Hardness of water
The hardness of water gives the concentration of carbonates (CO32-) and strong bases, in other words its alkalinity.
Different scales are used in different countries; in the UK, English degrees (°e) are used and in the USA, ppm, also called American degrees. 1°e is equivalent to 2.39 mg/L of hydroxide ion (HO-) or 4.21 mg/L of carbonate ion, or 8.56 mg/L of bicarbonate ion (HCO3-).
To take into account the bicarbonate ions (HCO3-), the total alkali strength is used. Total hardness is expressed as the sum of the calcium and magnesium concentrations.
Thealkalinity of water is closely related to its hardness and therefore to its corrosive character and capacity to produce scale in pipes.
A calcium bicarbonate molecule in solution. Total alkali strength indicates bicarbonate ions. © Epop, Wikimedia public domain