Chapter 22 - Chapter 22 An Introduction to...

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Unformatted text preview: Chapter 22 An Introduction to Electroanalytical Chemistry Electroanalytical chemistry encompasses a group of quantitative analytical methods that are based upon the electrical properties of a solution of the analyte when it is made part of an electrochemical cell. Electroanalytical methods have certain general advantages over other types of procedures often specific for a particular oxidation state of an element. Instrumentation is relatively inexpensive. Provide information about activities rather than concentrations of chemical species. ELECTROCHEMICAL CELLS A dc electrochemical cell consists of two electrical conductors called electrodes , each immersed in a suitable electrolyte solution. For a current to develop in a cell, it is necessary (1) that the electrodes be connected externally by means of a metal conductor, (2) that the two electrolyte solutions be in contact to permit movement of ions from one to the other, and (3) that an electron transfer reaction can occur at each of the two electrodes. Conduction in a Cell Charge is conducted by three distinct processes in various parts of the cell 1. In the copper and Zinc electrodes, as well as in the external conductor, electrons serve as carriers, moving from the zinc through the conductor to the copper. 2. Within the solutions the flow of electricity involves migration of both cations and anions. In the half cell on the left, zinc ions migrate away from the electrode, whereas sulfate and hydrogen sulfate ions move toward it; in the other compartment, copper ions move toward the electrode and anions away from it. Within the salt bridge, electricity is carried by migration of potassium ions to the right and chloride ion to the left. continued 3. A third process occurs at the two electrode surfaces. Here, an oxidation or a reduction reaction provides a mechanism to provide a complete circuit for the flow of charge. The two electrode processes are described by the equations Zn(s) Zn 2+ + 2e- Cu 2+ + 2e- Cu(s) Solution Structure; The Double Layer Electrochemical measurements involve heterogeneous systems because an electrode can only donate or accept electrons from a species that is present in a layer of solution that is immediately adjacent to the electrode. Thus, this layer may have composition that differs significantly from that of the bulk of the solution. The surface of the metal electrode is shown as having an excess of positive charge as a consequence of an applied positive potential. continued The charged solution layer consists of two parts: (1) a compact inner layer (d to d 1 ), in which the potential decreases linearly with distance from the electrode surface and (2) a diffuse layer (d 1 to d 2 ), in which the decrease is exponential....
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Chapter 22 - Chapter 22 An Introduction to...

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