So why not calcium reactor media in ATO ?

[k2-]

Pardon my curious nature but :

We try to change the PH of salt water by co2 and try to melt calcium reactor media (mostly between 6.1 - 6.6) - RODI water coming out of my system usually ph's around 5.8 -

Question - Will it be crazy to put this media in ATO system to feed for alkalinity and magnesium or better yet dissolve it and dose it or a inline reactor system less cumbursome than dealing with co2 etc ?

 

[bushdoc]

pH of highly purified water is around 7.0, but it can change if CO2 is dissolved in it.
Also it is difficult for various reasons to accurately measure pH of RODI water and due to it‘s lower buffering capacity it can also fluctuate fast.

 

[A_Blind_Reefer]

Enter kalkwasser. I think you’d be hard pressed reinventing the wheel on maintaining a proper dose and salinity going that route. Kalk is already pretty much this and is cheap,

 

[Randy Holmes-Farley]

Pardon my curious nature but :

We try to change the PH of salt water by co2 and try to melt calcium reactor media (mostly between 6.1 - 6.6) - RODI water coming out of my system usually ph's around 5.8 -

Question - Will it be crazy to put this media in ATO system to feed for alkalinity and magnesium or better yet dissolve it and dose it or a inline reactor system less cumbursome than dealing with co2 etc ?

Just a clarification on terminology: it is not melting. it is dissolving. It's equally bad to describe what happens to an ice cube sitting on a table as dissolving.

That said, one can dissolve calcium carbonate in fresh water, without CO2, or to a greater extent with CO2. But the amount that will dissolve is substantially less than in seawater.

The pH of RO/DI is not key here. You cannot accurately measure it, and just a tiny bit of CO2 will lower the pH, while not actually adding enough CO2 to actually disoslve much calcium carboante.

I describe the limitations to these processes here:

Chemistry And The Aquarium: Calcium Carbonate As A Supplement

Randy discusses the importance of calcium supplementation using a variety of techniques .

reefs.com

Dissolution of CaCO3 in Water Prior to Addition​

The best way, in my opinion, to use calcium carbonate as a supplement is to dissolve it in fresh water prior to addition. In this sense, it can be used rather like limewater. One can rig up an automatic evaporation replacement system using appropriate pumps and float switches, and just use water saturated with CaCO3 instead of limewater. Alternatively, one can simply pour the saturated water into the tank each day. Unfortunately, the fact that you can add it this way is a mixed blessing. One reason that you can add it this way is that there is so little present that the carbonate does not drive up the pH too much.


So how much goes into solution? This question is rarely addressed directly, and it is because of one big complication: carbon dioxide from the atmosphere. In the case of limewater, it is partially destroyed by atmospheric carbon dioxide (producing insoluble CaCO3 from the dissolved calcium and hydroxide). In the case of calcium carbonate, however, the solubility is actually increased by mixing with carbon dioxide. The reason that the solubility is increased is that the carbon dioxide enters the water, becomes carbonic acid (equation 1), and largely combines with carbonate ions to form two bicarbonate ions (equation 2):

1. CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3–
2. H+ + CO32- ⇔ 2HCO3–

The net effect is that the concentration of carbonate ions declines: since the solubility of calcium carbonate is governed by the multiplication product of the calcium and carbonate concentrations (equation 3), more calcium carbonate can dissolve to regain saturation.

1. KSP = [Ca2+][CO32-]

Knowing the KSP and some other constants, it is a textbook calculation to determine how much calcium carbonate can dissolve in pure water in the absence of atmospheric carbon dioxide. Pankow (Aquatic Chemistry Concepts; 1991) carries out this calculation for calcite (a slightly less soluble form of calcium carbonate than aragonite).

For those really interested in the chemical details, this calculation is actually much more complicated than it would first appear (i.e., more complicated than for a simple salt like NaCl). You cannot simply solve equation 3 for [Ca2+] and [CO32+]. You need to take into account the fact that some of the carbonate that comes from dissolution will be converted into bicarbonate (HCO3–) and even carbonic acid (H2CO3). This conversion permits more CaCO3 to dissolve before the carbonate concentration rises too high to dissolve any more. One also needs to take into account the fact that calcium can exist as CaOH+, which effectively lowers the calcium concentration (though not very extensively at pH values below 11).

From this calculation, we find that the solution at equilibrium contains about 6 ppm calcium and 0.3 meq/L alkalinity, and results in a pH of 10.0. If we correct this result for aragonite instead of calcite (which is slightly more soluble), we get about 10 ppm calcium and 0.5 meq/L alkalinity, with a pH of just over 10 (which is what about what I got when I initially dissolved both AragaMIGHT and Southdown aragonite sand in RO/DI water). For comparison, full strength limewater contains about 820 ppm calcium and 41 meq/L alkalinity.

The calculation is even more involved when atmospheric carbon dioxide is allowed to enter the system. Thankfully, Pankow has again done the calculations for us. In equilibrium with normal atmospheric carbon dioxide, the solubility is increased by about a factor of 3, with the alkalinity about 1 meq/L and the calcium about 20 ppm. In this case, the pH drops to about 8.3 as the carbon dioxide enters the system. Confirming Pankow’s calculation, this result is about what I got when I let both AragaMIGHT and Southdown aragonite sand sit in RO/DI water for a few days). Still, these calcium and alkalinity values are about 40X lower than for saturated limewater, so are likely not enough to satisfy the needs of most reef tanks.

At one point I had the bright idea of adding aragonite to seltzer (soda water) bought at the grocery store to really boost the solubility and maybe have a nice, liquid additive. Seltzer has far more carbon dioxide in it than water in contact with normal air (which is why it goes flat when open), and that extra carbon dioxide will cause a great deal more calcium carbonate to dissolve (at 3.5 atmospheres CO2, the solution would contain more than 10 meq/L alkalinity and would be similar to limewater in potency, but much lower in pH). If only I had been able to mix them!! Instead, it ended up a science experiment for the kids, with the added aragonite sand providing a perfect surface for the carbon dioxide to turn into the gas phase and erupt from the bottle as a fountain of water, gas, and sand!

 

[k2-]

So reading through it - it sounds like our key drawback is amount of water required to obtain the level of replenishment one might expect in their tanks - specially when compared to lime water - thanks makes sense @Randy

 

[Randy Holmes-Farley]

So reading through it - it sounds like our key drawback is amount of water required to obtain the level of replenishment one might expect in their tanks - specially when compared to lime water - thanks makes sense @Randy

You're welcome.

Happy Reefing.