# Mohr’s method

The chloride ion concentration of the MgCl_{2} and CaCl_{2} 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 MgCl
_{2}and CaCl_{2}weighing) - 4 x 100 ml beakers (for pipet rinsing)
- 6 x 100 ml beakers (for MgCl
_{2}and CaCl_{2}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 MgCl
_{2}and CaCl_{2})

### Chemicals

- Distilled water
- MgCl
_{2}(+6H_{2}O) - CaCl
_{2}(+2H_{2}O) - AgNO
_{3}0.1 N (Standardized silver nitrate solution) - NaHCO
_{3} - K
_{2}Cr_{2}O_{7}(1.25 g K_{2}Cr_{2}O_{7}dissolved in 25 ml distilled water)

## Method

- Preparation of ~1 N original solutions of MgCl
_{2}and CaCl_{2} - Preparation of diluted solutions of MgCl
_{2}and CaCl_{2}(1/20 dilution) - Titration of diluted solutions with standardized silver nitrate solution
- Calculation of the concentration of CaCl
_{2}and MgCl_{2}in the original solutions - Calculation of the density of CaCl
_{2}and MgCl_{2}original solutions - Calculation of the weight of CaCl
_{2}and MgCl_{2}original solutions (to introduce in the buffer solutions)

## Detailed protocol

**1. Preparation of ~1 N original solutions of MgCl _{2} and CaCl_{2} **

**500 ml MgCl**_{2}(MgCl

_{2}, 6 H

_{2}O = 203 g/mol)

- 101.65 g MgCl
_{2}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 CaCl _{2}**

(CaCl_{2}, 2 H_{2}O = 147.02 g/mol)

- 73.51 g CaCl
_{2}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 MgCl _{2} and CaCl_{2}**

- Dilute original solutions of MgCl
_{2}and CaCl_{2}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 MgCl
_{2}and CaCl_{2}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 MgCl
_{2}and CaCl_{2}(with 25 ml volumetric pipets) into 50 ml erlenmeyer flasks (3 replicates for each solution). - Add small magnetic stir bars.
- Add a pinch of NaHCO
_{3}and some drops (2-3 drops) of K_{2}Cr_{2}O_{7}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 MgCl
_{2}and CaCl_{2}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 CaCl _{2} and MgCl_{2} in the original solutions**

- Calculate the average volume of AgNO
_{3}used for the titration of the three replicates of diluted solutions:

volume of AgNO_{3} (0.1 N) = **V**

- Calculate the amount (in mol) of AgNO
_{3}reacting:=**x**x 0.1 mol**V**

- Calculate the amount (in mol) of chloride ions reacting:

- for 25 ml of diluted solution:
/ 2 mol MgCl**x**_{2}and CaCl_{2} - for 1 l of diluted solution :
/ 2 x 40 mol MgCl**x**_{2}and CaCl_{2} - for 1 l of original solution:
/ 2 x 40 x 20 mol MgCl**x**_{2}and CaCl_{2}

- for 25 ml of diluted solution:

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

**5. Calculation of the density of CaCl**

_{2}and MgCl_{2}in the original solutionsWeigh at least 3 aliquots of 20 ml of original solution in beakers and calculate the average weight (

*, g)*

**W**

**Density**(g/ml) =**W**

**/ 20****6. Calculation of the weight of original CaCl**

_{2}and MgCl_{2}solution(for ½ l of buffer solutions)

**Weight**(g) =**((molarity / normality) x density x 1000) / 2***0.05474 / 2 mol MgCl*

**Molarity**=_{2}and 0.01075/2 mol CaCl

_{2}for ½ l of buffer solution