Direct Potentiometry:

In direct potentiometry, we measure a potential difference between two electrodes immersed in a solution and connecting the cell so formed to a voltage measuring device (potentiometer, vacuum tube voltmeter). The cell created through two electrodes (half-cells) and the solution is known as an electrochemical cell (a galvanic cell). A net cell reaction could be considered as the sum of the two half-cell reactions in which the each half-cell reaction is the representation of the actual chemical process that occurs at the individual electrode of the galvanic cell. A half-cell reaction always includes the electrons transferred.

Usually, one of the electrodes (half-cell) is chosen such that its potential is invariant and is termed as reference electrode. The potential of the other electrode is then a function of the concentration (more correctly activity) of the species involved in the electron transfer process (a redox reaction), through the Nernst Equation. This electrode is termed the indicator electrode. Under these conditions the cell emf is given by

E_cell  = (E_ind - E_ref ) + E _j                                                                     .... (1.42)

where

E_cell is emf of galvanic cell,

E_ind = half-cell potential of the indicator electrode,

E_ref= half-cell potential of the reference electrode,

E _j = liquid- junction potential developed at the interface between two electrolytes.

Concentration of the solution is determined by a single measurement of cell potential applying the Nernst Equation or from the calibration curve drawn for E_cell vs. logarithm of concentration of the species of interest, from the potentials measured for a series of solutions of known concentrations.

It is important that the indicator electrode should respond selectively to the species of analytical significance and this practical requirement has stimulated the development of many types of indicator electrodes. A common type of indicator electrode for potentiometric measurements consists of a metal in contact with a solution containing its ion. Such an electrode responds to the redox potential of a solution as established by the metal ion/metal redox couple. An inert metal, such as platinum, immersed in a solution containing an appropriate redox couple for example, Fe³⁺ and Fe^2+, can also be used to form an indicator electrode.

Several electrodes with good selectivity for specifications are based on the development of potential across a membrane. Electrodes of this category are referred to since ion-selective electrodes.

The glass electrode commonly used for pH measurement is an ion selective membrane electrode. The refine selectivity and wide pH range of the conventional glass electrode makes the pH determination of the solution as an important analytical application of the direct potentiometry. The instrument used for this purpose is known as pH-meter which measures the potential of a glass electrode with respect to the external reference electrode and displays the output on a meter scale calibrated to read directly in pH (or with a digital pH read out) the potential of the glass electrode changes by 59 mV per unit change in pH.