Potentiometry using ion-selective electrodes

Ion-selective electrodes, when immersed in test solutions, develop a potential which should be directly related to the logarithm of the activity of a specific hydrated ion, but as noted earlier, the response can be distorted due to interaction with other ions in the test sample. 

In biological systems, most of the elements are present as complexes, rather than as free ions, hence direct potentiometric measurements provide little information except for major electrolytes such as Fl+, Na+, K+, Cl- and possibly Ca2+, Mg2+. The introduction of ion-selective microelectrodes (e.g. tips 1 jtm diameter) has allowed these major electrolyte ions to be determined in single cells. 

Distinction between the amount of a particular ion present in a free form, as distinct from complexed content, can sometimes be achieved by observing the 

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As noted above, voltage is developed between two electrodes in an electrochemical cell. The voltage depends upon the kinds and concentrations of dissolved chemicals in the solutions contacted by the electrodes. In some cases this voltage can be used to measure concentrations of some substances in solution. This gives rise to the branch of analytical chemistry known as potentiometry. Potentiometry uses ion-selective electrodes or measuring electrodes whose potentials relative to a reference electrode vary with the concentrations of particular ions in solution. The reference electrode that serves as the ultimate standard for potentiometry is the standard hydrogen electrode shown in Figure 8.13. In practice, other electrodes, such as the silver/silver chloride or calomel (mercury metal in contact with are... 

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