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

 * 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)

500 ml CaCl2
 * 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.

(CaCl2, 2 H2O = 147.02 g/mol)

2. Preparation of ~0.05 N diluted solutions of MgCl2 and CaCl2 3. Titration of diluted solutions with standardized silver nitrate solution
 * 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.
 * 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.
 * 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 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''
 * 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