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Potentiometry: Electrodes & Titrations
Master Electrochemical Analysis. Complete notes on Electrochemical Cells, Reference Electrodes (SHE, Calomel), Indicator Electrodes, and Potentiometric Titration Curves.
Potentiometry is an electrochemical method of analysis where the potential (voltage) difference between two electrodes is measured under conditions of zero current flow. It is widely used to determine the concentration of a specific ion (like H+ for pH) in a solution.
The Electrochemical Cell:
A complete potentiometric cell consists of two half-cells (Electrodes) connected by a Salt Bridge (which completes the circuit without mixing the solutions).
Cell Formula: Ecell = Eindicator - Ereference
By knowing the constant potential of the reference electrode and measuring the total cell potential, we can easily calculate the potential of the indicator electrode.
A Reference Electrode is an electrode that has a known, constant, and stable electrode potential, which is completely independent of the concentration of the analyte solution.
1. Standard Hydrogen Electrode (SHE)
It is the Primary reference electrode. Its potential is arbitrarily set to exactly 0.00 V at all temperatures.
- Construction: Platinized platinum wire immersed in a 1M H⁺ solution, with H₂ gas bubbled at 1 atm pressure.
- Drawback: Highly inconvenient to use and maintain H₂ gas pressure.
2. Saturated Calomel Electrode (SCE)
It is a Secondary reference electrode. Most widely used due to its ease of preparation.
- Construction: Mercury (Hg) in contact with Calomel paste (Hg₂Cl₂) and filled with saturated Potassium Chloride (KCl) solution.
- Potential: +0.244 V at 25°C.
3. Silver/Silver Chloride Electrode
Another popular Secondary reference electrode.
- Construction: A silver wire coated with a layer of solid Silver Chloride (AgCl), immersed in a saturated KCl solution.
- Potential: +0.199 V at 25°C. Highly stable at high temperatures.
An Indicator Electrode is an electrode whose potential varies directly with the concentration (or activity) of the specific ion being analyzed in the solution.
- 1. Metal Electrodes: E.g., Platinum (Pt) is used for Redox titrations because it is inert. Silver (Ag) wire is used for Argentometric (precipitation) titrations.
- 2. Ion-Selective Electrodes (ISE) - Glass Electrode: The most common indicator electrode used for pH measurement (H⁺ ion concentration).
Practical Alert: Why use SCE over SHE?
Although the Standard Hydrogen Electrode (SHE) is the ultimate reference, it is rarely used in routine lab work. Maintaining pure Hydrogen gas at exactly 1 atmospheric pressure is highly dangerous and impractical. That’s why the Saturated Calomel Electrode (SCE) is preferred as it is robust, safe, and gives a highly stable potential.
In potentiometric titrations, we do not use chemical indicators. We record the Potential (E) after each addition of titrant Volume (V). The end point is found graphically using three main methods:
| Method | Graph Plotted | End Point Indicator |
|---|---|---|
| Normal Curve | Potential (E) vs Volume (V) | The midpoint of the steep vertical part (S-shaped curve). |
| First Derivative | ΔE / ΔV vs Volume (V) | The Peak Maximum indicates the exact end point. |
| Second Derivative | Δ²E / ΔV² vs Volume (V) | The point where the curve crosses the Zero line (X-axis). Extremely precise. |
- Colored/Turbid Solutions: Highly useful where visual indicators fail to show a color change.
- Clinical Analysis: Measurement of blood pH and blood gases using ion-selective electrodes.
- Titrations: Can be applied to all types of titrations (Acid-base, Redox, Precipitation, and Complexometric).
- Mixture Analysis: Can titrate a mixture of strong and weak acids in a single titration.