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Potentiometry is the field of electroanalytical chemistry in which potential is measured under the conditions of no current flow. The measured potential may then be used to determine the analytical quantity of interest, generally the concentration of some component of the analyte solution. The potential that develops in the electrochemical cell is the result of the free energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition has been satisfied.
This concept is typically introduced in quantitative analysis courses in relation to electrochemical cells that contain an anode and a cathode. For these electrochemical cells, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell.
If the reaction is conducted under standard state conditions, this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must utilize the Nernst equation to determine the cell potential.
Physical phenomena which do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion concentration gradients across a semi-permeable membrane. This can also be a potentiometric phenomena, and is the basis of measurements that use ion-selective electrodes.
See the documents on ion-selective electrodes and pH meters.
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