calcium carbonate confusion

[Greg Grasson]

I am new to reef keeping, and am trying to understand the relationship between calcium, carbonate, and calcium carbonate and am very confused. Hopefully someone in here can explain this to me. I thought that in seawater, there are calcium ions, and carbonate ions. If too much calcium, carbonate, or both are added to the water (the water is already saturated with one, or both), the two will combine to form calcium carbonate and precipitate out of solution. I had the understanding that calcium carbonate is formed when the two combine, either through the skeletal building process of corals, or by precipitating out of solution. I recently purchased some B-ionic. In the instructions it said that if adding more product does not increase alkalinity or calcium levels, the water is saturated with calcium carbonate. Is there calcium, carbonate, AND calcium carbonate in the water? If so, what is the importance of the calcium carbonate?

[Randy Holmes-Farley]

thought that in seawater, there are calcium ions, and carbonate ions.

So far so good. :)

If too much calcium, carbonate, or both are added to the water (the water is already saturated with one, or both), the two will combine to form calcium carbonate and precipitate out of solution.

Yes, but you can add other things that increase carbonate (CO3--) besides carbonate itself. Bicarbonate (HCO3-) is one, and hydroxide (OH-) is another.

I had the understanding that calcium carbonate is formed when the two combine, either through the skeletal building process of corals, or by precipitating out of solution.

That is correct. :)

In the instructions it said that if adding more product does not increase alkalinity or calcium levels, the water is saturated with calcium carbonate. Is there calcium, carbonate, AND calcium carbonate in the water? If so, what is the importance of the calcium carbonate?



I'm not sure what words they use exactly on the label, but saturation is a complicated issue for calcium carbonate in seawater.

The correct wording would be "saturated with respect to calcium carbonate". It is the calcium and carbonate ions that together are saturated, and it is calcium carbonate that is the solid product of the precipitation (which is no longer in solution).

That said, nearly every reef aquarium is already supersaturated with respect to calcium carbonate, as is the ocean.

I discuss that a bit in this article (That as a beginner you may find useful):

Reef Aquarium Water Parameters (a summary general article)
http://www.reefkeeping.com/issues/2004-05/rhf/index.htm

"Magnesium's primary importance is its interaction with the calcium and alkalinity balance in reef aquaria. Seawater and reef aquarium water are always supersaturated with calcium carbonate. That is, the solution's calcium and carbonate levels exceed the amount that the water can hold at equilibrium. How can that be? Magnesium is a big part of the answer. Whenever calcium carbonate begins to precipitate, magnesium binds to the growing surface of the calcium carbonate crystals. The magnesium effectively clogs the crystals' surface so that they no longer look like calcium carbonate, making them unable to attract more calcium and carbonate, so the precipitation stops. Without the magnesium, the abiotic (nonbiological) precipitation of calcium carbonate would likely increase enough to prohibit the maintenance of calcium and alkalinity at natural levels."

What the folks at ESV presumably mean is that if you push the supersaturation too high, you can overcome these natural ways of preventing calcium carbonate precipitation, and you get excessive abiotic precipitation of CaCO3.

[Greg Grasson]

Thankd Randy. Something else I was wondering, how do people maintain dKH levels of 11 or 12 with calcium levels in the 450-500 range? My calcium level is about 450, and dKH between 7 and 8. If I try adding the alkalinity portion of B-ionic, it does'nt seem to have any effect. I'm affraid to keep adding more because I already screwed up once before and had to change almost all the water.

[Randy Holmes-Farley]

It is easier to maintain simultaneously high calcium and alkalinity at lower pH, such as comes with use of a CaCO3/CO2 reactor. I do not know how high your pH is, but more of the B-ionic alkallinity part (or just baking soda) will raise the alkalinity unless the pH rises too high (increasing abiotic precipitation).

[Greg Grasson]

Oh, ok. Maybe I just did'nt add enough of it. Like I said, I just did'nt want to screw up the water. My PH is around 8.0.

[Randy Holmes-Farley]

I'd try adding more for a while and see what happens. :)

[Greg Grasson]

Thanks a lot Randy.

[Randy Holmes-Farley]

You're welcome. :)

Good luck!