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DrX Whiz Niraj
Conductometry: Principles & Titrations
Master Electrochemical Analysis. Complete notes on Conductance, Conductivity Cell (Cell Constant), and Conductometric Titration Curves.
Conductometry is an electrochemical method of analysis that measures the electrical conductance of an electrolytic solution. In this method, the end point of a titration is determined by measuring the change in conductance during the reaction.
Ohm's Law & Conductance (G)
Conductance is the ease with which electric current flows through a solution. It is the reciprocal of Resistance (R).
Unit: Ohm⁻¹ (Mho) or Siemens (S).
Specific Conductance (k)
It is the conductance of a solution placed between two electrodes of 1 cm² area, kept 1 cm apart.
Unit: Siemens cm⁻¹ (S cm⁻¹).
Important Note on Ionic Mobility:
The conductance of a solution depends purely on the number and mobility of the ions present.
Rule of Thumb: Hydrogen ions (H⁺) and Hydroxyl ions (OH⁻) have exceptionally high mobility compared to all other ions (like Na⁺, Cl⁻). Therefore, their presence or absence causes major changes in conductance.
The device used to measure the conductance of a solution is called a Conductivity Cell. It is made of glass and consists of two Platinum Electrodes coated with Platinum black (spongy platinum) to prevent polarization effects.
Cell Constant (K):
The ratio of the distance between the electrodes (l) to their cross-sectional area (a) is fixed for a given conductivity cell. This is known as the Cell Constant.
Formula: Cell Constant (K) = l / a
Unit: cm⁻¹
The principle of conductometric titration is based on the fact that during a titration, ions of one mobility are replaced by ions of another mobility, leading to a change in the overall conductance of the solution.
A. Strong Acid vs Strong Base (e.g., HCl vs NaOH)
This gives a classic V-shaped curve.
- Initial: Conductance is very high due to highly mobile H⁺ ions from HCl.
- Addition of NaOH: The fast-moving H⁺ ions react with OH⁻ to form unionized water. They are replaced by slow-moving Na⁺ ions. Hence, conductance falls rapidly.
- Equivalence Point: Reached at the minimum conductance (lowest point of the V).
- After Equivalence Point: Further addition of NaOH adds highly mobile OH⁻ ions and Na⁺ ions. Conductance rises sharply again.
| Advantages of Conductometry over Volumetric Analysis |
|---|
| 1. No Indicator Needed: It can be successfully used for colored, highly turbid, or opaque solutions where visual indicators (like Phenolphthalein) fail. |
| 2. Very Dilute Solutions: Accurate results can be obtained even with highly dilute solutions where standard titrations fail. |
| 3. Weak Acid-Weak Base: It is the best method for Weak Acid vs Weak Base titrations, which do not give sharp end points with chemical indicators. |
| 4. Mixture Analysis: A mixture of a strong acid and a weak acid can be titrated simultaneously against a strong base. |
Practical Alert: Temperature Control
Conductance is highly sensitive to temperature. A 1°C rise in temperature increases the conductance by approximately 2%. Therefore, during conductometric titrations, the temperature of the solution must be kept strictly constant (often using a water bath/thermostat) to avoid false readings.