Total Hardness

Abstract

The total hardness of water results from divalent cations—mainly from calcium and magnesium—expressed as equivalent calcium carbonate. The total hardness equivalence of 1 mg/L calcium is 2.5 mg/L, while 1 mg/L magnesium equates to 4.12 mg/L. Hardness and alkalinity often are similar in concentration in waters of humid regions, but hardness frequently exceeds alkalinity in waters of arid regions. Hardness generally is less important than alkalinity as a biological factor, but it is quite important in water supply and use. High concentrations of calcium and magnesium in water containing appreciable alkalinity lead to scale formation when the water is heated or its pH increases. This leads to clogging of water pipes and scale accumulation in boilers and on heat exchangers. The Langelier Saturation Index often is used to determine if water has potential to cause scaling. Divalent ions also precipitate soap increasing soap use for domestic purposes and in commercial laundries. The traditional method for softening water is to precipitate calcium as calcium carbonate and magnesium as magnesium hydroxide by the lime-soda ash process. Water also may be softened by passing it through a cation exchange medium such as zeolite.