Reef Chemistry Question of the Day #83 Carbonate Ion Pairs

Randy Holmes-Farley · Dec 11, 2014

Reef Chemistry Question of the Day #83

Carbonate is one of the more important ions in a reef aquarium. It is part of the calcium carbonate skeleton of hard corals, for example.

Curiously, when carbonate is present in seawater, it is mostly not present as free, independent carbonate ions.

Rather, it forms ion pairs to other ions in the seawater, and they hold together for a short period of time before coming apart and then reforming.

Which ion forms the largest fraction of ion pairs with carbonate in seawater, based on the percentage of carbonate ions involved?

A. Bicarbonate
B. Sodium
C. Chloride
D. Magnesium
E. Sulfate
F. Calcium
G. Potassium

Good luck!

.

beaslbob · Dec 11, 2014

a
(wild guess)

baitrunner · Dec 12, 2014

Carbonate forms ion pairs primarily with magnesium, forming soluble MgCO[SIZE=-1]3[/SIZE].

Steven@ · Dec 12, 2014

After googling it, it appears magnesium is the correct answer... I would have guest Chloride.

Randy Holmes-Farley · Dec 15, 2014

And the answer is...D. Magnesium

In seawater, some of the ions form fairly strong ion pars to other ions. Carbonate is one of these.

These ion pairs are formed between ions of opposite charge, and the higher the charge the more likely they are to form.

In normal 35 ppt seawater, more than half of the carbonate is ion paired to magnesium. Sodium ion pairs, calcium ion pairs and free carbonate about evenly split the remainder.

Think of these ion pairs as soluble, temporary molecules of magnesium carbonate. They last for a brief time, then separate.

The most important aspect of this is that since the free carbonate is low (less than 1/6 of the total carbonate) and it is this free carbonate that causes precipitation of calcium carbonate, calcium carbonate ends up being a lot more soluble in seawater than in fresh water. That is, there is more total calcium and carbonate in solution in seawater than is possible in fresh water.

Happy Reefing!