Units of Alkalinity
The various units used for alkalinity are themselves cause for confusion. The clearest unit, and that used by most scientists is milliequivalents per L (meq/L). For a 1 millimolar solution of bicarbonate, the alkalinity is 1 meq/L. Since carbonate takes up two protons for each molecule of carbonate, it "counts" twice, and a 1 millimolar solution of carbonate has an alkalinity of 2 meq/L.
A unit that is used by many kits and some industries involves representing alkalinity in terms of the amount of calcium carbonate that would need to be dissolved in fresh water to give the same alkalinity. Typically, it is reported as ppm
calcium carbonate. Of course, it has nothing to do with calcium, and there may be no carbonate in the water at all. Nevertheless, it is frequently used. Since calcium carbonate weighs 100 grams/mole (100 mg
/mmole), then a solution that has an alkalinity of 100 ppm
calcium carbonate equivalent contains 100 mg
/L calcium carbonate divided by 100 mg
/mmole calcium carbonate = 1 mmol/L calcium carbonate equivalent. Since carbonate has 2 equivalents per mole, this 100 ppm
of alkalinity is equivalent to 2 meq/L. So to convert an alkalinity expressed as ppm
CaCO3 to meq/L, divide by 50.
Finally there is the German term dKH
(degrees of carbonate hardness), or just KH
(carbonate hardness).Strictly speaking, it is the same as the carbonate alkalinity (AC
in equation 8). Unfortunately, it is a very confusing term, as it has nothing to do with hardness. Further, it has been corrupted by the marine aquarium hobby to mean the same as total alkalinity, and every test kit that tests for dKH
with a single titration is giving total alkalinity. The only kit that I am aware of that even makes a distinction between carbonate alkalinity and total alkalinity is one of the Seachem kits (Reef Status: Magnesium, Carbonate, & Borate) and it thankfully doesn’t use the term dKH
at all. Consequently, most hobbyists should think of dKH
as simply another measure of total alkalinity. The results obtained with such a kit (dKH
) can be divided by 2.8 to yield the alkalinity in meq/L.
For those who are mathematically challenged, here is an alkalinity conversion table for all three units. Alkalinity Conversion Table