Experiment Overview

The Mohr method is a direct precipitation titration used to determine chloride ion concentration. It involves titrating a halide solution with silver nitrate (\(\ce{AgNO3}\)), using potassium chromate (\(\ce{K2CrO4}\)) as an indicator.

What This Experiment Demonstrates

• Selective precipitation behavior of silver nitrate
• pH dependent equilibrium shifts
• Endpoint detection using an indicator

Apparatus Required

• Burette
• Pipette
• Conical flask
• Volumetric flasks
• White tile
• Amber storage bottle

Chemicals Required

• Silver nitrate solution
• Potassium chromate
• Chloride solution (e.g. NaCl)
• Calcium carbonate
• Distilled water

Theory

In the initial phase of the titration, silver ions added from the burette react exclusively with the chloride ions present in the conical flask. A white precipitate of silver chloride is formed:

\[\ce{Ag^+ + Cl^- \rightarrow AgCl \downarrow } (K_{sp} = 1.8 \times 10^{-10})\]

The low solubility product ensures that the concentration of free \(Ag^+\) ions remains extremely low as long as \(Cl^-\) ions are present. As the titration approaches the equivalence point, the chloride ion concentration drops rapidly, causing a sharp increase in the concentration of free silver ions.

Once the chloride ions are essentially exhausted, the concentration of \(Ag^+\) increases until the solubility product of silver chromate is exceeded:

\[\ce{2Ag^+ + CrO4^{2-} \rightarrow Ag2CrO4 \downarrow} (K_{sp} = 1.2 \times 10^{-12})\]

This results in the formation of a reddish-brown precipitate, indicating the endpoint of the titration.

Note on pH:

• If too acidic (\(pH < 6.5\)): Chromate ions convert to dichromate (\(\ce{Cr2O7^{2-}}\)), delaying the endpoint.
• If too alkaline (\(pH > 10\)): Silver ions react with hydroxide ions to form silver oxide (\(\ce{Ag2O}\)), producing a black precipitate that masks the endpoint.

Method

1. Pipette a known volume of the sample (or standard NaCl solution) into a conical flask.
2. Adjust the pH of the solution to between 7 and 10 using \(\ce{NaHCO3}\) or \(\ce{CaCO3}\) if necessary.
3. Add a few drops of potassium chromate indicator.
4. Titrate with standardized silver nitrate solution while continuously swirling.
5. Identify the endpoint when a faint, persistent reddish-brown coloration appears.
6. Perform a blank titration using distilled water and a small amount of \(\ce{CaCO3}\) to determine \(V_{blank}\).

Observations

The solution initially forms a cloudy white precipitate. As the titration progresses, temporary reddish flashes may appear but disappear upon swirling. The endpoint is reached when a permanent faint red-brown coloration persists.

Calculations

• Use stoichiometry to determine mole ratios from the balanced equations.
• Correct the titrant volume: \(V_{corrected} = V_{titrant} - V_{blank}\).
• Calculate the concentration of chloride ions in the sample (e.g. in mg/L or ppm).

Result

The concentration of chloride ions in the sample is determined from the volume of silver nitrate used, after correcting for the blank titration.

Video Demonstration

Video courtesy of Universitat Politècnica de València - UPV (YouTube). Video used for educational purposes under YouTube’s embedding policy.