<a href=showthread.php?s=&postid=13706589#post13706589 target=_blank>Originally posted</a> by Phillybean
Hey Tom,
I'm curious to know the math on how to "determine magnesium by subtracting out the calcium determined in the calcium test"
Does this tell you the magnesium in the tank with-out using a magnesium test?
Man, the article has been up for less than 12 hours and already someone asks. OK, stay with me cause it is somewhat complicated (the main reason I didn't explain it in the article itself).
Usually, total hardness is measured in ppm of Hardness
as CaCO<sub>3</sub>. That is somewhat of a convention as the main reason people measure hardness is in water softening. It includes mainly calcium and magnesium but can also include other divalent cations. In seawater, calcium and magnesium are the two main hardness ions and we can get a pretty clear determination of their total content with this test.
When you run the test on typical seawater you get a reading of around 6630 ppm of hardness as calcium carbonate. Fine and dandy but it doesn't differentiate between what is calcium and what is magnesium. Now we need to do a little math.
Say we test for calcium and get a reading of 440 ppm as Ca. Well if we multiple that number by 2.5 we convert Ca as Ca to Ca as CaCO<sub>3</sub>. Here, 440 ppm as Ca as Ca x 2.5=1000 ppm as CaCO<sub>3</sub>. To find magnesium we first subtract the Ca as CaCO<sub>3</sub> from our total hardness measurement. Let's say it was 6630 ppm. That means we have 6630 ppm - 1000 ppm = 5630 ppm of Mg as CaCO<sub>3</sub>. We're still not there yet as we want to know Mg as Mg. We then divide the Mg as CaCO<sub>3</sub> by 4.17 ( the conversion factor for changing Mg as CaCO<sub>3</sub> to Mg as Mg). This gives us 1350 ppm and that is our true magnesium content of the water sample.
In a nutshell
Find the Total hardness
Find the calcium with a calcium test kit and multiply by 2.5
Subtract that value form the total hardness
Divide that remainder by 4.17
The answer it the Magnesium content of the sample.
Have Fun!