Hardness

Definition

Hardness is defined as the concentrations of calcium and magnesium ions expressed in terms of calcium carbonate. Concentration is measured in degrees hardness. There are many different definitions around the globe, such as German hardness, French hardness, English Hardness and American Hardness.

1 ° dh equals

• 10 mg/l CaO = 7.14 mg/l Ca = 17.8 mg/l CaCO3
• 7.19 mg/l MgO = 4.34 mg/l Mg
• 0.18 mmol/l Earthalkali-ions

Reaction of Water Hardness Compounds

Water hardness compounds react with fatty acids and some vegetable oils with a high free fatty acid content to form hard magnesium and calcium soaps. These soaps result in a white sticky layer or a white powdery residue both of which are difficult to remove.
Therefore seawater should not be used for the initial cleaning of these cargoes.
Freshwater hardness varies depending on the source of supply:

Example

Seawater hardness