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Can you measure Ca or Mg from GH?

5552 Views 6 Replies 4 Participants Last post by  KevinC
Or at least estimate the quantity of Ca or Mg?
Just curious.

It'll save $$$ for extra test kits.
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You need to know your GH and Ca to know your Mg levels. The last part of this note speaks of the hard water areas.


In order to find magnesium hardness, you must take a hardness reading with a GH test.

Lamotte's calcium test is a GH test with an inhibiting agent that prevents magnesium from affecting the test. There's no way you can find the magnesium level by using the calcium test alone.

The calcium reading of the calcium test is in a CaCO3 equivalence. CaCO3 is 40.04320% calcium. So if the kit reads 75 ppm as CaCO3,
then 75 * .4004320 = 30.0324 ppm actual calcium.

For magnesium, take your GH reading minus your calcium reading. For example, if your GH reading is 100 ppm as CaCO3, then 100 - 75 = 25 ppm magnesium as CaCO3. If you want to express this as actual magnesium, multiply by .2428391. So 25 * .2428391 = 6.0709775 ppm actual magnesium.

(In case you're interested, the .2428391 comes from the assumption that calcium and magnesium react equally on the GH test. Calcium weighs 40.078 grams per mole; magnesium weighs 24.305 grams per mole. Therefore, calcium weighs 1.64896 times more than magnesium (40.078 / 24.305). Take your 40.04320% calcium in calcium carbonate, and divide by 1.64896 = 24.28391%)

hardness kit:
calcium hardness kit:

Hard water in the US

According to the United States Geological Survey, 85% of US homes have hard water. The softest waters occur in parts of the New England, South Atlantic-Gulf, Pacific Northwest, and Hawaii regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes, Pacific Northwest, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. Hardest waters (greater than 1,000 mg/L) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.
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Oh snap! you brought out the 'mole' for this.
thanks.. very informative.
Your're welcome MG!
Ca test kit Hagen...

Hagen have a Ca test Kit and explain how to find MG
(2 nutrients for the price of 1)

I’m using that test kit and I’m happy with it...

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Hagen have a Ca test Kit and explain how to find MG
(2 nutrients for the price of 1)
Hi G

Maybe I'm misreading what you are saying a little.:icon_smil

You still need a GH test kit plus a Ca test kit to measure Mg to get "(2 nutrients for the price of 1)."

General Hardness mg/L (CaCO3) - Calcium Hardness mg/L (CaCO3) = Magnesium Hardness mg/L (CaCO3)
For those who want to DIY:

The difference between a Ca and GH test kit is the pH and the indicator.

If you adjust the pH to >12 (use a few drops of plain old Drano for DIY), the Mg precipitates and only the Ca will react with the titrant (EDTA) and indicator.

The preferred indicator is hydroxynaphthol blue, as it retains its color change at the higher pH. The Eriochrome black T indicator typically used for GH does not have as definite a color change at pH>12.

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