‘Hardness’ is defined as the total concentration of alkaline earth ions (Ca2+, Mg2+, Sr2+ and Ba2+) in water. The Ca2+ and Mg2+ ions dominate the alkaline earth ions. Therefore, one can refer to total hardness as the total concentration of the Ca2+ and Mg2+ ions in solution. Hardness is expressed as mg CaCO3/L water sample (ppm CaCO3). Read MANTECH's hardness method abstract here.