Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. Figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is
Ref(samp) || [A]samp | [A]int || Ref(int)
where the membrane is represented by the vertical slash (|) separating the two solu- tions containing analyte. Two reference electrodes are used; one positioned within the internal solution, and one in the sample solution. The cell potential, therefore, is
Ecell = ERef(int) – ERef(samp) + Emem + Elj ……….11.6
where Emem is the potential across the membrane. Since the liquid junction poten- tial and reference electrode potentials are constant, any change in the cell’s potential is attributed to the membrane potential.
Interaction of the analyte with the membrane results in a membrane potential if there is a difference in the analyte’s concentration on opposite sides of the mem- brane. One side of the membrane is in contact with an internal solution containing a fixed concentration of analyte, while the other side of the membrane is in contact with the sample. Current is carried through the membrane by the movement of ei- ther the analyte or an ion already present in the membrane’s matrix. The membrane potential is given by a Nernst-like equation
where [A]samp and [A]int are the concentrations of analyte in the sample and the internal solution, respectively, and z is the analyte’s charge. Ideally, Emem should be zero when the concentrations of analyte on both sides of the membrane are equal. The term Easym, which is called an asymmetry potential, accounts for the fact that the membrane potential is usually not zero under these conditions.
Substituting equation 11.7 into equation 11.6, assuming a temperature of 25 °C and rearranging gives
where K is a constant accounting for the potentials of the reference electrodes, any liquid junction potentials, the asymmetry potential, and the concentration of ana- lyte in the internal solution. Equation 11.8 is a general equation, and applies to all types of ion-selective electrodes.
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