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9.6.1: Kinetic Techniques versus Equilibrium Techniques

  • Page ID
    222705
  • In an equilibrium method the analytical signal is determined by an equilibrium reaction that involves the analyte or by a steady-state process that maintains the analyte’s concentration. When we determine the concentration of iron in water by measuring the absorbance of the orange-red \(\text{Fe(phen)}_3^{2+}\) complex, the signal depends upon the concentration of \(\text{Fe(phen)}_3^{2+}\), which, in turn, is determined by the complex’s formation constant. In the flame atomic absorption determination of Cu and Zn in tissue samples, the concentration of each metal in the flame remains constant because each step in the process of atomizing the sample is in a steady-state. In a kinetic method the analytical signal is determined by the rate of a reaction that involves the analyte or by a nonsteady-state process. As a result, the analyte’s concentration changes during the time in which we monitor the signal.

    In many cases we can choose to complete an analysis using either an equilibrium method or a kinetic method by changing when we measure the analytical signal. For example, one method for determining the concentration of nitrite, \(\text{NO}_2^-\), in groundwater utilizes the two-step diazotization re-action shown in Figure 13.1.1 [Method 4500-NO2 B in Standard Methods for the Analysis of Waters and Wastewaters, American Public Health Association: Washington, DC, 20th Ed., 1998]. The final product, which is a reddish-purple azo dye, absorbs visible light at a wavelength of 543 nm. Because neither reaction in Figure 13.1.1 is rapid, the absorbance—which is directly proportional to the concentration of nitrite—is measured 10 min after we add the last reagent, a lapse of time that ensures that the concentration of the azo dyes reaches the steady-state value required of an equilibrium method.

    Figure 13.1.1 . Analytical scheme for the analysis of NO-2 in groundwater. The red arrows highlights the nitrogen in \(\text{NO}_2^-\) that becomes part of the azo dye.

    We can use the same set of reactions as the basis for a kinetic method if we measure the solution’s absorbance during this 10-min development period, obtaining information about the reaction’s rate. If the measured rate is a function of the concentration of \(\text{NO}_2^-\), then we can use the rate to determine its concentration in the sample [Karayannis, M. I.; Piperaki, E. A.; Maniadaki, M. M. Anal. Lett. 1986, 19, 13–23].

    There are many potential advantages to a kinetic method of analysis, perhaps the most important of which is the ability to use chemical reactions and systems that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic control: chemical kinetic techniques, in which we measure the rate of a chemical reaction; radiochemical techniques, in which we measure the decay of a radioactive element; and flow injection analysis, in which we inject the analyte into a continuously flowing carrier stream, where its mixes with and reacts with reagents in the stream under conditions controlled by the kinetic processes of convection and diffusion.

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