English English.   Español  Español.

The application of Appropriate Technology

Articles for Keyword "water hardness"

An Introduction To Water Hardness

Posted on Oct 25, 2011

A document providing an introduction to the causes and treatment of hard water, including magnetic water softeners. Some Simple Chemistry When ionic substances dissolve in water they are split into their constituent ions. For example when NaCl (sodium chloride, common salt) dissolves in water it produces Na+ (the positive or cation) and Cl– (the negative or anion). These ions stay in the solution unless something happens to make them precipitate out as a solid; for example if water is evaporated from a salt solution, the solution becomes saturated and salt crystals will precipitate. The process of dissolving is therefore not a chemical reaction and can be reversed by physical, rather than chemical, means. The Causes Of Hardness Most hardness in water is caused by the presence of dissolved calcium (Ca2+) and magnesium (Mg2+) ions. Other cations, such as Al3+ and Fe3+ can contribute to hardness, however their presence is less critical. The most common, and troublesome, form of hardness is caused by the presence of calcium bicarbonate (Ca(HCO3)2) which is picked up by rain water passing through lime stone (CaCO3). As rain water falls it dissolves carbon dioxide (CO2) from the air and becomes slightly acidic because carbonic acid (H2CO3) is formed: In the above equation g=gas, l=liquid and aq=aqueous (i.e. dissolved in water). In the following equation s= solid. CaCO3 is not very soluble in water however, when the dilute acid runs through the lime stone a reaction occurs that creates calcium bicarbonate which is readily soluble: Thus the rain water has picked up Ca2+ and HCO3– (bicarbonate) ions and become hard. When hard water is heated the previous two reactions are reversed and calcium carbonate, water and carbon dioxide are formed: Since calcium carbonate is much less soluble in water than calcium bicarbonate it precipitates out of solution as a solid known as scale or lime scale. Because this type of hardness is easily removed (i.e. by simple heating) it is known as temporary hardness. Scale normally appears around heating elements and hot water systems. However, if the water is exceptionally hard scaling may occur in cold water pipes [1]. Other types of temporary hardness are caused by the presence of Mg2+ ions and the precipitation of magnesium hydroxide (Mg(OH)2) can contribute to scaling problems. Combinations of Ca2+ and Mg2+ ions with chloride (Cl–), sulphate (SO42-) and nitrate (NO32-) ions are known as permanent hardness. For example in some areas CaSO4 may cause considerable hardness. Permanent hardness can not be removed by boiling. The term hardness total hardness is used to describe the combination of calcium and magnesium hardness. However, hardness values are usually quoted in terms of CaCO3 because this is the most common cause of scaling. The standard classifications are given below [1]: Hardness mg/L as CaCO3 moderate 60-120 hard 120-180 very hard more than 180 Problems Caused By Hardness Excessive soap is needed for washing (i.e. soap will not lather). Some modern detergents work less efficiently because anions (also known as surfactants) which are meant to hold dirt particles in suspension react with Ca2+ and Mg2+ instead...

Read More