Conductometry is an electroanalytical technique used to measure the electrical conductance of an electrolyte solution. Conductance depends on the concentration of ions, their mobility, and the nature of the solvent. Conductometric methods are widely used in pharmaceutical analysis, water quality testing, titrations, and the study of ionization of strong and weak electrolytes. Because conductometry does not require an indicator, it is particularly useful for titrations involving colored or turbid solutions.
Conductive Solutions
Electrolytic solutions conduct electricity due to the presence of free ions. When an electric field is applied, cations move toward the cathode while anions migrate toward the anode. The ability of a solution to conduct electricity depends on the type of electrolyte.
Strong Electrolytes
Strong electrolytes completely ionize in aqueous solution. Examples include HCl, NaOH, KCl, and NaNO3. Because they produce a large number of ions, their conductivity is high and varies linearly with concentration.
Weak Electrolytes
Weak electrolytes partially ionize. Examples include acetic acid and ammonium hydroxide. Their conductivity increases significantly on dilution because ionization increases, a key feature used in titrimetric analysis.
Ohm’s Law
Ohm’s law states that the current flowing through a conductor is directly proportional to the applied voltage (V = IR). Conductivity is the reciprocal of resistance and forms the basis for conductometric measurements.
Important Definitions
(a) Conductance
Conductance (G) is the reciprocal of resistance (R), expressed as G = 1/R. It indicates how easily electricity passes through a solution. The SI unit is siemens (S).
(b) Specific Resistance
Specific resistance (ρ) is the resistance of a solution contained between two electrodes 1 cm apart with a cross-sectional area of 1 cm2. It is the inherent resistive property of the solution.
(c) Specific Conductance
Specific conductance (κ) is the reciprocal of specific resistance. It is the conductance of a solution placed between electrodes with unit distance and area. Specific conductance increases with the concentration of ions.
(d) Electrical Conductivity
Electrical conductivity is the measure of a solution’s ability to conduct electric current. It depends on ion type, valence, concentration, and temperature.
(e) Equivalent Conductance
Equivalent conductance (Λeq) is the conductance of a solution containing one gram-equivalent of an electrolyte, placed between electrodes 1 cm apart.
(f) Molecular Conductance
Molecular conductance (Λm) refers to the conductance of a solution containing one mole of an electrolyte. It is useful for studying the ionization behavior of weak electrolytes.
Effect of Dilution
Dilution has different effects on strong and weak electrolytes:
- Strong electrolytes: Conductance increases slightly with dilution because ionization is complete.
- Weak electrolytes: Conductance increases sharply as dilution promotes ionization.
This difference is important in determining dissociation constants and for conductometric titrations.
Instrumentation and Conductance Measurement
A conductometric setup includes:
- Conductance meter
- Conductivity cell
- Electrodes
- Conductance bridge
- Thermostatic bath (optional)
Stable measurements require constant temperature because conductivity increases with temperature.
Conductance Bridge
A conductance bridge is used to measure resistance accurately. It uses alternating current (AC) to avoid polarization of electrodes. The bridge balances resistance and capacitance components to yield precise conductance values.
Conductivity Cell
The conductivity cell consists of two platinum electrodes coated with platinum black to reduce polarization. The cell constant (K) relates measured conductance to specific conductance and depends on electrode distance and surface area.
Electrodes
Electrodes in conductometry must:
- Be chemically inert
- Have stable surface characteristics
- Maintain consistent cell constant
Platinized platinum electrodes are most commonly used.
Conductivity Water
Conductivity water is highly pure water with extremely low ionic content used for calibration and dilution. It prevents interference and ensures accurate readings.
Principle of Conductometric Titration
Conductometric titration measures conductance as a reagent is gradually added to a solution. The conductance changes due to the formation or removal of ions. The endpoint is determined by plotting conductance against volume of titrant. The intersection of straight-line segments indicates the equivalence point.
Types of Conductometric Titrations
(a) Acid–Base Titrations
During acid–base titrations, conductance changes due to the replacement of highly mobile ions (H⁺ or OH⁻) with less mobile ions. For strong acid–strong base systems, conductance decreases initially, reaches a minimum, and then rises as excess titrant increases ion concentration.
(b) Precipitation Titrations
These involve formation of an insoluble precipitate. For example, in titrating Cl⁻ with Ag⁺, conductance drops as ions precipitate, then rises when excess titrant adds free ions to solution.
(c) Redox (Oxidation–Reduction) Titrations
Conductance changes reflect the consumption or formation of ions during oxidation–reduction reactions. These titrations are useful when color indicators are unsuitable.
(d) Complexometric Titrations
Complex formation between metal ions and ligands like EDTA causes variations in ion concentration, allowing end-point identification based on conductance changes.
Applications of Conductometric Titrations
- Determination of strong and weak acids and bases
- Estimation of halides (Cl⁻, Br⁻, I⁻)
- Determination of sulfate and other precipitating ions
- Purity testing of water and pharmaceuticals
- Determination of dissociation constants of weak electrolytes
- Monitoring industrial processes
High Frequency Method
High-frequency AC circuits are used for highly conductive or concentrated solutions where conventional conductance measurements become inaccurate due to electrode polarization. This method improves precision at high ionic strength.
Advantages of Conductometric Titrations
- No need for chemical indicators
- Suitable for colored or turbid samples
- Applicable to weak electrolyte titrations
- High accuracy and reproducibility
- Operates in aqueous and non-aqueous media
- Simple, rapid, and low-cost instrumentation
Disadvantages of Conductometric Titrations
- Temperature variations affect conductivity
- Requires calibrated cell constants
- Not suitable for non-ionic species
- Less sensitive than some other electroanalytical methods
Detailed Notes:
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