Now for the fun part. When we measure hardness, what exactly are "they"
refering to when "they" say 100 parts per million? Actually it doesn't matter
if we are talking about carbonate hardness (KH) or general hardness (Calcium and
Magnesium). Let's start with Calcium though. Is this 100 milligrams (mg =ppm)
of calcium per liter OR is it the molecule that "they" decide is the standard?
As it turns out, after a few phone calls and some checks of the various test
kits it's a 100 parts per million (100 mg/liter) of Calcium Carbonate (CaCO3).
That would actually only be 40 parts per million of calcium (Calcium is 40% of
the weight of calcium carbonate). Calcium carbonate is the "standard" that they
use for their calicum/magnesium test kits as well as their carbonate test kits.
Now I'm not sure how the heck they get that much calcium carbonate into solution
but that's what they call their standard. One hundred parts per million CaCO3
also just happens to be a 1 millimolar (mMolar) solution of CaCO3. Now we are
on a molecule to molecule basis. Knowing this you can substitute a 1 mMolar
Calcium Chloride (CaCl2) solution and obtain the same answer with your test kit
as you would with a 1 mMolar solution of CaCO3. However a 1 mmolar solution of
CaCl2 would be 110 <ppm>. The same measurements also apply on the carbonate
side of the equation or when substituting magnesium (Mg) for calcium. Our test
kits actually measure on a "molecule or molar" basis but they calibrate the
results to calcium carbonate. If you were going to make test solutions that
would react the same with your GH hardness kit you would make them all the same
molarity (ie 1mMolar CaCO3(100ppm) = 1 mMolar MgCl (203.3 ppm) = 1 mMolar CaOH
(74.09 ppm).
Gary Lange
Rainbowfish Study Group
gwlang at ccmail.monsanto.com
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