Reef Chemistry Question of the Day #157 Limewater/kalkwasser

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#157[/HASHTAG]

Limewater (aka kalkwasser) that is saturated (that is, it has dissolved all of the calcium hydroxide into fresh water that it can) at 25 deg C contains about 41 meq/L (114 dKH) of alkalinity and 808 ppm calcium.

That calcium level isn't even twice the level in seawater, but the alkalinity is more than 16 times as high as seawater (420 ppm calcium and about 7 dKH alkalinity).

So how can it be a suitable way to add both calcium and alkalinity to a reef aquarium without creating an imbalance?

Good luck!



































.

 

[JimWelsh]

The 41 meq/L of alkalinity equals about 20.5 mM of CO3--. The 808 ppm of calcium equals 808 / 40.078 = about 20.2 mM of Ca++. Thus, there is an equimolar concentration of calcium ions and carbonate ions in the limewater, which are consumed in equal parts to form CaCO3.

 

[TD13]

^^^^^^^^ What he said.

 

[Josh King]

Reef Chemistry Question of the Day [HASHTAG]#157[/HASHTAG]

Limewater (aka kalkwasser) that is saturated (that is, it has dissolved all of the calcium hydroxide into fresh water that it can) at 25 deg C contains about 41 meq/L (114 dKH) of alkalinity and 808 ppm calcium.

That calcium level isn't even twice the level in seawater, but the alkalinity is more than 16 times as high as seawater (420 ppm calcium and about 7 dKH alkalinity).

So how can it be a suitable way to add both calcium and alkalinity to a reef aquarium without creating an imbalance?

Good luck!



































.

My simple answer, which may be incorrect; is that Alk is depleted faster than Calcium. Unless there are many stony corals, which then consume Calcium at a very fast rate. Please feel free to correct me if I am wrong. I am still new to Kalk, but I am currently using 2 little fishes Kalk in a sealed ATO container. I use it on a timer with a Tom's pump dosing early morning when pH is lowest.

 

[beaslbob]

it isn't. Which is why you have to add it slowly so carbon dioxide brings it back into balance.

 

[Randy Holmes-Farley]

And the answer is...

The amount being added needs to replace what is being lost during coral growth, regardless of how large the reservoir of each may be in seawater. It just turns out that seawater has a large amount of calcium and a small amount of alkalinity, so the seawater ratio does not match that of additives needed to replace what is lost.

Calcium carbonate (that corals deposit) and limewater (kalkwasser) both contain the same relatively fixed ratio of calcium to alkalinity. About 1 meq/L (2.8 dKH) for each 20 ppm of calcium (there are minor adjustments to this for magneisum, but let's ignore that). So limewater is adding the same ratio that corals use (roughly)

But the composition of seawater is not related to calcium carbonate at all, and has a relatively much larger amount of calcium. At 420 ppm calcium, for seawater to have the same ratio as calcium carbonate or limewater, the alkalinity would have to be 21 meq/L (59 dKH).

Since seawater obviously has a lot lower alkalinity (7 dKH), all balanced calcium and alkalinity additives (whether limewater, a two part, or dissolving CaCO3 in a reactor) seem too add much more alkalinity than calcium, relative to what is naturally present in seawater.