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Originally Posted by banderbe What I do not understand is if there is always a fixed ratio of Ca to Mg in 1 dGH, or if 1 dGH can be made up completely of Mg, or completely of Ca. |
GH tests are EDTA chelation tests. Calcium and Magnesium cations react the same on these tests. No, there is no fixed ratio of Ca to Mg in 1 dGH. If it is made up completely of Mg or completely of Ca, it will read the same on a GH test. If you do a test on tap water, there is no way of knowing how much is Mg and how much is Ca unless you test using an ion selective probe or have your water company test it for you using whatever method they use.
My understanding is water softeners that use table salt (sodium chloride) exchange calcium and magnesium ions with sodium ions. I don't like them because they don't actually
remove ions, just exchange them, so they produce water very rich in sodium.
I don't know of any calcium test kits that only test for calcium and not magnesium at the same time. American Marine developed a
"dip and read" ion selective probe meter for calcium. However, it costs $250, and is really aimed more at reef tanks (i.e., tanks with very high calcium levels). Usually ion selective probes need to be used with ionic strength adjusters (and sometimes pH adjusters) so I'm not sure how American Marine got around this. I have to admit if you gave me an ion selective Ca probe, meter, and ionic strength adjuster, I would have no idea how to use it. I sure would like to learn but my Google searches haven't turned up anything I can make sense out of.