Coulometry is a technique in analytical chemistry that determines the quantity of a substance by measuring the total electric charge passing through it. It measures how much matter participates in a chemical reaction by tracking the flow of electrons. ^[1-4]

Basic Principles

The foundation of coulometry lies in the direct relationship between the amount of substance involved in a chemical reaction and the total electric charge passed through it. According to Faraday’s Laws of Electrolysis, the amount of a substance produced during electrolysis is directly proportional to the quantity of electric charge passed through and is expressed by: ^[1-3]

Q = nFN_A

​Where:

– Q: Total charge in coulombs

– n: Number of moles of electrons transferred per mole of substance

– F is Faraday’s constant (approximately 96,485 C/mol)

– N_A: Number of moles of the substance reacted

This equation tells us that the total charge Q is proportional to the amount of substance N_A that reacts. By measuring Q, we can determine the exact quantity of a substance, provided we know the number of electrons involved in the reaction.

Types

There are two main types of coulometry, each with its approach to measuring Q: ^[1]

1. Controlled-Current Coulometry (Amperostatic Method)

In this method, the current I is kept constant throughout the reaction. The charge is calculated using the simple equation

Q = I x t

Where 

t: time of the chemical reaction 

Since the current is fixed, measuring the time gives us the total charge. This technique is especially useful when a fixed current ensures the reaction proceeds to completion, such as in the Karl Fischer titration for water content analysis.

2. Controlled-Potential Coulometry (Potentiostatic Method)

In this method, the electrode potential is held constant while the current varies during the reaction. We calculate the total charge by integrating the current over time.

Q = ∫ Idt

This approach allows selective control over the reaction by maintaining a fixed potential, ensuring that only the desired reaction occurs. Researchers commonly use it to precisely determine metal ions or to study specific redox processes.

Applications ^[2]

1. Quantitative Analysis

• Determines trace amounts of water in oils, fuels, and pharmaceuticals using Karl Fischer titration.
• Measures small quantities of metals such as silver, copper, or lead in environmental samples and industrial products using electrodeposition.
• Quantifies specific organic compounds through controlled oxidation or reduction processes in controlled-potential coulometry.

2. Quality Control and Purity Testing

• Ensures accurate composition in products like pharmaceuticals, food, and chemicals under controlled conditions (current or potential)
• Checks the purity of substances by calculating the exact charge required for their complete reaction under controlled conditions (current or potential).