Potentiometry: Electrochemical Cell, Principle and Applications
Introduction
Potentiometry is an electrochemical analytical technique used to determine the concentration or activity of specific ions in a solution by measuring the potential difference between two electrodes.
The measurement is performed under conditions where almost no current flows through the electrochemical cell. This helps determine the electrode potential without significantly disturbing the chemical system.
Potentiometry is commonly used in pH measurement, ion concentration determination and pharmaceutical analysis.
1. Principle of Potentiometry
The principle of potentiometry is based on measuring the potential difference between two electrodes immersed in a solution.
The two electrodes are:
Indicator Electrode: Develops a potential that depends on the concentration or activity of the ion being measured.
Reference Electrode: Provides a stable and known electrode potential.
The potential difference between these electrodes is measured using a high-impedance voltmeter or potentiometer.
The measured potential depends on the activity of the ion in the solution and can be related to its concentration under suitable conditions.
Nernst Equation:
For a general electrode reaction involving an ion, the electrode potential is described by the Nernst equation:
E = E° − (RT/nF) ln Q
Where:
E = Electrode potential
E° = Standard electrode potential
R = Universal gas constant
T = Absolute temperature in Kelvin
n = Number of electrons transferred
F = Faraday constant
Q = Reaction quotient
At 25°C, for a simple electrode reaction involving a metal ion:
E = E° + (0.0591/n) log [Mⁿ⁺]
This equation shows that electrode potential changes with the activity of the ion being measured.
[IMG 1 — Horizontal scientific infographic illustrating the principle of potentiometry: indicator electrode and reference electrode immersed in a sample solution, both connected to a high-impedance voltmeter displaying the potential difference.]
2. Electrochemical Cell
An electrochemical cell used in potentiometry consists of two electrodes immersed in an electrolyte solution and connected to a potential-measuring instrument.
Main Components:
1. Indicator Electrode
Its potential changes according to the activity of the specific ion present in the solution. Examples include glass electrodes and metal electrodes.
2. Reference Electrode
It maintains a stable and known potential throughout the measurement. Common examples include the silver/silver chloride electrode and saturated calomel electrode.
3. Electrolyte Solution
The sample solution contains the ions whose concentration or activity is to be determined.
4. Salt Bridge or Liquid Junction
In suitable cell arrangements, it provides ionic contact between solutions and helps maintain electrical continuity.
5. Potentiometer or High-Impedance Voltmeter
Measures the potential difference between the indicator and reference electrodes while drawing negligible current.
[IMG 2 — Horizontal labelled diagram of a potentiometric electrochemical cell showing indicator electrode, reference electrode, sample solution, salt bridge, electrical connections and high-impedance voltmeter.]
3. Types of Electrodes Used in Potentiometry
Electrodes used in potentiometric analysis are broadly classified into two main types.
A. Reference Electrodes
Reference electrodes provide a stable and reproducible potential.
1. Standard Hydrogen Electrode (SHE)
It consists of a platinum electrode in contact with hydrogen gas and a solution containing hydrogen ions. Its standard electrode potential is assigned a value of 0.00 V.
2. Saturated Calomel Electrode (SCE)
It consists of mercury, mercurous chloride (calomel) and saturated potassium chloride solution. It provides a stable reference potential.
3. Silver/Silver Chloride Electrode (Ag/AgCl)
It consists of a silver wire coated with silver chloride and immersed in a chloride-containing solution. It is widely used as a reference electrode.
B. Indicator Electrodes
Indicator electrodes respond to changes in the activity of the ion being analysed.
1. Metal Electrodes
These electrodes are made from metals such as silver or copper and respond to the activity of their corresponding ions under suitable conditions.
2. Glass Electrode
A glass electrode is commonly used for measuring hydrogen ion activity and determining the pH of a solution.
3. Ion-Selective Electrodes
These electrodes selectively respond to particular ions, such as fluoride, potassium or calcium ions.
[IMG 3 — Horizontal educational infographic comparing reference electrodes (standard hydrogen, saturated calomel and silver/silver chloride) with indicator electrodes (metal, glass and ion-selective electrodes).]
4. Working of Potentiometry
The general procedure for potentiometric measurement involves the following steps:
Prepare the sample solution containing the ion to be analysed.
Select a suitable indicator electrode according to the ion being measured.
Select an appropriate reference electrode.
Connect both electrodes to a high-impedance measuring instrument.
Immerse the electrodes in the sample solution.
Allow the potential reading to become stable.
Record the potential difference.
Determine the ion concentration using a suitable calibration curve or the Nernst equation.
For potentiometric titration, the potential is recorded after successive additions of titrant. A plot of potential against titrant volume is used to identify the endpoint.
5. Applications of Potentiometry
1. pH Measurement
Potentiometry is widely used to determine the pH of solutions using a glass electrode and a reference electrode.
2. Determination of Ion Concentration
Ion-selective electrodes are used to determine the concentration or activity of specific ions, such as fluoride, calcium and potassium.
3. Potentiometric Titrations
It is used in acid-base, redox, precipitation and complexometric titrations to identify endpoints through changes in electrode potential.
4. Pharmaceutical Analysis
It helps determine the concentration of ionic substances and supports quality control of pharmaceutical products.
5. Water Analysis
Used to measure pH and monitor selected ions in drinking water, wastewater and other water samples.
6. Biological and Clinical Analysis
Suitable ion-selective electrodes are used for measuring electrolytes such as sodium and potassium in biological samples.
[IMG 4 — Horizontal educational infographic showing applications of potentiometry: pH measurement, ion concentration analysis, pharmaceutical quality control, water testing and clinical electrolyte analysis.]
6. Advantages of Potentiometry
High Sensitivity: Can detect small changes in ion activity.
No Visual Indicator Required: Suitable for coloured and turbid solutions.
Simple Measurement: Potential can be measured directly using an appropriate instrument.
Wide Applications: Useful for pH measurement and different types of titrations.
Small Sample Requirement: Many potentiometric measurements require relatively small sample volumes.
7. Limitations and Precautions
Electrodes must be properly calibrated before measurement.
Maintain suitable temperature conditions because electrode potential changes with temperature.
Keep electrodes clean and properly stored.
Allow sufficient time for the potential reading to stabilize.
Select electrodes appropriate for the ion being measured.
Other ions may interfere with ion-selective electrode measurements.
Maintain proper electrical contact and avoid contamination of the sample.
Use a high-impedance measuring instrument to minimize current flow.
Key Takeaways
Potentiometry measures the potential difference between two electrodes.
It generally uses an indicator electrode and a reference electrode.
The Nernst equation relates electrode potential to ion activity.
Glass electrodes are commonly used for pH measurement.
Ion-selective electrodes help determine specific ion concentrations.
Potentiometry is useful in pharmaceutical analysis, water testing, clinical analysis and potentiometric titrations.