Potentiometry is an electrochemical analytical technique used to measure the electrical potential of electrodes immersed in a solution. The measured potential is related to the concentration of ions present in the sample. Potentiometric methods are widely used in pharmaceutical analysis, water testing, pH measurement, environmental monitoring, and titrations involving acids, bases, redox substances, precipitates, and complex-forming reagents.
In potentiometry, no current is allowed to flow through the cell, ensuring that the measured potential reflects the chemical composition of the solution without disturbing equilibrium. The technique is accurate, rapid, non-destructive, and suitable for automated systems.
Electrode Potential
Electrode potential is the measurable potential difference between an electrode and a reference electrode. This potential arises due to the tendency of ions to gain or lose electrons at the electrode surface. The potential is governed by the Nernst equation, which relates electrode potential to ion concentration and temperature.
Potentiometry uses this principle to determine ion concentration by measuring the potential difference of an electrochemical cell under zero-current conditions.
Types of Analyses in Potentiometric Methods
- Direct Measurement: Electrode potential is read directly to determine ion concentration.
- Acid–Base Titrations: Used for strong and weak acids and bases.
- Redox Titrations: Useful for oxidizing and reducing agents.
- Precipitation Titrations: Used for chloride, silver, and other precipitate-forming ions.
- Complexometric Titrations: Applied in determination of metals using EDTA or other ligands.
Potentiometry Instrumentation
A potentiometric setup typically includes:
- A high-impedance voltmeter
- A reference electrode
- An indicator electrode
- A sample solution
- Stirrer or magnetic mixer
The voltmeter must have high internal resistance to ensure minimal current flow, preserving the electrochemical equilibrium in the solution.
Electrodes
Two electrodes are essential in potentiometry:
- Reference Electrode: Provides a stable, known potential.
- Indicator Electrode: Responds to the concentration of ions in solution.
Reference Electrodes
Reference electrodes serve as stable baseline electrodes and maintain a constant potential. Common examples include:
- Saturated Calomel Electrode (SCE): Contains mercury and mercurous chloride; stable and reliable.
- Silver/Silver Chloride Electrode (Ag/AgCl): Widely used due to simplicity and low toxicity.
- Hydrogen Electrode: Primary reference electrode, though rarely used due to operational difficulty.
A good reference electrode must have constant potential, thermal stability, and long operational life.
Indicator Electrodes
Indicator electrodes develop a measurable potential that varies with the activity of the analyte ion. Types include:
1. Metal–Metal Ion Electrodes
- Used for metal ions such as Cu²⁺, Zn²⁺, Ag⁺.
- The electrode potential changes as the concentration of metal ions changes.
2. Inert Metal Electrodes
- Platinum or gold electrodes
- Used in redox titrations where no direct metal–ion equilibrium exists
3. Ion-Selective Electrodes (ISEs)
ISEs are widely used for selective determination of specific ions. Examples include:
- Glass electrode: For pH measurement
- Fluoride-selective electrode
- Potassium-selective electrode
- Calcium-selective electrode
Potentiometric Titrations
Potentiometry is extremely useful in titrimetric analysis. During titration, the potential is plotted against the volume of titrant added. A sharp change in potential indicates the end point.
Reactions in Potentiometric Titrations
1. Neutralization Reactions
Widely used for strong and weak acids and bases. Glass electrode is typically used as indicator.
2. Redox Reactions
Performed with inert electrodes such as platinum. Used for titration of Fe²⁺, Fe³⁺, Ce⁴⁺, MnO₄⁻ and other redox species.
3. Precipitation Reactions
Used to determine halides (Cl⁻, Br⁻, I⁻) by titration with silver nitrate. Silver electrode is commonly used.
4. Complexation Reactions
EDTA titrations fall under this category. Metal–metal ion electrodes or specific ion-selective electrodes are used.
5. Potentiometric Titrations in Non-Aqueous Media
Used when analytes are poorly soluble in water or water-based titrations give inaccurate results. Organic solvents such as glacial acetic acid or methanol may be used.
Methods of Detecting End Point
- First derivative method: Endpoint corresponds to the maximum slope.
- Second derivative method: Endpoint occurs where curvature changes sign.
- S-shaped titration curve: Endpoint identified at the inflection point.
- Gran plot method: Useful for weak acids and bases.
Karl Fischer Titration
Karl Fischer titration is a specific application of potentiometry used to determine water content in samples. It is highly accurate and sensitive, even at very low moisture levels.
The reaction involves iodine, sulfur dioxide, base, and an alcohol. Water reacts stoichiometrically with iodine, and the endpoint is detected potentiometrically.
Advantages of Potentiometric Titrations
- Accurate and highly sensitive
- Suitable for colored and turbid solutions
- Useful for dilute samples
- Allows automation and digital data handling
- No indicators required
- Can be applied in aqueous and non-aqueous media
Applications of Potentiometry
- Determination of pH using glass electrode
- Analysis of metal ions
- Determination of halides by precipitation titration
- Quality control analysis in pharmaceuticals
- Determination of water content (Karl Fischer method)
- Environmental monitoring of pollutants
- Food analysis and ionic strength determination
Detailed Notes:
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