Mohr's method

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.

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.

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

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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