Electrochemical Methods of Analysis PDF

8  ELECTROCHEMICAL METHODS OF ANALYSIS These are physicochemical methods in which we measure some electrical quantity or property of a solution of the analyte, which has quantitative relation with the concentration of the analyte to be determined. The measurements are carried out using certa...

8  ELECTROCHEMICAL METHODS OF ANALYSIS These are physicochemical methods in which we measure some electrical quantity or property of a solution of the analyte, which has quantitative relation with the concentration of the analyte to be determined. The measurements are carried out using certain instruments; thus, the methods are also denoted by instrumental methods. These methods can be classified into: I-Methods depending on the movement of ions in an electric field without occurrence of redox reaction at the electrode surface i.e. no electron transference e.g. conductometry. II- Methods depending on measuring of current or voltage between two electrodes where redox reactions take place at the electrodes surfaces i.e. electron transfer occurs e.g. polarography and Potentiometry, respectively. 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 that: (1) The electrodes be connected externally by means of a metal conductor (2) The two electrolyte solutions be in contact to permit movement of ions from one to the other (3) An electron transfer reaction can occur at each of the two electrodes. 9 1- Galvanic electrochemical cell e.g. Daniel cell: 10 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 nitrate ions move toward it; in the other compartment, copper ions move toward the electrode and nitrate ions away from it. Within the salt bridge, electricity is carried by migration of sodium ions to the right and chloride ion to the left. 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) Zn2+ + 2e- & Cu2+ + 2e- Cu(s) The net cell reaction that occurs in the cell is the sum of the two half-cell reactions Zn(s) + Cu2+ Zn2+ + Cu(s) 2- Electrolytic electrochemical cell Electrolytic cells consume electrical energy, e.g. the cell under discussion could be made electrolytic by connecting the negative terminal of a dc power supply to the zinc electrode and the positive terminal to the copper electrode. If the output of this supply was made somewhat greater than 1.1 V, the two electrode reactions would be reversed, and the net cell reaction would become Cu(s) + Zn2+ Cu2+ + Zn(s) It is used in electroplating e.g. with AuCl3 when current is passed the Au is precipitated on the electrode while Cl2 gas is evolved. 11 N.B.: 1- The cathode of an electrochemical cell is the electrode at which reduction occurs, while the anode is the electrode where oxidation takes place. The copper electrode is the cathode while the zinc electrode is the anode. In contrast, where this same cell is operated as an electrolytic cell, the copper electrode would be the anode and the zinc electrode the cathode. 2- Salt bridge: is agar saturated with salt that should be strong electrolyte and whose 2 ions have the same ionic mobility e.g. KCl 12  Schematic Representation of Cells To simplify the description of cells, chemists often employ a shorthand notation, e.g. the cells shown before can be described by Zn/Zn(NO3)2 (a Zn 2+ = 0.0100) // Cu(NO3)2 (a Cu 2+ = 0.0100)/Cu 1- By convention, the anode and information about the solution with which it is in contact is always listed on the left. While, the cathode and information about the solution with which it is in contact is always listed on the right. 2- Single vertical lines (|) represent phase boundaries across which potential differences may develop. 3- The double vertical lines (||) indicates the salt bridge. 4- Because the potential of a cell is dependent upon activities of the cell components, it is common practice to provide activity or concentration data for the cell constituents in parentheses. 5- A comma (,) to separate species in the same phase. 13