pH, Ca, Ca(OH)2-- general solubility questions

[zily]

Hey there,

I was wondering if anyone could help me. In quantitative analysis, i mentioned to my teacher something about aquariums (we were doing solubility in acidic solutions), and now he wants me to make a little presentation about it for class.

Upon thinking about stuff, I've become very confused. I guess I'll get to it.

I want to do the presentation on the headaches of maintaining calcium concentrations while still keeping the pH above 8.1.

I learned that calcium, with respect to carbonate ions is supersaturated (if I heard correctly), and I guess i'll starting asking questions.

1. What is the ideal concentration of CO3-2 in a tank? Is that directly related to the KH?

2. Often when you mess up your tank (and go crazy with the turbo calc), that's calcium carbonate precipitating, right?

3. If too much Super Buffer is added (carbonate and bicarbonate), it precipitates as calcium carbonate and the pH drops from the release of the hydrogen from the bicarbonates-- is that right?

4. Does magnesium have anything to do with what I was talking about?

Thanks for the help guys! I no longer have my fishtank, as I had to sell it so I could move to college station, but I guess I am a chemistry major and I liked the challenge of the tank. maybe i'll start a nanoreef...

-daniel

[Randy Holmes-Farley]

What is the ideal concentration of CO3-2 in a tank? Is that directly related to the KH?


Alkalinity (dKH) is a measure of bicarbonate and carbonate ions. The pH controls the ratio between the two of these. I detail what alkalinity is in this article:


What is Alkalinity
http://www.advancedaquarist.com/issu.../chemistry.htm
Often when you mess up your tank (and go crazy with the turbo calc), that's calcium carbonate precipitating, right?

Typically, yes. Adding a high pH solution also can cause temporary formation of magnesium hydroxide. I discuss such issues in this article:

What is that Precipitate in My Reef Aquarium?
http://reefkeeping.com/issues/2005-07/rhf/index.htm

If too much Super Buffer is added (carbonate and bicarbonate), it precipitates as calcium carbonate and the pH drops from the release of the hydrogen from the bicarbonates-- is that right?

Yes.

Does magnesium have anything to do with what I was talking about?

Yes. It can poison the growing CaCO3 surface, preventing further precipitation. This article discusses that:

Magnesium
http://www.advancedaquarist.com/issues/oct2003/chem.htm

[zily]

Thank you for the articles, they were very informative.

I still have one question though,

1. What is the concentration (in molarity) of Carbonate and Bicarbonate in seawater?

After reading your article, I came up with a method ( that I hope is correct) to determine the concentration.

I assumed that out of 100 meq/L of alkalinity, 89.2 meq/L comes from bicarbonate, and 6.7 meq/L comes from carbonate, which would mean there is a total of 89.2 mM of bicarbonate and 3.35 mM.

Would that put seawater at (alk=4.0 meq/L)
bicarbonate ~ 3.568 mM
carbonate ~.134mM
?

Once again, thank you so much for your help
-daniel

[zily]

Too late to edit, I did want to add something

If I made the correct calculations, that would mean that calcium carbonate would be supersaturated 356 times (the most reasonable ksp I found for calciumcarbonate was 3.76*10^-9)

multiplying .01 (400mg/L Ca) and .000134 (.134mM of carbonate) would give 1.34*10^-6.

(1.34*10^-6) / (3.76*10^-9) = 356.

Is there something I'm doing wrong? I thought your article mentioned it was only 5 or 6 times supersatureated, not 356.

-daniel

[Randy Holmes-Farley]

1. What is the concentration (in molarity) of Carbonate and Bicarbonate in seawater?

Depends on the pH, but about 2.3 mM, with most present as HCO3- and a fraction (5-20% as CO3--).

Calcium carbonate is about 4x supersaturated in normal seawater. I discuss that in this article:

Calcium
http://www.advancedaquarist.com/issues/mar2002/chem.htm