The concentration of a salt solution can be measured using a variety of techniques, including titration using Mohr's Method.
Titration with Mohr's Method
One common method to measure the concentration of a salt solution involves titration, specifically using Mohr's method. This technique relies on a chemical reaction to determine the unknown concentration of a solution. Here's a breakdown:
Principles of Mohr's Method
- Reaction: Mohr's method uses the reaction between silver ions (Ag+) and halide ions (like chloride, Cl-), found in many salt solutions.
- Endpoint: The titration process involves adding a solution of silver nitrate (AgNO₃) of known concentration to the unknown salt solution.
- Indicator: A chromate indicator is used to determine the endpoint. Initially, the reaction favors the formation of silver halide precipitate. When all of the halide ions have reacted, the silver ions start to react with the chromate ions, forming a red-brown silver chromate precipitate. The appearance of this color marks the endpoint of the titration.
Equipment Required
- Burette: A burette is used to accurately dispense the known solution of silver nitrate. The burette is mounted on a burette stand.
- Pipettes: 10 and 20 mL pipettes are needed to accurately measure the volume of the unknown salt solution.
- Reagents:
- Silver nitrate solution of known concentration.
- Chromate indicator.
Procedure
- Preparation: A known volume of the salt solution is measured using a pipette and placed in a titration flask. A few drops of chromate indicator are added to the salt solution.
- Titration: The silver nitrate solution is added slowly from the burette into the flask containing the salt solution. The flask is swirled to ensure mixing.
- Endpoint: The addition of the silver nitrate solution is continued until the first permanent appearance of the red-brown silver chromate precipitate appears.
- Calculation: The volume of silver nitrate solution used is recorded, and this, along with the known concentration of the silver nitrate, is used to calculate the concentration of the unknown salt solution.
Example
Let's say you're measuring the concentration of a sodium chloride (NaCl) solution.
- You add 20mL of your sodium chloride solution to the flask.
- You add the chromate indicator to the flask.
- You slowly add the silver nitrate solution from the burette.
- You stop once the red-brown precipitate appears and record the volume of silver nitrate used from the burette (e.g. 22.4 mL)
- Using the concentration of silver nitrate and volume used, you can calculate the concentration of your unknown sodium chloride solution.
Summary
| Method | Principle | Equipment |
|---|---|---|
| Mohr's Method | Titration using AgNO₃; chromate indicator to show endpoint | Burette, stand, pipettes, silver nitrate, chromate indicator |
In essence, Mohr's Method offers a direct way to quantify the amount of halide ions, and therefore, the concentration of many salt solutions.
