STANDARDISATION OF AgNO3 SOLUTION (MOHR’S METHOD)

Principle:

Silver nitrate reacts with sodium chloride and forms a white precipitate of silver chloride

            AgNO₃ + NaCl                 AgCl + NaNO₃

When all the chloride ions are precipitated as AgCl, excess of AgNO₃ reacts with potassium chromate indicator and forms a reddish brown precipitate of silver chromate. 

            2 AgNO₃ + K₂CrO₄                   Ag₂CrO₄ + 2 KNO₃

Procedure:

1.      Weigh accurately about 1.5 g. of NaCl (Equivalent weight = 58.46) transfer to a 250 ml volumetric flask.

2.      Dissolve with a little quantity of distilled water and make up the volume with distilled water to the mark. Mix the solution thoroughly.

3.      Fill the burette with the given silver nitrate solution.

4.      Pipette out exactly 25 ml of the NaCl solution into a 250 ml conical flask.

5.      Add few drops of potassium chromate (5% aqueous solution) and mix the content of the flask.

6.      Run down the AgNO₃ solution from the burette into the conical flask in small quantities till the reddish brown precipitate begins to appear, which is the end point.

7.      Repeat the above experiment till concordant titration values are obtained.

8.      Carryout a blank with 25 ml of distilled water and same quantity of K₂Cr₂O₄ to get reddish brown colour when titrated with AgNO₃.

9.      Deduct the blank reading with previous reading obtained for NaCl solution.

Calculation:

Volume of NaCl pipetted out                     = V₁ ml

Normality of NaCl (by calculation)          = N₁

Volume of AgNO₃                                        = V₂ ml

                                                                    V₁   X  N₁

Normality of AgNO₃  (N₂)               =

                                                                           V₂

Result: The strength of the given AgNO₃ solution = _____ N