Reef Chemistry Question of the Day #50 Dolomite

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #50

When added to a calcium carbonate/carbon dioxide reactor, dolomite can be a suitable way to maintain existing magnesium levels. It is not, however, a suitable way to raise low magnesium levels. Which of the following is the primary reason it is not suitable for substantially raising magnesium?


A. Because it is too slow to dissolve
B. Because it adds to much sulfate to the water
C. Because it adds too much alkalinity to the water
D. Because it is too fast to dissolve

Good luck!





























.

 

[Randy Holmes-Farley]

No takers on this one yet?

 

[niftyjosh]

I know mine hardly dissolves so I'll go with A.

 

[jedimaster1138]

C -- Dolomite (Ca,Mg)(CO₃)₂ is made up of twice as much carbonate as calcium carbonate CaCO3 so if you were to use dolomite to raise Mg levels substantially, you're also going to raise calcium and carbonate dramatically higher, well, 2x higher, which is probably a bad thing for your reef.

 

[Randy Holmes-Farley]

Any one else before I give the answer?

 

[DFW]

A...

 

[Randy Holmes-Farley]

And the answer is....C. Because it adds too much alkalinity to the water
(although A might also be correct if you wanted fast results)

Dolomite is (Ca,Mg)(CO₃)₂. It varies a bit in the calcium to magnesium ratio, but the point here is that every magnesium is accompanied by a carbonate.

Consequently, it is fine if you are maintaining magnesium against tank demand, where one magnesium ion is used exactly in proportion to one carbonate ion.

However, this method is unsuitable if the goal is to raise magnesium levels. The problem is that for every magnesium ion released from the dolomite, 2 units of alkalinity (one carbonate ion) are also released:


MgCO₃ → Mg²⁺ + CO₃^2-


Consequently, if one wants to raise magnesium by 100 ppm, the alkalinity will necessarily rise by 8.2 meq/L = 23 dKH. The only way around this problem is to add a mineral acid (not vinegar) to the aquarium to reduce the alkalinity, and that may be more problematic than just adding a magnesium supplement in the first place.

Happy Reefing!