22: An Introduction to Electroanalytical Chemistry
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In Chapters 6–21 we examined a wide range of spectroscopic techniques that take advantage of the interaction between electromagnetic radiation and matter. In this chapter we turn our attention to electrochemical techniques in which the potential, current, or charge in an electrochemical cell serves as the analytical signal.
Although there are only three fundamental electrochemical signals, there are many possible experimental designs—too many, in fact, to cover adequately in an introductory textbook. The simplest division of electrochemical techniques is between bulk techniques, in which we measure a property of the solution in the electrochemical cell, and interfacial techniques, in which the potential, current, or charge depends on the species present at the interface between an electrode and the solution in which it sits. The measurement of a solution’s conductivity, which is proportional to the total concentration of dissolved ions, is one example of a bulk electrochemical technique. A determination of pH using a pH electrode is an example of an interfacial electrochemical technique. Only interfacial electrochemical methods receive further consideration in this textbook. In this chapter we provide an introduction to electrochemistry, introducing ideas relevant to understanding the specific electroanalytical methods introduced in Chapters 23–25.
- 22.1: Electrochemical Cells
- The electrochemical cell consists of two half-cells, each of which contains an electrode immersed in a solution of ions whose activities determine the electrode’s potential. A salt bridge that contains an inert electrolyte, such as KCl, connects the two half-cells. The ends of the salt bridge are fixed with porous frits, which allow the electrolyte’s ions to move freely between the half-cells and the salt bridge. This movement of ions in the salt bridge completes the electrical circuit.
- 22.2: Potentials in Electroanalytical Cells
- If an electrochemical cell is at equilibrium, there is no current and the potential is fixed. If we change the potential, current flows as the system moves to its new equilibrium position. Alternatively, we can pass a current through the cell and effect a change in potential. If we choose to control the potential, then we must accept the resulting current, and we must accept the resulting potential if we choose to control the current.
- 22.3: Electrode Potentials
- The potential of an electrochemical cell is the difference between the potential at the cathode and the potential at the anode where both potentials are defined in terms of a reduction reaction (and are called reduction potentials).
- 22.4: Calculation of Cell Potentials from Electrode Potentials
- The potential of an electrochemical cell is the difference between the electrode potentials of the cathode and the anode.
- 22.5: Currents in Electrochemical Cells
- Most electrochemical techniques rely on either controlling the current and measuring the resulting potential, or controlling the potential and measuring the resulting current; only potentiometry measures a potential under conditions where there is essentially no current. Understanding the relationship between current and potential is important. Although, the experimentally measured potentials may differ from their thermodynamic values for a variety of reasons that we outline here.
- 22.6: Types of Electroanalytical Methods
- We divide electrochemical techniques into static techniques and dynamic techniques. In a static technique we do not allow current to pass through the electrochemical cell and, as a result, the concentrations of all species remain constant. Dynamic techniques, in which we allow current to flow and force a change in the concentration of species in the electrochemical cell, comprise the largest group of interfacial electrochemical techniques.