Reef Chemistry Question of the Day #70 Calcium and Alkalinity

Randy Holmes-Farley

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Reef Chemistry Question of the Day #70

You have a soft coral tank that you have been keeping for about 2 years. It is doing quite well. You maintain calcium and alkalinity through water changes alone.

Right after your most recent water change the calcium is measured to be 440 ppm and the alkalinity is 9.0 dKH (3.2 meq/L).

After a few days without any water changes, the calcium has declined to 431 ppm.

Before you run your alkalinity test, you make a prediction. What should the alkalinity be?

Assume alkalinity dropped only due to calcification by coralline algae or other calcifying organisms in the tank, and that the measurements you made so far were perfectly accurate.

A. 6.9 dKH (2.5 meq/L)
B. 7.6 dKH (2.7 meq/L)
C. 8.1 dKH (2.9 meq/L)
D. 8.7 dKH (3.1 meq/L)

Good luck!

























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Cory

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I think C is the closest.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...B. 7.6 dKH (2.7 meq/L) Birdsnest is today's winner!

The general rule is that for each 1 meq/L (2.8 dKH) of alkalinity that is consumed by calcification, calcium declines by 18-20 ppm. The low end at 18 assumes that the corals or coralline algae depositing the calcium carbonate deposit a fairly high proportion of magnesium (about 4% in skeleton by weight), and the 20 end assumes none (it is the exact ratio present in pure calcium carbonate). Coralline is around 18 and corals typically fall in between 18 and 20 ppm.

So, here we dropped by 9 ppm calcium. We'll assume for the math that the ratio is 18 ppm calcium per meq/L. If we used 20, the difference is small.

That would indicate that the alkalinity should decline by about 9/18 * 1 meq/l 0.5 meq/L or 9/18 * 2.8 dKH = 1.4 dKH drop in alkalinity.

So the final alkalinity is 3.2 - 0.5 = 2.7 meq/L or 9 dKH - 1.4 dKH = 7.6 dKH.

The main point of this questions to to get people to understand there is an approximately fixed ration and the numbers do not generally move independently very much. Another point is to see how much faster alkalinity appears to move than calcium. That is because there is much more calcium in seawater, acting as a "buffer" against changes in calcium ion.

These have more:

for a description of the ratio and why it might not always hold true:
When Do Calcium and Alkalinity Demand Not Exactly Balance? by Randy Holmes-Farley - Reefkeeping.com


for amounts of magnesium in corals:
Aquarium Chemistry: Magnesium In Reef Aquaria ? Advanced Aquarist | Aquarist Magazine and Blog
 
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Randy Holmes-Farley

Randy Holmes-Farley

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I think answer is D.
9/440*431= 8.81

What is that formula naecO? I've never seen that.

That formula assumed that both declined by the same percentage. It would hold for evaporation or dilution, but not for calcification.

During calcification, alkalinity declines by a much bigger percentage in seawater than does calcium. :)
 

naecO rM

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It's very interesting.
Could you explain please if for each 2.8 dKH calcium is declined by 18-20.
I just bot cleare with Ca level 420 ppm and KH 8. It's looks as for each 1 dKH going about 50 ppm of Calcium.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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It's very interesting.
Could you explain please if for each 2.8 dKH calcium is declined by 18-20.
I just bot cleare with Ca level 420 ppm and KH 8. It's looks as for each 1 dKH going about 50 ppm of Calcium.

That 420/8 ratio may be what is present in your saltwater, but the ratio I quote is for what gets deposited in calcium carbonate skeletons.

Calcium carbonate has the formula of CaCO3. That means there is one calcium ion for each one carbonate ion.

Calcium weighs 40 grams per mole, so if one millimole of calcium carbonate dissolved in water, there would be 40 mg (1 millimole) of calcium and 1 millimole of carbonate.

Since one carbonate counts as two units of alkalinity, that carbonate provides 2 millimoles (= milliequivalents) of alkalinity.

So, we have 40 mg/L (= ppm) of calcium and 2 meq/L of alkalinity.

If we convert that alkalinity into dKH units, it is 2.8 dKH.

So we get 40 ppm calcium for each 2.8 dKH of alkalinity, or 20 ppm of calcium for each 1.4 dKH of alkalinioty.

The reason it is not exactly 20 is that some magnesium (and a tiny amount of strontium) get into the calcium carbonate in place of calcium, so the actual calcium is about 18-20, depending on how much of these others are incorporated.

Does that make sense?
 

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