pH and Alkalinity
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      Why are calcium and total alkalinity measured in ppm CaCO3?

      Both calcium and total alkalinity are expressed as units of calcium carbonate (CaCO3). Here's why.

      Molecular weight of Calcium Carbonate (100 g/mol)

      Calcium carbonate (CaCO3) is used as the standardized basis for measuring factors like calcium hardness and total alkalinity because of its molar weight. CaCO3 is 100 g/mol, and water is 1000 g/Liter.  This makes for relatively easy math when calculating mg/L, which is the metric equivalent to parts-per-million (ppm). 

      Since calcium carbonate is an ion pair between calcium (calcium hardness) and carbonate (alkalinity), the unit works well to measure both factors in water chemistry.