In modern pharmaceutical and chemical laboratories, electrochemical analysis stands as a trusted ally, providing accuracy, sensitivity, and cost-effectiveness. By linking the invisible world of ions with measurable electrical signals, methods like conductometry, potentiometry, and polarography have revolutionized how analysts determine purity, concentration, and reactivity of drugs. Unit 5 explores these fascinating techniques, each harnessing the interplay of electricity and chemistry in unique ways.
Download UNIT 5 – Electrochemical Methods of Analysis – Probing Chemistry Through Electricity Notes
Get simplified revision notes for this unit:
⬇️
Download Unit 5 Notes PDF
Conductometry: Measuring Ions Through Conductivity
Introduction: The Current Flow of Knowledge
Conductometry revolves around the principle that electrolyte solutions conduct electricity due to the movement of ions. The ease of this conduction—termed conductance—directly relates to the ionic concentration and mobility, making it a valuable analytical tool.
The Conductivity Cell
At the heart of conductometry lies the conductivity cell, which typically consists of two platinum electrodes, often coated with platinum black to minimize polarization effects. The cell is immersed in the test solution, and a small alternating current is passed to measure conductance.
Conductometric Titrations
One of the most important applications is conductometric titration, where the conductance of the solution is measured continuously during a titration process. Unlike visual indicators, the endpoint is determined by observing sudden changes in conductance.
Strong acid vs. strong base: Conductance decreases sharply as ions neutralize, then rises with excess base.
Weak acid vs. strong base: Conductance initially increases as weak acid converts into its salt, then rises again after neutralization.
Applications
Determining purity of water and pharmaceutical solutions.
Estimating strong and weak acids, bases, and salts.
Useful in colored or turbid solutions where indicators fail.
Conductometry thus provides a simple, precise, and interference-free method of analysis.
Potentiometry: Reading Chemistry in Electrode Potentials
The Electrochemical Cell
Potentiometry is based on the measurement of the electromotive force (EMF) of an electrochemical cell without drawing significant current. The system consists of a reference electrode with a fixed potential and an indicator electrode whose potential varies with the analyte concentration.
Reference Electrodes
Reference electrodes serve as a stable baseline for potential measurement:
Standard Hydrogen Electrode (SHE): The ultimate reference, though rarely used due to handling difficulties.
Silver/Silver Chloride Electrode (Ag/AgCl): Simple, stable, and widely used in pharmaceutical analysis.
Calomel Electrode: Contains mercury and mercurous chloride, offering reliable potentials across different concentrations.
Indicator Electrodes
These respond to analyte activity and include:
Metal Electrodes: Such as platinum or gold, ideal for redox systems.
Glass Electrode: Famous for its sensitivity to hydrogen ion activity, making it the standard in pH measurement.
Determining Endpoints in Potentiometric Titrations
During titrations, the potential is recorded against the titrant volume. A sharp change in EMF signals the endpoint. This method avoids reliance on visual indicators, making it effective for colored, opaque, or biologically active solutions.
Applications
Measurement of pH in drug solutions.
Estimation of halides, metal ions, and weak acids/bases.
Quality control in pharmaceutical industries.
Potentiometry stands out for its non-intrusive, accurate, and versatile applications across chemistry and pharmacy.
Polarography: A Drop at a Time
Principle of Polarography
Invented by Jaroslav Heyrovský, polarography measures the current that flows in an electrochemical cell as a function of applied voltage. It relies on diffusion-controlled electrode reactions, where analyte ions are reduced or oxidized at a dropping electrode.
The technique is governed by the Ilković equation, which relates diffusion current to factors like electrode area, diffusion coefficient, and concentration of the analyte.
Electrodes in Polarography
Dropping Mercury Electrode (DME): Mercury drops from a capillary at regular intervals, providing a constantly renewable electrode surface. Its reproducibility and resistance to contamination made it a classic choice.
Rotating Platinum Electrode: A modern alternative, offering stable surface area and faster diffusion, making it suitable for more complex analyses.
Applications
Determination of trace metals such as lead, zinc, cadmium, and copper in pharmaceuticals.
Study of vitamins and certain organic compounds.
Useful in detecting drug impurities at micro-level concentrations.
Polarography remains a sensitive and selective method, particularly valuable when ultra-trace analysis is required.
Why Electrochemical Methods Matter Today
In today’s pharmaceutical and environmental laboratories, these electrochemical methods form the backbone of analysis.
Conductometry ensures water purity and accurate titrations.
Potentiometry delivers unmatched reliability in pH measurement and complex titrations.
Polarography opens a window to trace-level detection of essential and toxic substances.
Together, they guarantee quality, safety, and compliance in drug development and manufacturing, aligning science with societal well-being.