PHD

PHDAnalytical chemistryElectroanalytical techniques


Colometry


Coulometry is an electrochemical technique used to determine the amount of a substance transformed during an electrochemical reaction. Named after Charles-Augustin de Coulomb, this method relies on the measurement of electricity consumed or produced (charge in coulombs). In this overview of coulometry, we will understand its basic principles, instruments, applications, and advantages.

Principles of colometry

The main idea of coulometry is based on Faraday's laws of electrolysis, which relate the amount of a substance converted at the electrode to the amount of electric charge passed through the solution. The basic equation governing this relation is:

m = (Q x M) / (nx F)

Where:

  • m = mass of the substance converted (in grams)
  • Q = total electric charge (coulomb)
  • M = molar mass of the substance (grams per mole)
  • n = number of electrons exchanged per molecule of the substance
  • F = Faraday constant (about 96485 C/mol)

To put this into perspective, let's visualize this process with a simple electrochemical cell:

Anode Cathode Electrolyte

In the diagram above, the electric charge flows from the anode to the cathode through the electrolyte solution. By integrating the current over the time period of the reaction, we can calculate the total charge (Q).

Types of colometry

There are two main types of coulometry: potentiostatic coulometry (also called controlled potential coulometry) and amperostatic coulometry (also called controlled current coulometry).

Potentiostatic colometry

In potentiostatic coulometry, a constant potential is applied to the electrochemical cell. The distinct advantage of this method is that the selectivity of the measurement can be controlled by strictly maintaining the potential only when the desired reaction occurs.

Current (I) = dQ/dt

This equation of current (I) shows how the charge (Q) changes with time (t). Integration of the current over time gives the total charge used for the transformation of the analyzer.

Emperostatic colometry

In amperostatic coulometry, a constant current is applied. This method is generally faster because it ensures a quick reaction rate. The total time required for the reaction to complete is used to calculate the respective charge passed, and hence, the amount of analyte.

Equipment

The basic setup for colometric measurements includes:

  • Power supply: To control potential or current.
  • Electrochemical cell: It consists of working, reference and counter electrodes.
  • Analytical balance: For weighing samples before and after electrolysis.
  • Data acquisition system: To record current and potential data over time.

Stages of colometric analysis

Here is the general procedure for a typical colometric analysis:

  1. Prepare the electrochemical cell by properly placing the analyte solution and the electrodes.
  2. Set the desired potential (for potentiostatic) or current (for amperostatic).
  3. Initiate electrolysis and record the current time data.
  4. Calculate the total charge passed using numerical integration (e.g., trapezoidal rule).
  5. Use the Faraday equation to determine the amount of reacted analyte.

Applications of colometry

Coulometry is widely used in various areas of analytical chemistry due to its accuracy and precision. Some common applications include:

  • Determination of water content: Karl Fischer coulometry specifically measures the water content in samples.
  • Metallurgical analysis: Determination of the minute quantities of metals such as copper, nickel and zinc in alloys.
  • Pharmaceuticals: Determination of active pharmaceutical ingredient concentrations.
  • Environmental analysis: Measuring pollutant ions in water bodies.

Benefits and limitations

Benefit

  • High Precision: Direct measurement of charge leads to high precision and accuracy.
  • Low detection limit: Able to detect very low concentrations of substances.
  • Minimal sample preparation required: Simple sample preparation compared to other analytical techniques.

Boundaries

  • Time consuming: Responses can take a long time, especially in the potentiostatic mode.
  • Equipment costs: Initial setup and maintenance can be expensive.
  • Selectivity: Sometimes selectivity can be a problem, especially in complex mixtures.

Conclusion

Coulometry stands out as an important analytical technique that provides accurate measurement of substances based on the fundamentals of electrochemistry. Its development has led to improvements that have paved the way for advanced applications in environmental, pharmaceutical, and industrial fields. With improvements in technology, coulometry remains a cornerstone technique in determining the structure and concentrations of various chemical entities.


PHD → 4.2.3


U
username
0%
completed in PHD


Comments