When hardness is expressed as ‘mg/l as CaCO3’, it’s calculated as if all the calcium and magnesium were present only as calcium carbonate.
Hard water is a mixture of calcium and magnesium, together with bicarbonate, sulphate, chloride, etc. 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 – a negatively charged ion. For stability, it has to be associated with a positively charged ion or cation. These ions could be hydrogen or a metal such as calcium, magnesium, sodium, potassium, etc.
This makes CaCO3 calcium carbonate.
Limestone and chalk are essentially calcium carbonate. It is virtually insoluble and forms as limescale when the soluble bicarbonate decomposes to carbonate.
Why is hardness calculated in terms of CaCO3 in PPM?
Hard water with 300 mg/l as CaCO3 would precipitate 300 milligrams of calcium carbonate per litre.
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. It also includes the bicarbonate ions such as chloride, sulphate and nitrate.