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 powder of MgCl2 and CaCl2 original solutions)

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

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

Detailed protocol

Preparation of ~1 N original solutions of MgCl₂ and CaCl₂

500 ml MgCl₂’’<br/

(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<br/

(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.

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.

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 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 solution of MgCl₂ and CaCl₂ with the silver nitrate solution (a drop at a time) up to the first appearance of a red-brown color (Fig. 1)

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

• Use the mean of the 3 titrations as a final result.

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 solution: AgNO₃ volume (0.1 N) = V
• Calculate the amount (in moles) of AgNO₃ reacting: x = V*0.1 mole
• Calculate the amount (in moles) of chloride ions reacting:
  • for 25 ml of diluted solution: x/2 mol MgCl₂ et CaCl₂
  • for 1 l of diluted solution : x/2 * 40 mol MgCl₂ et CaCl₂
  • for 1 l of original solution: x/2 * 40 *20 mol MgCl₂ et CaCl₂

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

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

Calculation of the weight of original CaCl₂ and MgCl₂ solution for ½ l of buffer solution

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

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