Coulometry is a form of quantitative analysis used to determine the concentration of a substance in solution — generally referred to as the analyte — by measuring the amount of electricity required to change the oxidation state of the substance. The oxidation state of the analyte can be reduced by adding electrons or increased by removing electrons. The transfer of electrons to or from a substance can take place chemically in reduction-oxidation reactions, but can also be achieved by electricity, which is a flow of electrons, using electrolysis. By measuring the amount of electrical charge transferred during the complete oxidation or reduction of the analyte by electrolysis, it is possible to calculate the amount that was present. This is in accordance with Faraday’s first law of electrolysis, which states that the amount of a substance transformed during electrolysis is in direct proportion to the amount of electricity transferred.
In chemistry, the unit of measurement for the quantity of a substance is normally the mole, which is defined as the number of atoms in 0.42 ounces (12 grams) of carbon-12. Electrons can also be measured in moles. It is therefore possible to calculate how many moles of electrons would be needed to convert, for example, a given amount of copper in solution from its +2 oxidation state to neutral copper metal. Two moles of electrons are required for each mole of copper: Cu2+ + 2e- -> Cu. Thus, in a solution containing an unknown amount of Cu2+ ions, the quantity present, measured in moles, can be determined by measuring the quantity of electrons, in moles, which are used in performing this conversion to completion.
Electrical charge is measured in coulombs, one coulomb being the amount of charge transferred by a current of one ampere in one second. One mole of electrons is equivalent to just over 96,485 coulombs. By measuring the amount of time taken for a known current to complete a reaction like the one above, the number of moles of electrons used can be calculated and from this, the number of moles of the analyte determined. This type of coulometry is known as controlled current coulometry or coulometric titration. The method requires a means of determining when the endpoint of the reaction has been reached, such as a chemical indicator.
Another type of coulometric analysis is controlled potential coulometry. In this case, a constant electrical potential is maintained and the current is measured. As the reaction proceeds, the current decreases and approaches zero when the reaction is complete. The number of moles of analyte can be calculated from the time taken for the current to drop to zero.