DCOM Volume I Appendix I

Appendix I: Measurement of Water Hardness

Hardness and its Measurement
Hardness is expressed in terms of mg/l by weight in terms of calcium carbonate. Water having hardness not exceeding 70 mg/l is termed ‗soft‘ and above that 'hard'. In public water supplies, it used to be customary to reduce carbonate hardness to 35 - 40 mg/l and total hardness to between 50 and 100 mg/l. However as indicated above this is no longer recommended unless hardness exceeds about 130 mg/l, but should still be practised for strictly industrial supplies of hard water.

Hardness = ∑ divalent cations = Ca2+ + Mg 2+ + Fe 2+ + Mn 2+ + Sr 2+.....

Principle cations causing hardness in water and major anions associated with them are as follows:

Table I.1: Principle cations and anions associated with water Hardness

Cations	Anions
Ca2+	HCO-3 or CO2-3
Mg2+	SO2-4
Sr2+	cl-
Fe2+	NO-3
Mn2+	SiO-3