Coulometry

In analytical electrochemistry, coulometry is the measure of charge (coulombs) transfer during an electrochemical redox reaction.

[1] It can be used for precision measurements of charge, but coulometry is mainly used for analytical applications to determine the amount of matter transformed.

The term coulometry was introduced in 1938 by Hungarian chemist László Szebellédy and Zoltan Somogyi.

Michael Faraday, known for his work in electricity and magnetism, made critical contributions to the field of electrochemistry.

In the earliest developments of coulometry, Faraday proposed the first instrument to measure charge by utilizing the electrolysis of water.

[6] Potentiostatic coulometry utilizes a constant electric potential and is a technique most commonly referred to as "bulk electrolysis".

Also called direct coulometry, the analyte is oxidized or reduced at the working electrode without intermediate reactions.

This constant potential is applied long enough to fully reduce or oxidize all of the electroactive species in a given solution.

Bulk electrolysis is often used to unambiguously assign the number of electrons consumed in a reaction observed through voltammetry.

It also has the added benefit of producing a solution of a species (oxidation state) which may not be accessible through chemical routes.

In indirect or secondary coulometry, the working electrode produces a titrant that reacts with the analyte.

When the analyte is completely consumed, endpoint detection is employed, preferably with an instrumental method for higher precision.

Using Faraday's Law, total charge can be used to determine the moles of the unknown species in solution.

The Karl Fischer reaction uses a coulometric titration to determine the amount of water in a sample.

It is used to find the amount of water in substances such as butter, sugar, cheese, paper, and petroleum.

The reaction involves converting solid iodine into hydrogen iodide in the presence of sulfur dioxide and water.

Pyridine is often used to prevent the buildup of sulfuric acid, although the use of imidazole and diethanolamine for this role are becoming more common.

Since this technique is used to determine the water content of samples, atmospheric humidity could alter the results.

This method is called surface coulometry and is performed by measuring the quantity of electricity needed to dissolve a well-defined area of the coating.

The results obtained by this coulometric method are similar to those achieved by other chemical and metallurgic techniques.

A type of clinical chemistry is measuring chloride levels in blood samples through a Cotlove chloridometer.

Measuring chloride levels allows for electrolyte stability, without this feature diseases such as hyperchoremia and hypochloremia would be harder to detect leaving body functions compromised.

[7] Coulometry can be used to measure the total antioxidant capacity (TAC) in blood and plasma through electrogenerated bromide.

An acid-base microtitorator utilizes the electrolysis of water, where protons or hydroxide ions are produced at the working electrode.

[6] Some advantages of using a microtitrator include the fast completion time of the titration due to the micro-scale.