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


  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. 


  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.

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



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


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

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

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