The Determination of Magnesium in Solutions Free from Other Divalent Cations

Equipment Required

1. pH/ION analyzer
2. Water hardness ion selective electrode
3. Accurate balance weighing to ±0005g
4. Magnetic stirrer

Reagents

1. Magnesium chloride stock standard

Standard Preparation

• molar Mg²+: dissolve 2.0331g magnesium chloride (analar MgCl₂.6H₂O) in a 1 liter volumetric flask with de-ionized water. Dilute to the mark. This is the stock standard.

 From the 0.01 molar Mg²+ standard prepare 10^-3 and 10^-4 molar Mg²+ by serial dilution with de-ionized water. 

Method

1. Determine the slope of the water hardness electrode by immersing the electrodes in 10^-4 molar Mg²⁺. Record the mV value (mV₁).
2. Immerse the electrodes in the 10^-3 molar Mg²⁺ and record the stable value (mV₂).

The slope is calculated by mV₂ - mV₁.

3. Dilute the sample 1 in 10 using de-ionized water.
4. To a 250-ml beaker add 50 ml of diluted sample and immerse the electrodes in this solution.
5. Set the pH/ION analyzer to the known addition mode.
6. When the reading is stable (this could take several minutes), freeze the display.
7. Add 5 ml of the 10^-2 molar Mg²⁺ standard and record the result of the sample concentration.

Calculation

Multiply the displayed result by 10 to account for the dilution factor.

Notes

Store the electrode between measurements in 10^-3 molar Mg²⁺.

For long term storage, rinse with de-ionized water and store dry.