Reef Chemistry Question of the Day #34 Limewater (kalkwasser)

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #34

If limewater (kalkwasser) is used over the course of a day to boost the alkalinity in a reef aquarium by 2.8 dKH (1 meq/L), the calcium level will have risen by how much?


A. 0 ppm
B. 5 ppm
C. 10 ppm
D. 20 ppm
E. 30 ppm
F. It cannot be determined because calcium and alkalinity additions are unrelated using this method

 

[Randy Holmes-Farley]

Anyone?

 

[candcreef]

I'll take a shot.....D 20ppm?

 

[ritter6788]

D. 20

 

[Chameleon]

Reef Chemistry Question of the Day #34

If limewater (kalkwasser) is used over the course of a day to boost the alkalinity in a reef aquarium by 2.8 dKH (1 meq/L), the calcium level will have risen by how much?


A. 0 ppm
B. 5 ppm
C. 10 ppm
D. 20 ppm
E. 30 ppm
F. It cannot be determined because calcium and alkalinity additions are unrelated using this method

Wowzers how high is the evap of that tank? That seems like a lot to raise with just kalwasser.

 

[Randy Holmes-Farley]

Wowzers how high is the evap of that tank? That seems like a lot to raise with just kalwasser.

That would take 2.5% of the water volume to be added in saturated limewater. That's on the high side, but not out of the range of folks experiences.

You definitely wouldn't want to add that all at once, or the pH would skyrocket.

 

[Steven@]

B. 5ppm

 

[leptang]

I chose F, based on live stock that consumes alk or Ca that takes place commonly from one tank to another in a days time. The amount may vary on the demand right?

 

[birdsnest]

D would be the correct proportion, but I'm w/leptang thinking that demand would skew that.

 

[Randy Holmes-Farley]

I chose F, based on live stock that consumes alk or Ca that takes place commonly from one tank to another in a days time. The amount may vary on the demand right?

Demand certainly varies tank to tank. The question is whether the relative demand for calcium and alkalinity is variable (or not) and what ratio this method adds (if fixed).

 

[Randy Holmes-Farley]

And the answer is...D. 20 ppm

Limewater (kalkwasser) is made by dissolving calcium hydroxide, Ca(OH)₂, into fresh water.

The ratio of calcium to hydroxide added is consequently 1 mole of calcium for 2 moles of hydroxide.

So if limewater is added such as to add 1 meq/L (milliequivalent per liter) of alkalinity (as hydroxide) it must add 1/2 milliequivalent per liter of calcium.

Calcium weighs 40 grams per mole, or 40 milliequivalents per millimole. So the calcium addition must have been 1/2 x 40 milligrams per liter, or 20 ppm.

Easier than working through that every time is just to remember that for limewater or calcium carbonate, the ratio of alkalinity to calcium in it is 1 meq/L (2.8 dKH) for every 20 ppm of calcium.

Since it is also true for calcium carbonate, it is the ratio of demand in an aquarium for the precipitation of calcium carbonate (as skeletons, or abiotic precipitation) with the slight deviation that some magnesium slips into the crystals in place of calcium, lowering that amount.

So when asked about the alkalinity to calcium demand in an aquarium, I usually state it as 18-20 ppm calcium for each 1 meq/L (2.8 dKH) of alkalinity.

There are other minor things which might impact this ratio, but they are not usually a big driver of the ratio unless the calcification demand by corals is very low. One is the accumulation of nitrate. If it is rising (not just present, but actually rising), then some alkalinity is depleted by that process.

This has more:

When Do Calcium and Alkalinity Demand Not Exactly Balance? by Randy Holmes-Farley - Reefkeeping.com


Happy Reefing.

 

[Electrobes]

I should have known this.. I need to reread the Reef Chemistry articles! Thanks for re-posting the articles.. it's nice to find up to date links and all. I have a lot of reading to do!!