CaCO3 equivalent is a unit of chemical equivalency that is familiar and has been used by water people for ages. It's also convenient because it's so easy to convert in your head to/from molarity. For any divalent molecule, be it CaCO3, MgSO4, CaCl2, CO3²⁻ or whatever, you can multiply your mmol/L value by 100 to get mg/L CaCO3 equivalent. For univalent, like NaHCO3 or bicarbonate ion, HCO3⁻ , you multiply by 50 (because two univalent ions are chemically equivalent to one divalent ion.)

There are other units of chemical equivalence used for water, for example grains per gallon, French degrees, German degrees, mEq/L, etc. All can be easily converted to CaCO3 equivalent.

Rickpatbrown wrote:How do you convert 40 ppm CaCO3 units alkalinity into molarity?

For bicarbonate (HCO3⁻ ) it would be 0.8 mmol/L, and for carbonate (CO3²⁻) it would be only 0.4 mmol/L.* And either would neutralize 0.8 mmol/L of a univalent acid like HCl,

Rickpatbrown wrote:I just realize that KHCO3 is 100 g/mol MW also! what sick kind world is this!? That's the other thing that confused me is people seem to just add ppm together and get get total molarity, lol. I guess it's because of this strange coincidence.

That is a coincidence that may lead to confusion. And you can't generally add ppm of hardness and ppm of alkalinity together to get anything meaningful. For example if you have 40 ppm hardness and 40 ppm alkalinity, it could all be due to 0.4 mmol/L (40 mg/L) of CaCO3. But it might also be due to 0.8 mmol/L (40 mg/L) of KHCO3 plus 0.4 mmol/L (48 mg/L) MgSO4. To convert mg/L of ion to/from chemical equivalence units you always have to know what ions you are dealing with.

Using equivalence units (like CaCO3 equivalent) for both hardness and alkalinity is handy if you want to get an idea about how much of your hardness is carbonate (aka temporary hardness) vs non-carbonate (non-scaling, aka permanent hardness.) When using equivalence units, your carbonate 'temporary' hardness is equal to the lesser of the two (total hardness vs alkalinity) values.

A little more info that might help is in this post from a while back: Good references on water treatment for coffee/espresso

And also M. Wellinger's presentation: http://scae.com/images/pdfs/AST-LIVE-20 ... Coffee.pdf



* Re carbonate and bicarbonate: Water below a pH of 9 or 10 will be almost all bicarbonate rather than carbonate. Even if you add CaCO₃, it reacts with water to form Ca²⁺ + (HCO₃⁻)₂. As the pH goes up or when temp goes up it shifts to Ca²⁺ + CO3²⁻ and may precipitate. With chemical equivalence units (as opposed to molarity) you don't need to know or care how much is carbonate vs bicarbonate.