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Old 01/18/2008, 04:54 AM   #26
Randy Holmes-Farley
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The exact ratio of carbonate to bicarbonate is determined instantly by pH. If the pH is not too high, there is not an excessive amount of carbonate relative to bicarbonate.

here's the ratio:



Figure 1. Relative fraction of carbon dioxide and carbonic acid (black), bicarbonate (white) and carbonate (red) in seawater as a function of pH.


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Old 01/18/2008, 04:55 AM   #27
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New sand and high pH seem to promote solidification of the sand.

There are two different theories about how sand beds solidify, and I am not sure which is correct (perhaps both). Such solidification most often happens in new aquaria.

The first possible explanation is simply that calcium carbonate is precipitating onto the sand grains, cementing them together. That would be promoted by high calcium, alkalinity, and especially pH (the higher it is, the more likely it is, but many sands are fine at pH values up to pH 8.5), and low magnesium. It would also be promoted by a lack of organisms stirring the sand, and a lack of a good coating of organics, phosphate, magnesium, and bacteria on the sand (for example, on new sand).

The second theory is that bacteria growing on the sand cement it together with organic or inorganic materials. Depending on what exactly was being deposited, that would be promoted by a lack of organisms stirring the sand, as well as the same factors above.


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Old 01/18/2008, 06:07 AM   #28
madadi
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well since i don't have many SPS corals yet, just frags, maybe its best i turn off my calcium reactor and just run kalk for a while till the corals get bigger and the calcium demand goes up. it is only a 90gal and sometimes i feel like a calcium reactor is not the best thing to use in a smaller tank. perhaps your two part calcium and alkalinity additive is more ideal.

this is the part of the article that grabbed my attention and got me worried that the source of my sand clumping problem was too fast of a kalk drip which was my attempt to counterbalance the low pH caused by the calcium reactor. that in a way makes no sense now because the pH drop caused by the calcium reactor should introduce plenty of free CO2 in the tank to react with the lime water and not produce calcium carbonate as the end product. im sure you can sort it out and make me understand what im reading and if its in fact true.

When Calcium Hydroxide solution (Kalkwasser) is slowly dripped into your aquarium, it captures free Carbon Dioxide present in the tank water and converts it to Bicarbonate ions (which is a good thing), like this:

Ca++ + 2(OH-) + 2(CO2) <==> Ca++ + 2(HCO3-)

If you drip too fast or if there is not enough Carbon Dioxide available in the water, your shiny new Bicarbonate ions will be converted to Carbonate ions (a bad thing), like this:

Ca++ + 2(OH-) + 2(HCO3-) <==> Ca++ + 2(CO3--) + 2 H2O

The Carbonate ions formed will make the Ca++ you are trying to add to your tank get wasted by the useless precipitation of Calcium Carbonate -- the white stuff you are seeing.

So, too rapid addition of Kalk may actually cause the Calcium and Alkalinity in your tank to go DOWN instead of UP (a bad thing), like this:

Ca++ + 2(HCO3-) + Ca++ + 2(OH-) <==> 2 CaCO3 + 2 H2O

In the above reaction, a Calcium ion and two Bicarbonate ions from the aquarium combine to form solid calcium carbonate -- the white stuff you are getting in your tank, which is really just a kind of sand.

This can happen even with a slow drip of Kalk if there is not enough CO2 in your water -- something you can't easily control.

To avoid this, try mixing and adding your Kalkwasser like this: pour 15ml of 5% Acetic Acid (or ordinary Distilled White Vinegar from the grocery store -- same thing) into a 1 liter (1 quart) container. Dissolve 1/2 teaspoon of lab-grade Ca(OH)2 (or commercial Kalkwasser mix) in the Acetic Acid, and then dilute to 1 liter (1 quart) volume with either RO/DI water, or even tank water.

15 ml is more Vinegar than some people are comfortable with, but I use it constantly with no problems. There should be no sediment in the mixture, or just a little bit at most. You can let the sediment settle out if you don't like the white flakes in your tank. I just drip the liquid and the sediment both into my tank.

Dissolving the Kalk powder in the Vinegar first will accomplish several very good things.

First, it will get more Calcium ions (Ca++) into the solution because you are dissolving the Ca(OH)2 in an acid instead of water, and forming Calcium Acetate, which exists as a dissociated equilibrium of free Calcium ions and Acetate ions.

Second, the Acetic Acid (Vinegar) provides an equivalent of all the CO2 you need to avoid precipitating the newly-added Calcium ions as useless white Calcium Carbonate powder.


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Old 01/18/2008, 06:33 AM   #29
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This can happen even with a slow drip of Kalk if there is not enough CO2 in your water -- something you can't easily control.

That sentence might scare folks, and it is misleading. Forget that whole section as it is totally unimportant. If the pH is not high, there is never a shortage of bicarbonate and CO2. If the pH is high for any reason, then there is a concern about precipitating calcium carbonate.

Dosing limewater too fast can cause the pH to be too high, especially locally in a sump or such. In a sand bed, the ph will not be above the tank, so the pH is not too high in it if the pH in the tank is 8.2.

That said, the pH may not need to get especially high for sand to harden, if the other contributing conditions are met (like new sand).


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Old 01/18/2008, 01:18 PM   #30
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one more and i promise i wont bug you, i appreciate your advice very much.
since you said pH can be higher locally if dosed too fast, such as in the sump.
in my situation, there isn't much flow in the area where the kalk water enters the sump. even though i don't dose it fast, i always get a thick calcium deposit on the float valve till it is useless and i have to constantly dip it in vinegar. do you think the fact that i add it to an area of the sump that has low flow, it can result in more calcium carbonate then if i introduced it in an area of high flow? i believe its ideal for it to be introduced in an area of high flow, but my valve wont let me do that unless i redo that whole area of the sump. also, should i turn off my calcium reactor and only dose kalk till my tanks calcium demands rise. i only have frags now. my sand is starting to clump again and i just replaced it 3 months ago! since its new sand i wish there was something more i can do then just get 100 nassarius snails. but it looks like i might just have to do that.


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Old 01/18/2008, 01:52 PM   #31
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i always get a thick calcium deposit on the float valve till it is useless and i have to constantly dip it in vinegar. do you think the fact that i add it to an area of the sump that has low flow, it can result in more calcium carbonate then if i introduced it in an area of high flow?

Absolutely.

FWIW, the low pH may be driven by your home CO2 as much or more than the CaCO3/CO2 reactor. So shutting it off may not solve the problem.

I'd look to adding the limewater more slowly somehow.


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Old 01/18/2008, 02:37 PM   #32
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its hard to drip kalk slower when im barely keeping up with evaporation so ill try this.
turn off CO2 to the calcium reactor. im going to put my kalk reactor back on, stir it more often to get the most kalk out of it. ill only drip 200mL per hour trough the kalk reactor to start and see how that will change the pH. as for the rest of the evaporation, i will just run a float switch to the same RO tank the kalk reactor is drawing the water from to maintain salinity. i guess its all trial and error for a while as i see how the alk and calcium are going to change as a result of the new set up. thanks again for your help it was a pleasure and i definitely learned a bunch about CO2 and kalk.


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Old 01/18/2008, 05:14 PM   #33
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You're welcome.

Good luck.


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