H and Ion Activity Measurements

The unique property of ion-sensitive electrodes is a membrane between the test solution and the electrode sensor that develops an electrical potential, or voltage, in response to a change in the concentration of a single ion. The pH electrode, for example, is shown schematically in Fig. 10.5. Other ion-sensitive electrodes differ in the composition of the membrane and in the salts necessary to develop the potential. In the pH electrode, a silver wire coated with AgCl dips into an HC1 solution. The HCI solution is separated from the test solution by a membrane of special glass, usually a lithium silicate. Differences in H+ activity across this glass membrane cause a difference in electrical potential, which can be measured by a sensitive potentiometer. 

The electrode potentials developed by this electrode are the membrane potential plus the potential of the Ag—AgCl—HCI reaction inside the electrode-. 

This reaction is reversible and, since the activities of Ag and AgCl can be taken as unity and the CP activity is fixed by the constant HC1 concentration, the Ag—AgCl potential (E° = 0.222 V at 25° C) is constant. This potential is accounted for when the electrode is standardized against a standard pH buffer solution. 

The electrode develops a second potential across the membrane separating the standard HC1 from the test solution. The tiny current flow required by the pH meter causes ion exchange at the inner and outer surfaces of the glass membrane and causes diffusion of ions across the glass membrane. The electrical current is of the order of nano- or picoamperes and diffusion of trace quantities of Na+ in the glass apparently carries the current. The potential of the pH electrode is 

A second or reference electrode is necessary to complete the electrical circuit. The reference electrode is sometimes welded to the pH electrode so that the pair look like a single electrode. Reference electrodes are too often taken for granted their spurious potentials are a common source of error in soil pH measurements. A typical reference electrode is also sketched in Fig. 10.5. The wire dipping into the liquid mercury makes electrical contact with the pH meter, and current flows from the electrode to the solution phase through the reversible reaction ( ° = 0.268 V at 25° C)  

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