Mohr’s method

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The chloride ion concentration of the MgCl2 and CaCl2 solutions is determined by a precipitation titration with calibrated silver nitrate solution. This procedure is known as Mohr’s method.

Contents

Equipment and chemicals

Equipment

  • 2 x 200 ml beakers (for MgCl2 and CaCl2 weighing)
  • 4 x 100 ml beakers (for pipet rinsing)
  • 6 x 100 ml beakers (for MgCl2 and CaCl2 density determination)
  • 2 x 500 ml volumetric flasks (for original solutions)
  • 2 x 100 ml volumetric flasks (for diluted solutions)
  • 1 x 5 ml automatic pipet
  • 1 x 25 ml volumetric pipet
  • 1 x 50 ml buret
  • 6 x 50 ml erlenmeyer flasks
  • 6 x small magnetic stir bars
  • 2 x funnels (for preparation of original solutions with powder of MgCl2 and CaCl2)


Chemicals

  • Distilled water
  • MgCl2 (+6H2O)
  • CaCl2 (+2H2O)
  • AgNO3 0.1 N (Standardized silver nitrate solution)
  • NaHCO3
  • K2Cr2O7 (1.25 g K2Cr2O7 dissolved in 25 ml distilled water)


Method

  1. Preparation of ~1 N original solutions of MgCl2 and CaCl2
  2. Preparation of diluted solutions of MgCl2 and CaCl2 (1/20 dilution)
  3. Titration of diluted solutions with standardized silver nitrate solution
  4. Calculation of the concentration of CaCl2 and MgCl2 in the original solutions
  5. Calculation of the density of CaCl2 and MgCl2 original solutions
  6. Calculation of the weight of CaCl2 and MgCl2 original solutions (to introduce in the buffer solutions)


Detailed protocol


1. Preparation of ~1 N original solutions of MgCl2 and CaCl2

500 ml MgCl2
(MgCl2, 6 H2O = 203 g/mol)

  • 101.65 g MgCl2 dissolved in distilled water in 500 ml volumetric flask,
  • make it up to the mark with distilled water.
  • Mix the solution up to total dilution.


500 ml CaCl2

(CaCl2, 2 H2O = 147.02 g/mol)

  • 73.51 g CaCl2 dissolved in distilled water in 500 ml volumetric flask, make it up to the mark with distilled water.
  • Be careful, the distilled water must be added little by little since the reaction emanate a lot of warmth. *Mix the solution up to total dilution and wait for cooling.


2. Preparation of ~0.05 N diluted solutions of MgCl2 and CaCl2

  • Dilute original solutions of MgCl2 and CaCl2 by a factor 20 by taking 5 ml of original solution with an automatic pipet, pipetting it into a 100 ml volumetric flasks, and making it up to the mark with distilled water.
  • N.B. Pour a part (~20 ml) of the original solution of MgCl2 and CaCl2 in 2 beakers rather than pipetting directly in the volumetric flasks.


3. Titration of diluted solutions with standardized silver nitrate solution

  • Pipette a 25 ml aliquot of diluted solutions of MgCl2 and CaCl2 (with 25 ml volumetric pipets) into 50 ml erlenmeyer flasks (3 replicates for each solution).
  • Add small magnetic stir bars.
  • Add a pinch of NaHCO3 and some drops (2-3 drops) of K2Cr2O7 indicator (diluted potassium chromate solution) in the erlenmeyer flasks.
  • Fill the 50 ml buret with the 0.1 N silver nitrate solution.
  • Titrate the diluted solutions of MgCl2 and CaCl2 with the silver nitrate solution (a drop at a time) up to the first appearance of a red-brown color (see Figure 1)
  • Use the mean of the 3 titrations as a final result.





Figure 1. Flasks before the titration endpoint (left) and at the endpoint (right)




4. Calculation of the normality of CaCl2 and MgCl2 in the original solutions

  • Calculate the average volume of AgNO3 used for the titration of the three replicates of diluted solutions:

volume of AgNO3 (0.1 N) = V

  • Calculate the amount (in mol) of AgNO3 reacting: x = V x 0.1 mol


  • Calculate the amount (in mol) of chloride ions reacting:
    • for 25 ml of diluted solution: x / 2 mol MgCl2 and CaCl2
    • for 1 l of diluted solution : x / 2 x 40 mol MgCl2 and CaCl2
    • for 1 l of original solution: x / 2 x 40 x 20 mol MgCl2 and CaCl2


Original solution normality (mol/l) = (V x 0.1 / 2) x 40 x 20 / 1000


5. Calculation of the density of CaCl2 and MgCl2 in the original solutions
Weigh at least 3 aliquots of 20 ml of original solution in beakers and calculate the average weight (W, g)

Density (g/ml) = W / 20


6. Calculation of the weight of original CaCl2 and MgCl2 solution
(for ½ l of buffer solutions)

Weight (g) = ((molarity / normality) x density x 1000) / 2

Molarity = 0.05474 / 2 mol MgCl2 and 0.01075/2 mol CaCl2 for ½ l of buffer solution

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