Softening Of Magnesium Hardness

As in the case of calcium hardness, magnesium can also be present in the form of

carbonate and noncarbonate hardness. The Ksp of Mg(OH)2 is a low value of 9(10 ). Thus, the hardness is removed in the form of Mg(OH)2. To remove the carbonate hardness of magnesium, a source of the OH— ion is therefore added to precipitate the Mg(OH)2 as shown in the following softening chemical reaction:

Mg (HCO3 )2 + 4OH— ^ Mg (OH )2i + 2CO2— + 2HOH (10.10)

The carbonate ions in the product side will pair with whatever cation the OH- was with in the reactant side of the reaction. If this cation is calcium, in the form of Ca(OH)2, then the product will again be the calcium carbonate precipitate.

In natural waters, the form of noncarbonate hardness normally encountered is the one associated with the sulfate anion; although, occasionally, large quantities of the chloride and nitrate anions may also be found. The softening reactions for the removal of the noncarbonate hardness of magnesium associated with the possible anions are as follows:

As shown in all the previous reactions, the removal of the magnesium hardness, both carbonate and noncarbonate, can use just one chemical. This chemical is normally lime, CaO.

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