Potentiometry salt bridge

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

Potentiometry requires a reference electrode, a working electrode and a potentialmeasuring instrument, e.g. voltmeter, otherwise known as a potentiometer. The test solution must be in direct contact with the working electrode, which is sometimes referred to as the chemical sensor as it is sensing the output of a chemical reaction. The reference electrode can also be placed in the test solution or can be brought into contact with the test solution via a salt bridge. The measured potential can be related to the concentration of the species being measured and this approach is called direct potentiometry. 

In potentiometry, the potential of a suitable indicator electrode is measured versus a reference electrode, i.e. an electrode with a constant potential. Whereas the indicator electrode is in direct contact with the analyte solution, the reference electrode is usually separated from the analyte solution by a salt bridge of various forms. The electrode potential of the indicator electrode is normally directly proportional to the logarithm of the activity of the analyte in the solution. Potentiometric methods have been and are still frequently used to indicate the end point of titrations. This use has been known since the end of the nineteenth century. Direct potentiometric determinations using ion-selective electrodes have been mainly developed in the second half of the twentieth century. 

Potentiometry has long been a significant part of instrumental chemical analysis. The experimental set-up is simple (Fig. 7.1). As reference electrodes, those of the second kind are useful. They are createdby a combination of a metal/metal ion electrode and a stock of a sparingly soluble salt of the participating metal Among the practical examples is the silver/silver chloride electrode with a salt bridge of potassium chloride solution, mentioned in Chap. 2, Sect 2.2.6. The instrument for potential measurement must be charcterized by a very high input resistance. So-called pH meters are well suited for this purpose since they... 

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