Questions
1 question per exam
Difficulty
Medium
Importance
Core for Pharmaceutical Analysis and Clinical Chemistry
Overview
Potentiometry and polarography are fundamental electrochemical analytical techniques used to determine analyte concentration by measuring electrical properties in a solution. Mastering these concepts is essential for understanding clinical diagnostic tools and pharmaceutical quality control, as they provide high precision in quantifying redox-active substances.
Principles of Potentiometry
Potentiometry involves measuring the potential difference between two electrodes—an indicator electrode and a reference electrode—under conditions of zero current. It is primarily used to detect the equivalence point in titration where the potential changes rapidly upon addition of titrant.
- Nernst equation governs the potential: E = E0 - (RT/nF)ln(Q)
- Indicator electrodes: Glass electrode (pH), Ion-selective electrodes (ISE)
- Reference electrodes: Saturated Calomel Electrode (SCE), Ag/AgCl electrode
- Based on the relationship between electrode potential and ion activity
Potentiometric Titrations
These titrations use the variation of cell potential to determine the endpoint instead of visual indicators, making them ideal for colored or turbid solutions. Results are typically analyzed using the first derivative (dE/dV) or second derivative (d2E/dV2) of the potential curve.
- Sharpest change in dE/dV marks the precise equivalence point
- Used for acid-base, redox, and precipitation titrations
- High accuracy in non-aqueous solvent systems
- Requires constant stirring and temperature control
Polarographic Analysis
Polarography is a type of voltammetry where the current is measured as a function of the potential applied to a dropping mercury electrode (DME). It is specifically designed for the qualitative and quantitative analysis of ions that undergo reduction or oxidation at the electrode surface.
- Ilkovic equation: id = 607 n D^0.5 m^2/3 t^1/6 C
- Characteristic feature is the 'S-shaped' polarographic wave
- Half-wave potential (E1/2) is independent of analyte concentration
- Limiting current is directly proportional to analyte concentration
Dropping Mercury Electrode (DME)
The DME consists of mercury dripping from a fine capillary, which provides a continuously renewed surface. This unique feature prevents electrode fouling and ensures reproducible diffusion-controlled current readings.
- Provides high hydrogen overpotential allowing wide cathodic range
- Renewable surface eliminates the poisoning effect
- The small size leads to significant current density
- Capillary characteristic defined by mass flow (m) and drop time (t)
Formula Sheet
E = E0 - (RT/nF)ln(Q)
id = 607 n D^0.5 m^2/3 t^1/6 C
Exam Tip
Always draw the S-shaped polarogram curve clearly and label the limiting current and half-wave potential to secure full marks in long-answer questions.
Common Mistakes
- Confusing the Nernst equation for potentiometry with the Ilkovic equation for polarography.
- Forgetting that the half-wave potential is a qualitative parameter, while limiting current is quantitative.
- Failing to mention the significance of zero current flow in potentiometric measurements.
More Revision Notes
Ready to test yourself?
Play topic-wise Potentiometry & Polarography questions in Aspirant Arcade — gamified MCQ practice.
Download Free