pHYSICAL cHEMISTRY - Ii

Experiment 3

Aim of the Experiment

To find the strength of the given NaCl solution by titrating against AgNO3 solution. Also evaluate the solubility product of sparingly soluble salt (AgCl) in water, potentiometrically.

Principle

Precipitation titration is based on a reaction that yields ionic compounds of limited solubility. The number of precipitating agents that can be used is limited due to the slow rate of formation of precipitates. The widely precipitation reagent is silver nitrate and precipitation titration based on silver nitrate is known as Argentometric methods.

The precipitation reaction between NaCl and AgNO3 is shown below

AgNO3 + NaCl ---> AgCl + NaNO3

Ag + + Cl- ----> AgCl (s) Ksp = 1.8 * 10 raised to -10 M Square

To measure the potential changes, the indicator electrode is coupled with a reference electrode using a salt bridge. The cell can be depicted for the precipitation titration as follows

Ecell = E0 Cell + 0.0591 log [ Ag +]

Where, E0 cell = 0.5565

In this precipitation titration, silver electrode is used as an indicator /working electrode. As the titration proceeds, i.e. when the AgNO3 is added to NaCl solution (taken in a beaker), Cl- concentration decreases and Ag+ concentration increases. Therefore, the half cell potential as well as the cell potential increases. Near the end point the rate of change in potential will be maximum due to completion of precipitation. On crossing the equivalence point, EMF changes in small increments and finally it reaches saturation.

Materials required

Potentiometer, std.0.1 N AgNO3 , xN NaCl, Distilled water

Procedure

1. Pipette out 25 mL given xN NaCl solution. Transfer it to 250 mL volumetric flask

2. Dilute it to distilled water up to the mark. shake it well

3. Take out 50 mL from it using measuring cylinder and transfer it to 100 mL beaker.

4. Switch on the potentiometer. Do calibration adjusting 1.018 V for water

5. Replace water beaker with xN sodium chloride. Insert Reference electrode and platinum electrode in it.

6. Fill 0.1 N Silver nitrate in 10 mL Micro Burette upto '0' mark

7. Note down the EMF observed for 0.0 mL 0.1 N Silver Nitrate addition.

8. Do Titration of 50 mL xN NaCl with 0.1 N Silver Nitrate dropwise

9. Note the reading for 1 mL addition of silver nitrate. At the End Recalibrate and switch of the potentiometer.

10. Take readings by addition of 1 mL aliquots till get high jump get observed in EMF.

11. Plot the graph -1 E versus V from set-1 table

12. Plot the graph -2 ΔE / ΔV versus V which will give V0 value. Now put this V0 value in graph-1 which will give Ecell.

Observation Table

Set-1

Result

1.Amount of AgNO3 required for precipitation of 50 mL diluted NaCl solution = __________ mL

2.Normality of NaCl solution = ___________N

3. Amount of NaCl in given solution =_________ gm

4. Solubility product of AgCl (Ksp)= ________ mol sqaure per lite square

5. Solubility of AgCl (S) = ____________ mol/lit

Conclusion

Solubility product of AgCl is Ksp = 1.8 * 10 raised to -10 M Square at 25 degree centigarde temperature.

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

2. E Journal on relevant topic.