How to Determine Concentration of Cl in Rock Salt Solution
Rock salt, also known as the mineral halite, is simply table salt, sodium chloride (NaCl). The classic way to determinate its chloride ion (Cl-) concentration is the Mohr method. It involves the titration of the sodium chloride solution with the solution of silver nitrate, and it follows the reaction NaCl+AgNO3=AgCl (precipitant) +NaNO3. A standard solution with the known concentration of the titrant (AgNO3) is slowly added using a burette to a flask with the sodium chloride solution of an unknown concentration. The reaction endpoint is determined with a chemical indicator.
Other People Are Reading
Things You'll Need
- Flask, 100ml
- Distilled water
- Potassium chromate (K2CrO4), 5 percent solution
- Burette
- Silver nitrate (AgNO3), standard 0.1 molar solution
- Dropper
Instructions
-
-
1
Put a 10 or 15ml aliquot of the sodium chloride solution with the unknown concentration in the flask.
-
2
Add about 20ml of distilled water to the flask.
-
-
3
Add 1ml of the potassium chromate solution.
-
4
Swirl the flask for about five seconds to mix the solution.
-
5
Fill out the burette with the standard solution of silver nitrate.
-
6
Write down the initial volume mark. For example, the initial volume mark is 2.5ml.
-
7
Add 2 to 3 drops of the titrant solution from the burette to the flask.
-
8
Swirl the flask for two to three seconds to mix the solution.
-
9
Observe the color of the precipitant.
-
10
Add the titrant solution (Steps 7 to 9) until you see a formation of the red precipitant. This is the titration endpoint. At that point where no chloride ions (Cl-) are left, an excess of silver nitrate starts to react with the indicator according to the equation: 2AgNO3+K2CrO4=Ag2CrO4+2KNO3. Ag2CrO4 is the red precipitant.
-
11
Record the volume mark of the solution in the burette. For example, the final volume mark is 26.4ml.
-
12
Subtract the initial volume mark (Step 6) from the final one (Step 11) to calculate the volume of the silver nitrate solution used for titration. In this example, the volume is 26.4ml-2.5ml=23.9ml.
-
13
Determine the concentration of the chloride ions (Cl-). Do this by multiplying the volume of the silver nitrate solution by its concentration and dividing by the aliquot volume of the sodium chloride solution (Step 1). In this example, assuming the aliquot volume being 15ml, the concentration of Cl- is 23.9mlx0.1 molar/15ml=0.16 molar.
-
1
