Definition: Hardness is a measure of the mineral, calcium and magnesium, content of the water. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Ideal range: Ideal calcium hardness content is 300 to 400 ppm as measured by a suitable kit.
Method of measurement: Hardness is most commonly measured by titration with an EDTA solution. A titration involves adding small amounts of a solution to a water sample until the sample changes color. You can titrate a sample for total hardness using a buret or hard water test kit. You can also measure calcium hardness separately from magnesium hardness by adjusting the pH and using different indicators.
Method of control: Lowering hardness, expressed carbonate hardness, is commonly carried out by a water softener, which is a form of chemical filtration employing an ion-exchange resin. The resin binds with mineral ions (Ca) reducing the hardness hence the mineral content of the water. The downside is that the salts used to un-bind to the minerals are back-flushed into the sewer system and pollute the environment. Another hardness control alternative is electro-chemical oxidation whereby the calcium is converted to the soft bicarbonate form, subsequently the heat point required to precipitate calcium in the carbonate form is increased substantially.
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