Estimation of Sodium Chloride by Argentometric Titration
Introduction
Sodium chloride (NaCl), commonly known as common salt, is an important inorganic pharmaceutical substance. Its estimation is necessary to determine the amount and purity of sodium chloride present in a given sample.
The quantitative estimation of sodium chloride can be performed by argentometric precipitation titration, commonly using Mohr's method.
In this method, standard silver nitrate solution is used as the titrant, while potassium chromate acts as an indicator.
1. Principle
The estimation of sodium chloride is based on the precipitation reaction between silver ions and chloride ions.
When standard silver nitrate solution is added to a sodium chloride solution, silver ions react with chloride ions to form a white precipitate of silver chloride.
Main reaction:
AgNO₃ + NaCl → AgCl↓ + NaNO₃
Silver ions + Chloride ions → Silver chloride (white precipitate)
After all the chloride ions have reacted, the first slight excess of silver ions reacts with potassium chromate indicator to form a reddish-brown precipitate of silver chromate.
Endpoint reaction:
2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄↓
The appearance of a permanent reddish-brown colour indicates the endpoint.
[IMG 1 — Horizontal educational infographic showing the principle of sodium chloride estimation by Mohr's method. Illustrate silver nitrate reacting with sodium chloride to form white silver chloride precipitate, followed by the formation of reddish-brown silver chromate at the endpoint.]
2. Reagents Required
1. Sodium Chloride Sample: The substance whose quantity or purity is being determined.
2. Silver Nitrate (AgNO₃): A standard solution used as the titrant. It reacts with chloride ions to form silver chloride precipitate.
3. Potassium Chromate (K₂CrO₄): Used as an indicator to detect the endpoint.
4. Distilled Water: Used for dissolving the sample and preparing the solution.
3. Procedure
The general procedure for estimating sodium chloride by Mohr's method is:
Accurately weigh a suitable quantity of sodium chloride sample.
Dissolve it in distilled water in a suitable volumetric flask.
Transfer a measured volume of the prepared solution into a conical flask.
Add the prescribed quantity of potassium chromate indicator.
Fill a burette with standardized silver nitrate solution.
Add silver nitrate slowly to the sample while continuously mixing.
Initially, white silver chloride precipitate forms.
Near the endpoint, add silver nitrate dropwise.
Observe the appearance of a permanent reddish-brown colour.
Record the volume of silver nitrate consumed.
[IMG 2 — Horizontal step-by-step laboratory infographic showing sodium chloride weighing and dissolution, transfer to conical flask, addition of potassium chromate, titration with silver nitrate from a burette, white AgCl precipitate and permanent reddish-brown endpoint.]
4. Endpoint and Observation
The endpoint is identified by the formation of a permanent reddish-brown precipitate of silver chromate.
Stage | Observation |
|---|---|
Before titration | Sodium chloride solution with yellow potassium chromate indicator |
During titration | Formation of white silver chloride precipitate |
At endpoint | Permanent reddish-brown silver chromate precipitate |
The endpoint should be observed carefully to avoid adding excess silver nitrate.
5. Calculation
The quantity of sodium chloride is calculated from the volume and concentration of silver nitrate consumed.
Molecular weight of NaCl = 58.44 g/mol
The reaction between silver nitrate and sodium chloride takes place in a 1:1 molar ratio.
For 0.1 M silver nitrate:
1 mL of 0.1 M AgNO₃ is equivalent to 5.844 mg of NaCl.
Formula:
Amount of NaCl (mg) = Volume of AgNO₃ used (mL) × 5.844
Example
Suppose 20 mL of 0.1 M silver nitrate solution is required to reach the endpoint.
Step 1: Identify the given values
Volume of AgNO₃ = 20 mL
Molarity of AgNO₃ = 0.1 M
Equivalence factor = 5.844 mg NaCl per mL
Step 2: Apply the formula
Amount of NaCl = 20 × 5.844
Step 3: Calculate
Amount of NaCl = 116.88 mg
Answer: The titrated sample contains 116.88 mg of sodium chloride.
Percentage purity:
Percentage purity = (Amount of NaCl found ÷ Weight of sample taken) × 100
If only an aliquot of the prepared sample is titrated, the result must first be multiplied by the appropriate dilution or aliquot factor to obtain the total amount.
6. Conditions and Precautions
Maintain the solution at approximately neutral or slightly alkaline pH, generally around 6.5–10.
Avoid strongly acidic conditions because they affect the chromate indicator equilibrium.
Avoid excessively alkaline conditions because silver may form other precipitates.
Use standardized silver nitrate solution for accurate results.
Add potassium chromate in the prescribed quantity.
Mix the solution properly during titration.
Add silver nitrate dropwise near the endpoint.
Use clean glassware and distilled water to avoid contamination.
7. Applications of Sodium Chloride Estimation
Purity Determination: Helps determine the purity of sodium chloride samples.
Pharmaceutical Quality Control: Supports the quality evaluation of sodium chloride used in pharmaceutical preparations.
Raw Material Testing: Helps verify the chloride content of sodium chloride raw materials.
Quantitative Analysis: Demonstrates the application of argentometric precipitation titration.
Laboratory Analysis: Used for chloride determination in suitable samples.
Key Takeaways
Sodium chloride can be estimated by argentometric precipitation titration using Mohr's method.
Standard silver nitrate is used as the titrant.
Potassium chromate acts as the indicator.
Chloride ions form a white precipitate of silver chloride.
A permanent reddish-brown silver chromate precipitate indicates the endpoint.
One millilitre of 0.1 M silver nitrate is equivalent to 5.844 mg of sodium chloride.
Proper pH control and careful endpoint detection are important for accurate estimation.