Aquarium GH calculation help

[Patrice]

Seem to me GH dont mean only calcium and magnesium. My calcium is 28.9 ppm and my magnesium is around 7 ppm meaning I shound have a GH of 35.9 ppm. But when i test the GH its at 100pm (5 drops). Whats going on?

Also, if you add 6gram of magnesium(epson salt) in 124 Liter you should have 4.5 ppm more but i have 20ppm more. What up whit that also?

 

[fort384]

GH is general hardness. It is testing for essentially all dissolved solids and ions present in your water, not just ca and mg.

 

[aqua_chem]

It's a measure divalent ions, the vast majority of which are Ca and Mg. It doesn't detect monovalent cations like sodium or potassium, which may be significant contributors to TDS (total dissolved solids) along with Ca and Mg. As far as your measurements go, there's several things you need to consider. First, parts per million is kind of a poor measurement for what you're doing. You can have ppm of Ca++, but you can also have GH measured in ppm, which is in ppm of CaO, but also has a contribution of Mg measured in molar equivalents of CaO in ppm (yuck). If you take that into account, you should be measuring 75-80 ppm GH. Further assuming you have the normal testing kit, one drop = 1 dGH, or ~89 ppm. Considering that you're using a test kit that's only really accurate to within 20 ppm or so and that your water conditions (which I'm assuming from the accuracy you looked up in a municipal report) vary seasonally, you should find that what you're seeing is pretty much what you would expect to see, quantitatively speaking.

 

[fort384]

This I did not realize. Thanks for explaining Mark. I thought GH was a measure of all ions.

 

[Patrice]

Wow. Thats exactly what I needed to know. Thank you. I like it when some one speak technical. Now all I have to do, is to understand it lol.