Electrochemical Cells for Electroanalysis

The reference electrode system is kept constant so that the composition of the cell up to the liquid junction does not change in the experiment. When the composition of the test electrolyte solution is changed, the cell EMF changes. These changes can be related to the concentration of a potential determining ion in the test solution via a calibration procedure. The system is also designed to keep the liquid junction potential constant and as small as possible. Thus, the observed potential drop across the cell can be written as 

As stated earlier, the reference electrode in a cell used for electroanalysis is designed so that its potential is independent of the composition of the test solution. There are several general properties that reference electrodes should have in order to be useful in analysis (1) they should be reversible with an electrode potential which is independent of time and reproducible (2) they should have a small temperature coefficient (3) they should be ideally non-polarizable with negligible effects from the flow a small current through the system and (4) they should be easily constructed. The most commonly used reference electrodes are those based on on the mercury calomel system and the silver silver chloride system. The electrolyte most commonly used in these systems is KCl. Relevant parameters for commonly used reference electrodes are given in table 9.4. 

The calomel electrode is based on reaction (9.2.42). Insoluble mercury(I) chloride (calomel) lies on top of the liquid mercury phase and ensures that the solution is saturated with Hg2. The potential of the electrode depends on the chloride ion activity as follows  

The most common electrolyte for this reference electrode is saturated KCl. This choice ensures that the liquid junction potential between the reference electrode 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

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