Amperometric titrations are electroanalytical methods in which the measured quantity is the electric current flowing through a solution during a titration. The current is monitored at a fixed applied potential while a titrant is added gradually. The change in current reflects the concentration of the electroactive species in solution. This technique is highly sensitive and especially useful for titrations involving oxidizable or reducible substances, including metal ions, halides, and organic compounds.
Unlike potentiometry, which measures voltage, amperometry focuses on current changes, making it ideal for detecting reaction endpoints even in colored, turbid, or complex mixtures. This method plays a significant role in pharmaceutical analysis, environmental chemistry, and industrial quality control.
Principle of Amperometric Titrations
The principle is based on measuring diffusion-controlled current at an indicator electrode when a constant potential is applied. As titrant is added, the concentration of the electroactive species changes, causing the measured current to change proportionally.
Before the equivalence point, the current is dominated by the analyte being oxidized or reduced at the electrode. After the endpoint, the current depends on the excess titrant if it is electroactive at the applied potential. A plot of current versus volume of titrant produces titration curves consisting of linear segments intersecting at the equivalence point.
Supporting electrolyte is used to minimize migration effects and ensure that mass transport occurs primarily by diffusion.
Instrumentation
A typical amperometric titration system consists of:
- A polarizable working electrode
- A reference electrode (e.g., Saturated Calomel Electrode)
- A counter electrode
- A voltage source to maintain constant potential
- A highly sensitive ammeter
- A titration assembly with burette and magnetic stirrer
Two main types of indicator electrodes are used: the Dropping Mercury Electrode (DME) and the Rotating Platinum Electrode (RPE).
1) Dropping Mercury Electrode (DME)
The DME is widely used due to its reproducible surface and broad negative potential range suitable for reduction reactions. It consists of a reservoir of mercury that flows through a narrow capillary, forming mercury drops at regular intervals.
Key features include:
- Constant renewal of electrode surface due to mercury drops
- High reproducibility of surface area
- Minimal contamination
- Ideal for reduction of metal ions and organic compounds
Because each drop forms a fresh electrode surface, the DME provides excellent sensitivity. However, concerns about mercury toxicity have encouraged alternative electrodes in modern practice.
2) Rotating Platinum Electrode (RPE)
The RPE is a versatile alternative to mercury-based electrodes. It consists of a platinum wire or disc rotated at controlled speeds to produce a uniform hydrodynamic layer. Rotation enhances mass transport and produces steady, reproducible currents.
Advantages include:
- No mercury handling or disposal issues
- Suitable for oxidation reactions
- Better control over current stability
- Rapid attainment of steady-state conditions
RPEs are widely used for amperometric titrations involving oxidizable analytes and in aqueous as well as non-aqueous media.
Titration Curves in Amperometry
Titration curves plot current against the volume of titrant added. Depending on the electrochemical behavior of the analyte and titrant, different curve patterns are observed.
(A) Only the Substance Titrated Is Reducible
If the analyte is reducible at the applied potential:
- The current decreases as titrant consumes the reducible analyte.
- After the endpoint, the analyte is fully consumed, and the current becomes nearly constant.
This type of curve is commonly observed in titrations of metal ions with complexing or precipitating agents.
(B) Only the Titrant Is Reducible
In this case:
- The current remains nearly zero until the equivalence point.
- After the endpoint, current increases because excess titrant contributes to the electrochemical reduction.
This type of curve is suitable for halide titrations with silver nitrate where Ag⁺ is reducible.
(C) Both Titrant and Analyte Are Reducible
When both species can undergo electrochemical reaction:
- Two linear portions with differing slopes appear before and after the endpoint.
- The intersection marks the equivalence point clearly.
These titrations often involve redox reactions in which both reactants are electroactive.
Advantages of Amperometric Titrations
- Highly sensitive and suitable for trace analysis
- No need for chemical indicators
- Can be used for colored or turbid solutions
- Useful for titrations of weak acids and bases
- Applicable to aqueous and non-aqueous systems
- Simple and rapid endpoint detection
Disadvantages of Amperometric Titrations
- Mercury electrodes pose environmental hazards
- Requires careful control of electrode potential
- Interfering species that are electroactive may affect accuracy
- Instrumental setup is more complex than classical titrations
Applications
- Determination of halides using silver nitrate titrations
- Estimation of metal ions such as lead, cadmium, and zinc
- Assay of pharmaceutical compounds with reducible groups
- Redox titrations involving oxidizing and reducing agents
- Determination of sulfide, cyanide, and other anions
- Study of reaction mechanisms and kinetic processes
- Trace analysis in environmental samples
Detailed Notes:
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