in the second step, however, is not instantaneous. To achieve a steady-state signal, such as that in Figure 13.1a, the absorbance is measured following a 10-min delay. By measuring the signal during the 10-min development period, information about the rate of the reaction is obtained. If the reaction’s rate is a function of the concentration of NO3–, then the rate also can be used to determine its concentration in the sample.
There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control: chemical kinetic techniques, in which the rate of a chemical reaction is measured; radiochemical techniques, in which a radioactive element’s rate of nuclear decay is measured; and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion.