Introduction:
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Hardness is the concentration of multivalent metabolic cations in t solution.
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At supersaturated conditions, the hardness cations will react with anions in
the water to form a solid precipitate.
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There are two types of hardness
1.
Carbonate hardness: The hardness that is equivalent to the alkalinity is termed
as carbonate hardness. Carbonate hardness is sensitive to heat and precipitates
readily at high temperatures:
(CaHCO3)2 → CaCO3 + CO2 + H2O
(MgHCO3)2 → MgOH2
+ 2CO2
2.
Non-carbonate hardness: Any remaining hardness is termed as non-carbonate
hardness.
Sources:
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Hardness is mostly from Calcium and Magnesium.
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Some also from iron and manganese (Fe2+, Mn2+) in their
reduced states.
Impacts:
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Soap consumption by hard water is an economic loss to the water user.
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Skin may fell rough and uncomfortable if hard water is used.
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The precipitate formed by hardness and soap adheres to surface of tubs, sinks
and dishwashers and may stain clothing, dishes and other items.
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The result of carbonate hardness precipitate may cause considerable economic
loss through fouling of water heaters and hot-water pipes.
Measurement:
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Hardness is measured by spectrophotometric techniques or chemical titration to
determine the quantity of calcium and magnesium ions in a given sample.
Use:
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Hardness test is commonly made for water that is intended for potables supplies
as drinking water and for certain industrial uses.
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Hardness may range from practically zero to several hundred, or even several
thousand, parts per million.
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Acceptability levels are as follows:
Soft <
50 mg/L as CaCO3
Moderately
Hard 50 - 150 mg/L
as CaCO3
Hard 150 –
300 mg/L as CaCO3
Very
Hard > 300 mg/L as CaCO3
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The public health service standards recommended a maximum of 500 mg/L of
hardness in drinking water.
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