Calculating magnesium from Calcium as Ca and THU as CaCo3

TeeJay87 · Jan 17, 2024

I just started using the Exact iDip for water testing. I’m trying to get Magnesium. Test results show:

Calcium = 450 as Ca
THU = 4642 as CaCo3

Can someone please help me calculate Magnesium with these numbers? I believe you have to convert Calcium to CaCo3, then subtract that from THU and use the THU difference to calculate Magnesium.

Randy Holmes-Farley · Jan 17, 2024

It does not seem as though the answer is likely to be accurate based on how low it is, but here goes:

4642 ppm CaCO3 equivalents x0.4 (fraction of CaCO3 that is Ca) = 1857 ppm calcium equivalents

1857 - 450 ppm calcium = 1407 ppm calcium equivalents that are not calcium

Assuming it is all magnesium and not anything else (such as strontium). We multiply by the weight ratio of magnesium ion to calcium ion to get magnesium:

1407 x 24.3/40 = 855 ppm magnesium

TeeJay87 · Jan 17, 2024

Randy Holmes-Farley said:
It does not seem as though the answer is likely to be accurate based on how low it is, but here goes:

4642 ppm CaCO3 equivalents x0.4 (fraction of CaCO3 that is Ca) = 1857 ppm calcium equivalents

1857 - 450 ppm calcium = 1407 ppm calcium equivalents that are not calcium

Assuming it is all magnesium and not anything else (such as strontium). We multiply by the weight ratio of magnesium ion to calcium ion to get magnesium:

1407 x 24.3/40 = 855 ppm magnesium

Thank you. I just read your RMM method for Mg, so I’m leaning towards it being an issue with the test kit/tester. I will test some mixed saltwater (IO) next to see what I get.

TeeJay87 · Jan 17, 2024

Randy Holmes-Farley said:
It does not seem as though the answer is likely to be accurate based on how low it is, but here goes:

4642 ppm CaCO3 equivalents x0.4 (fraction of CaCO3 that is Ca) = 1857 ppm calcium equivalents

1857 - 450 ppm calcium = 1407 ppm calcium equivalents that are not calcium

Assuming it is all magnesium and not anything else (such as strontium). We multiply by the weight ratio of magnesium ion to calcium ion to get magnesium:

1407 x 24.3/40 = 855 ppm magnesium

So I just tested a bucket of saltwater I had mixing for a couple of days and using your calculations I got 882 Mg (from 436.8 Ca and THU of 4723 CaCo3). I realized I should check the salinity and it was almost 40 ppt so I diluted it to about 35 ppt and retested. Results gave me 785 Mg (from 396 Ca and THU 4221 CaCo3). So atleast the tests gave roughly consistent decreases in results when the bucket was diluted a bit (Ca @ 90.6% and Mg @ 89.0% of pre diluted values).

All of the saltwater in my aquarium (100 gallons) and in the mixed bucket comes from the same 200 gallon box of IO sea salt. Per IO: “When dissolved in pure water to a specific gravity of 1.026, Instant Ocean® should provide a solution that has 400 mg/L calcium ion and 1320 mg/L magnesium ion.”

Does the “400mg/L calcium ion” mean my 396 of Ca in the diluted 35 ppt bucket is almost spot on?

Have you ever seen a test kits this far off for magnesium? It seems very unlikely the IO sea salt is this far off. Not sure where to go from here. I could reach out to the test kit manufacturer. I could ignore this Mg test kit altogether. I could keep testing and just look at trends.

Randy Holmes-Farley · Jan 17, 2024

Is the kit designed for seawater?

Heres a copy and paste from one of my articles;

GH (general hardness)

Hardness is a characteristic of water due to the presence of dissolved calcium and magnesium. Water hardness is responsible for most scale formation in pipes and water heaters (calcium and magnesium carbonates, typically), and forms insoluble solids when it reacts with soaps. Hardness is often expressed in grains per gallon, parts per million or milligrams per liter, all as calcium carbonate equivalents. It is a measure often used in freshwater aquarium systems, but not often in marine systems, where its values are very large. Seawater has a total (general) hardness of about 6.3 g/L (6,300 ppm) of calcium carbonate equivalents. These linked articles detail aspects of magnesiumand calcium in seawater.