Coulometry is a quantitative analysis method that measures the electricity needed to change the oxidation state of an analyte. It can be achieved through electrolysis, and the amount of analyte present can be calculated using Faraday’s first law of electrolysis. Coulometry can be controlled by current or potential, and the amount of analyte can be determined by measuring the time it takes for the reaction to complete.
Coulometry is a form of quantitative analysis used to determine the concentration of a substance in solution, usually referred to as an 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 occur 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 electric 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 directly proportional to the amount of electricity transferred.
In chemistry, the measure of the amount of a substance is usually 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 then possible to calculate how many moles of electrons would be required to convert, say, a given amount of copper in solution from its +2 oxidation state to neutral metallic copper. Two moles of electrons are needed for each mole of copper: Cu2+ + 2e- -> Cu. Thus, in a solution containing an unknown amount of Cu2+ ions, the amount present, measured in moles, can be determined by measuring the amount of electrons, in moles, that are used to perform this conversion upon completion.
Electric charge is measured in coulombs, one coulomb is the amount of charge transferred by a current of one ampere in one second. One mole of electrons equals just over 96.485 coulombs. By measuring the time it takes for a known current to complete a reaction like the one above, one can calculate the number of moles of electrons used and from this determine the number of moles of analyte. This type of coulometry is known as controlled current coulometry or coulometric titration. The method requires a means of determining when the end point 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 electric 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 it takes for the stream to drop to zero.
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