What is hardness. What is it made from? Why do they express it as CaCO3? Why do they express it as ppm.
The simple answer is that hard water comprises a mixture of calcium and magnesium, together with bicarbonate, sulphate, chloride, etc. When its hardness is expressed as mg/l as CaCO3, it’s calculated as if all the calcium and magnesium were present only as calcium carbonate.
If more detailed explanation is needed:
Ca – is calcium, C carbon and O oxygen. CO3 means 3 atoms of oxygen are combined with 1 atom of carbon to form carbonate. Carbonate is an acid radical (or anion) – it is negatively charged, so for stability, it has to be associated with a positively charged ion (or cation) like hydrogen (to form carbonic acid) or a metal such as calcium, magnesium, sodium, potassium, etc.
So CaCO3 is calcium carbonate. Limestone and chalk are essentially calcium carbonate – so is limescale in its simplest form. It is virtually insoluble and it forms as limescale when the bicarbonate (temporary hardness), which is soluble, decomposes to carbonate.
A hard water with 300 mg/l as CaCO3 would (in theory) precipitate 300 milligrams of calcium carbonate (CaCO3) per litre of hard water if all of the hardness precipitated.
In reality, the hardness is not entirely due to calcium – magnesium is usually present to some extent and other multivalent cations. When the hardness is expressed as CaCO3, it is calculated as if the magnesium, etc. were there as calcium. Also bicarbonate is not the only anion present, there will be chloride, sulphate and nitrate to some extent (permanent hardness). But the expression of hardness assumes its all there as carbonate.
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