Coulometry
Coulometry is an electrochemical technique used to determine the amount of a
substance by measuring the total charge (in coulombs) passed during a chemical
reaction. It is based on Faraday's Laws of Electrolysis, which relate the amount of
material transformed during an electrochemical reaction to the amount of electric
charge passed.
Types of Coulometry
1. Controlled-Potential Coulometry (Potentiostatic Coulometry):
o The working electrode's potential is kept constant, and the current
decreases over time.
o Used for reactions where precise control of potential is required to
oxidize or reduce a single species.
2. Controlled-Current Coulometry (Amperostatic Coulometry):
o The current is maintained constant, and the potential changes over
time to sustain the reaction.
o Simple but may lead to undesired side reactions.
Instrumentation in Coulometry
A basic coulometric setup includes the following components:
1. Electrochemical Cell:
o Contains the analyte solution and supporting electrolyte.
o Includes two electrodes (working and counter electrode), and
sometimes a reference electrode.
2. Working Electrode:
o Where the electrochemical reaction of interest occurs.
o Made of inert materials such as platinum or gold.
3. Counter Electrode:
o Completes the circuit by facilitating the opposite reaction to maintain
current flow.
4. Reference Electrode (in Controlled-Potential Coulometry):
o Maintains a stable potential to ensure accurate control of the working
electrode's potential.
o Common types: Silver/Silver Chloride (Ag/AgCl), Saturated Calomel
Electrode (SCE).
5. Potentiostat or Galvanostat:
o Potentiostat controls the working electrode's potential.
o Galvanostat controls the current.
6. Charge Measuring Unit:
o Integrates the current over time to calculate the total charge passed.
7. Stirring Mechanism:
o Ensures homogeneous mixing of the solution for consistent reaction
rates.
8. Software/Data Acquisition System:
o Records current, potential, and time to calculate total charge and
analyze results.
Functioning of Coulometry
1. Preparation:
o The analyte solution is placed in the electrochemical cell with a
supporting electrolyte to enhance conductivity.
o The electrodes are immersed in the solution.
2. Electrolysis Process:
o A known current or potential is applied across the working electrode.
o The electrochemical reaction occurs, converting the analyte to its
oxidized or reduced form.
3. Charge Measurement:
o The total charge passed is measured and related to the amount of
substance reacted using Faraday’s laws: Q=n⋅F⋅z where Q = charge
(coulombs), nnn = moles of substance, F = Faraday constant (96485
C/mol), z= number of electrons transferred.
4. Completion:
o The reaction is continued until the analyte is fully converted, indicated
by the stabilization of current in controlled-potential coulometry.
Applications of Coulometry
1. Quantitative Analysis:
o Determination of trace amounts of substances (e.g., metal ions,
halides).
o High sensitivity makes it ideal for pharmaceutical and environmental
analysis.
2. Purity Testing:
o Assessment of sample purity by determining the exact amount of a
single component.
3. Water Analysis:
o Karl Fischer Coulometry for precise determination of water content in
samples.
4. Electroplating and Corrosion Studies:
o Investigation of deposition rates and material degradation.
5. Battery Research:
o Determining charge capacities and reaction mechanisms in batteries.
6. Chemical Synthesis Monitoring:
o Used to quantify intermediates and end products in synthetic
processes.
Limitations of Coulometry
1. Interferences:
o Requires the reaction to be highly specific; side reactions can lead to
errors.
2. Time-Consuming:
o Reactions may take significant time to reach completion, especially at
low current.
3. High Accuracy Demands:
o Requires precise calibration of instruments and control of
experimental conditions.
4. Electrode Maintenance:
o Electrodes can become fouled or degraded, affecting performance.
5. Sample Preparation:
o Needs well-prepared and homogeneous samples to avoid
inconsistencies.
6. Not Suitable for All Systems:
o Limited to redox-active analytes with well-defined electrochemical
behavior
Advantages of Coulometry
High precision and accuracy.
Minimal reagent consumption in coulometric titrations.
Applicable to trace-level analysis.
Direct relationship with fundamental physical laws.
