Question

Give reasons why polyphosphates are used as water softening agents.

Answer 1

Water Softening:

1. Water softening is the process of removing the calcium, magnesium, and other metal cations from hard water.
2. The water is softened through ion exchange or the addition of chemicals that form insoluble precipitates.
3. Polyphosphates are utilized as water softeners because they produce soluble complexes with the cationic species of hard water that cause hardness.
4. $\underset{\mathrm{Calcium}\mathrm{sulphate}}{2{\mathrm{CaSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{polymeta}\mathrm{phosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]\left(\mathrm{s}\right)}\to \underset{\mathrm{soluble}\mathrm{complex}\mathrm{salt}}{{\mathrm{Na}}_2\left[{\mathrm{Ca}}_2{\left({\mathrm{PO}}_3\right)}_6\right]}+\underset{\mathrm{Sodium}\mathrm{sulphate}}{2{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right)}$
5. $\underset{\mathrm{Magnessium}\mathrm{sulphate}}{2{\mathrm{MgSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{polymeta}\mathrm{phosphate}}{{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]\left(\mathrm{s}\right)}\to \underset{\mathrm{Soluble}\mathrm{complex}\mathrm{salt}}{{\mathrm{Na}}_2\left[{\mathrm{Mg}}_2{\left({\mathrm{PO}}_3\right)}_6\right]}+\underset{\mathrm{Sodium}\mathrm{sulphate}}{2{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{s}\right)}$

Therefore, polyphosphates are used as water softening agents.