sodium hexa meta phosphate Na(PO₃)₆ or Na₆P₆O₁₈ is called Calgon . It is used for softening hard water. Calgon ionizes to give a complex anion:The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon.This results in the removal of calcium and magnesium ions from hard water in the form of a complex with Calgon. The water is softened and sodium ions are released into water.
Na₂[Na₄(PO₃)₆] + 2Ca²⁺ ------> Na₂[Ca₂(PO₃)₆ + 4Na⁺