Q. Write phosphate conditioning method for water softening with suitable chemical reactions. How it is different from soda lime method.

Ans. 

Phosphate conditioning: In high pressure boilers, scale formation can be avoided by adding sodium phosphate, which reacts with hardness causing substance of water, forming non adherent and easily removable soft sludge of calcium and magnesium phosphates, which can be removed by blow down operation.

3MCl₂ + 2Na₃PO₄ = M₃ (PO₄)₂ + 6NaCl

Where   M = Ca⁺² or Mg⁺²

Mainly employed phosphates are:

1.      Sodium dehydrogenate phosphate (acidic) Na₂HPO₄

2.      Disodium hydrogen phosphate (weakly acidic)  Na₂HPO₄

3.      Trisodium hydrogen phosphate (alkaline) Na₃PO₄.

Difference between phosphate conditioning and Lime-Soda method