The hardness of water is mainly caused by the presence of dissolved salts of calcium (Ca2+) and magnesium (Mg2+). On the basis of the associated anions, hardness is commonly classified as carbonate hardness and non-carbonate hardness.
1. Carbonate Hardness
Carbonate hardness is the portion of water hardness associated with carbonate and bicarbonate ions. It is commonly called temporary hardness because it can be removed, to a significant extent, by boiling when bicarbonate hardness is present.
It is mainly caused by:
- Calcium bicarbonate, Ca(HCO3)2
- Magnesium bicarbonate, Mg(HCO3)2
2. Non-Carbonate Hardness
Non-carbonate hardness is the portion of hardness associated with anions other than carbonate and bicarbonate. It is commonly called permanent hardness because it is not removed simply by boiling the water.
It is mainly caused by salts such as:
- Calcium sulfate, CaSO4
- Magnesium sulfate, MgSO4
- Calcium chloride, CaCl2
- Magnesium chloride, MgCl2
3. Difference Between Carbonate and Non-Carbonate Hardness
| Basis | Carbonate Hardness | Non-Carbonate Hardness |
|---|---|---|
| Definition | Hardness associated mainly with carbonate and bicarbonate alkalinity. | Hardness associated with calcium and magnesium salts whose anions are not carbonate or bicarbonate. |
| Common name | Temporary hardness | Permanent hardness |
| Main salts | Ca(HCO3)2 and Mg(HCO3)2 | CaSO4, MgSO4, CaCl2 and MgCl2 |
| Associated ions | Mainly HCO3− and carbonate-related alkalinity | Mainly SO42−, Cl− and other non-carbonate anions |
| Removal by boiling | Bicarbonate hardness can be substantially removed by boiling. | Not removed by ordinary boiling. |
| Example | Ca(HCO3)2 | CaSO4 |
| Common treatment | Boiling, lime treatment and other suitable water-softening methods. | Ion exchange, lime-soda treatment, reverse osmosis and other suitable softening methods. |
4. Removal of Carbonate Hardness by Boiling
Calcium bicarbonate decomposes on heating to form insoluble calcium carbonate, which precipitates out of the water:
Ca(HCO3)2 → CaCO3↓ + CO2↑ + H2O
Magnesium bicarbonate also decomposes on heating and ultimately forms insoluble magnesium hydroxide along with carbon dioxide and water:
Mg(HCO3)2 → Mg(OH)2↓ + 2CO2↑
5. Important Formula
In water chemistry, total hardness is often considered in relation to carbonate and non-carbonate hardness:
More precisely, carbonate hardness is limited by the available alkalinity, so it is often expressed as the lesser of total hardness and alkalinity, when both are expressed in equivalent units such as mg/L as CaCO3.
Conclusion
Carbonate hardness is mainly associated with calcium and magnesium bicarbonates and is traditionally referred to as temporary hardness. Non-carbonate hardness is mainly caused by calcium and magnesium sulfates, chlorides, and similar salts and is commonly called permanent hardness. The distinction is important because the two types of hardness may require different approaches for removal and water treatment.