# Basic Principle of Potentiometry Potentiometry is an important electroanalytical method used for the qualitative and quantitative determination of ions and other chemical species in a solution. The basic principle of potentiometry is the measurement of the potential difference, or electromotive force (EMF), between two electrodes immersed in a solution, under conditions where essentially no current flows through the electrochemical cell. The measured potential is related to the activity or concentration of the ion being determined. Thus, potentiometry provides a convenient method for determining the concentration of a particular ionic species by measuring electrical potential rather than measuring the amount of current. A potentiometric cell generally consists of two electrodes: an indicator electrode and a reference electrode. The indicator electrode responds selectively to the activity of the ion of interest, whereas the reference electrode has a stable and known potential that remains practically constant during the measurement. Common reference electrodes include the saturated calomel electrode and the silver/silver chloride electrode. Indicator electrodes may be metallic electrodes, such as platinum or silver electrodes, or ion-selective electrodes such as the glass electrode used for pH measurement. When the two electrodes are connected to a high-resistance voltmeter or potentiometer and immersed in the test solution, a potential difference develops between them. This potential arises because of electrochemical processes occurring at the electrode–solution interfaces. Since the measuring instrument has very high electrical resistance, only a negligible current flows through the cell. Therefore, the system is considered to be at or very close to equilibrium. Under these conditions, the measured cell potential can be related to the thermodynamic activity of the species involved. The fundamental relationship governing potentiometric measurements is the Nernst equation. For a general reduction