13: Kinetic Methods
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There are many ways to categorize analytical techniques, several of which we introduced in earlier chapters. In Chapter 3 we classified techniques by whether the signal is proportional to the absolute amount of analyte or the relative amount of analyte. For example, precipitation gravimetry is a total analysis technique because the precipitate’s mass is proportional to the absolute amount, or moles, of analyte. UV/Vis absorption spectroscopy, on the other hand, is a concentration technique because absorbance is proportional to the relative amount, or concentration, of analyte.
A second way to classify analytical techniques is to consider the source of the analytical signal. For example, gravimetry encompasses all techniques in which the analytical signal is a measurement of mass or a change in mass. Spectroscopy, on the other hand, includes those techniques in which we probe a sample with an energetic particle, such as the absorption of a photon. This is the classification scheme used in organizing Chapters 8–11.
An additional way to classify analytical techniques is by whether the analyte’s concentration is determined under a state of equilibrium or by the kinetics of a chemical reaction or a physical process. The analytical methods described in Chapter 8–11 mostly involve measurements made on systems in which the analyte is at equilibrium. In this chapter we turn our attention to measurements made under nonequilibrium conditions.
- 13.1: Kinetic Techniques versus Equilibrium Techniques
- 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.
- 13.2: Chemical Kinetics
- The earliest analytical methods based on chemical kinetics—which first appear in the late nineteenth century—took advantage of the catalytic activity of enzymes. Despite the diversity of chemical kinetic methods, by 1960 they no longer were in common use. By the 1980s, improvements in instrumentation and data analysis methods compensated for these limitations, ensuring the further development of chemical kinetic methods of analysis.
- 13.3: Radiochemistry
- Atoms that have the same number of protons but a different number of neutrons are isotopes. Although an element’s different isotopes have the same chemical properties, their nuclear properties are not identical. The most important difference between isotopes is their stability. The nuclear configuration of a stable isotope remains constant with time. Unstable isotopes, however, disintegrate spontaneously, emitting radioactive particles as they transform into a more stable form.
- 13.4: Flow Injection Analysis
- In this section we consider the technique of flow injection analysis in which we inject the sample into a flowing carrier stream that gives rise to a transient signal at the detector. Because the shape of this transient signal depends on the physical and chemical kinetic processes that take place in the carrier stream during the time between injection and detection, we include flow injection analysis in this chapter.
- 13.5: Problems
- End-of-chapter problems to test your understanding of topics in this chapter.
- 13.6: Additional Resources
- A compendium of resources to accompany topics in this chapter.
- 13.7: Chapter Summary and Key Terms
- Summary of chapter's main topics and a list of key terms introduced in this chapter.
Thumbnail: Determination of a reaction’s intermediate rate from the slope of a line tangent to a curve showing the change in the analyte’s concentration as a function of time.