"Coulometry: Fundamentals & Applications"
- 1. Coulometry: Basics and Applications Coulometry is an analytical technique used in electrochemistry. It involves measuring the electricity consumed or produced during an electrolysis reaction. Coulometry relies on exhaustive electrolysis of the analyte. By “exhaustive,” we mean that the analyte is completely oxidized or reduced at the working electrode or reacts entirely with a reagent generated at the working electrode.
- 2. Principles of Coulometry Coulometry relies on Faraday’s law: Q = nFNA (Q): Total charge (in coulombs) passing through the cell. (n): Number of electrons per mole of analyte. (F): Faraday’s constant (96487 C mol⁻¹). (NA): Moles of analyte.
- 3. Advantages of Coulometric Titration Coulometric titration has the advantage that constant current sources for the generation of titrants are relatively easy to make. The electrochemical generation of a titrant is much more sensitive and can be much more accurately controlled than the mechanical addition of titrant using a burette drive. For example, a constant current flow of 10 μA for 100ms is easily generated and corresponds to about 10 micrograms of titrant. The preparation of standard solutions and titer determination is of course no longer necessary. Chemical substances that are unstable or difficult to handle because of their high volatility or reactivity in solution can also very easily be used as titrants. Examples are bromine, chlorine and Karl Fischer reagents (iodine). Coulometric titration can also be performed under inert atmosphere or be remotely controlled e.g. with radioactive substances.
- 4. TYPES OF COULOMETRIC METHODS
- 6. Controlled-potential coulometry involves maintaining the working electrode at a constant potential. This potential is carefully chosen to allow the reduction or oxidation of the analyte without simultaneously reducing or oxidizing other species in the solution. The resulting current flowing through the cell is directly proportional to the concentration of the analyte. To achieve accurate results, all current must be used for analyte oxidation or reduction. 100% current efficiency is essential. As electrolysis progresses, the analyte concentration decreases, leading to a reduction in current.
- 8. In controlled-current coulometry, a constant current is passed through the electrolytic cell. The goal is to determine the concentration of the analyte based on the total charge produced during electrolysis. Unlike controlled-potential coulometry, where the current decreases over time, controlled-current coulometry maintains a constant current. As a result, analysis times are typically shorter During electrolysis, the analyte’s concentration continuously decreases. To maintain a constant current, the potential at the working electrode must adjust until another oxidation or reduction reaction occurs. However, this secondary reaction may lead to a current efficiency of less than 100%.
- 9. Applications of Coulometry 1) Determination of Electrons Involved in Redox Reactions Controlled-potential coulometry allows us to determine the number of electrons involved in a redox reaction. By performing a controlled-potential coulometric analysis using a known amount of a pure compound, we can precisely calculate the number of electrons exchanged during the reaction. 2) Carbonate Concentration Determination Coulometer analysis is used to quantify carbonate concentration in diverse samples, including pure carbonates, soils, rocks, and liquids. The technique measures the carbon dioxide evolved from acidified samples and uses this information to determine the carbonate content in the original sample
- 10. 3) Quantitative Analysis of Inorganic and Organic Compounds Both controlled-potential and controlled-current coulometric methods are employed for this purpose. These methods allow accurate determination of the moles of analyte based on the total charge passed during electrolysis. 4) Determination of Specific Elements coulometry techniques have been successfully used to determine over 50 elements. Notably, they are frequently employed for the determination of uranium and plutonium due to their minimal interference with the sample.
- 11. 5) Water Content Determination In Karl Fischer titration, a controlled-current coulometric approach is employed to determine the concentration of water in various samples. This method is particularly sensitive and useful for detecting low water concentrations (on the order of milligrams per liter). 6) Thickness of Metallic Coatings Coulometry is used to determine the thickness of metallic coatings by measuring the quantity of electricity needed to dissolve the coating. This information is crucial for quality control and surface treatment processes.
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