Hard water is the type of water that has high mineral content (in contrast with soft water). Hard water minerals primarily consist of calcium (Ca2+), and magnesium (Mg2+) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate (CaSO4), in the form of other mineral deposits. The predominant source of magnesium is dolomite (CaMg(CO3)2).
More exact measurements of hardness can be obtained through a wet titration. The total water 'hardness' (including both Ca2+ and Mg2+ ions) is read as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate (CaCO3) in the water. Although water hardness usually only measures the total concentrations of calcium and magnesium (the two most prevalent, divalent metal ions), iron, aluminium, and manganese may also be present at elevated levels in some geographical locations.
Classification of Water by Hardness Content
Concentration mg/L CaCO3 Description
0 - 75 soft
75 - 150 moderately hard
150 - 300 Hard
300 and up Very hard
Temporary hardness
Temporary hardness is caused by a combination of calcium ions and bicarbonate ions in the water. It can be removed by boiling the water or by the addition of lime (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.
The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water:
CaCO3(s) + H2CO3(aq) ⇋ Ca2+(aq) + 2HCO3-(aq)
More exact measurements of hardness can be obtained through a wet titration. The total water 'hardness' (including both Ca2+ and Mg2+ ions) is read as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate (CaCO3) in the water. Although water hardness usually only measures the total concentrations of calcium and magnesium (the two most prevalent, divalent metal ions), iron, aluminium, and manganese may also be present at elevated levels in some geographical locations.
Classification of Water by Hardness Content
Concentration mg/L CaCO3 Description
0 - 75 soft
75 - 150 moderately hard
150 - 300 Hard
300 and up Very hard
Temporary hardness
Temporary hardness is caused by a combination of calcium ions and bicarbonate ions in the water. It can be removed by boiling the water or by the addition of lime (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.
The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water:
CaCO3(s) + H2CO3(aq) ⇋ Ca2+(aq) + 2HCO3-(aq)
Permanent hardness
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. Despite the name, permanent hardness can be removed using a water softener or ion exchange column, where the calcium and magnesium ions are exchanged with the sodium ions in the column.
Measurement:
The method is based on the reaction of heavy metal ions and alkaline earth metal ions (mostly Ca and Mg) with EDTA in basic solution. The sample is titrated with EDTA solution of known concentration until the initially obtained violet colour of the solution has turned to stabile blue colour. Eriochrome black -T serves as an indicator with this measurement.
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. Despite the name, permanent hardness can be removed using a water softener or ion exchange column, where the calcium and magnesium ions are exchanged with the sodium ions in the column.
Measurement:
The method is based on the reaction of heavy metal ions and alkaline earth metal ions (mostly Ca and Mg) with EDTA in basic solution. The sample is titrated with EDTA solution of known concentration until the initially obtained violet colour of the solution has turned to stabile blue colour. Eriochrome black -T serves as an indicator with this measurement.
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