Classification of hardness can be classified based anion that has association with metal ion namely carbonate hardness and non-carbonate hardness. In the carbonate hardness, calcium and magnesium ions associated with CO32-and HCO3-. In the non-carbonate hardness, calcium and magnesium ions associated with SO42-, Cl-and NO3-. Carbonate hardness very sensitive to heat and easy to make sediment at high temperatures such as the following reaction:
Ca(HCO3)2 → CaCO3 sediment + CO2 + H2O
Mg(HCO3)2 → Mg(OH)2 sediment + 2CO2
Therefore, carbonate hardness can be called as temporary hardness. Non-carbonate hardness can be called as permanent hardness because calcium and magnesium that bind with sulfate and chloride, does not make sediment and hardness values does not change despite in high temperature.
Part of the total hardness is equivalent to bicarbonate that participates in the presence of carbonate alkalinity in accordance with carbonate hardness. Since the alkalinity and hardness are determined in CaCO3, carbonate hardness can be calculated by following formula:
If the total alkalinity < total hardness so carbonate hardness = total alkalinity.
If the total alkalinity ≥ total hardness so carbonate hardness = total hardness.
Non-carbonate hardness = total hardness - carbonate hardness.
If total alkalinity exceeds total hardness so some of anion constituent of alkalinity (bicarbonate and carbonate) associated with single valence cations (monovalent), for example, potassium (K+) and natrium (Na+) that are not detected in determination of hardness. Conversely, if total hardness exceeds total alkalinity so some of cation constituent of hardness (calcium and magnesium) bind to sulfate (SO42-), chloride (Cl-), silicate (SiO32-) or nitrate (NO3-) that are not detected in determination of alkalinity. Therefore the relationship between hardness and alkalinity is not always positive; or greater hardness is not always accompanied with higher alkalinity and vice versa.