Reef Chemistry Question of the Day #200 Calcification and The Big Three Parameters

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #200

Natural seawater has roughly the following parameters at 35 ppt:

420 mg/L calcium
7 dKH (2.5 meq/L) alkalinity
1280 mg/L magnesium.

Let's imagine a typical coral reef aquarium with a mixture of different types of hard corals and coralline algae. Which of the following sets of elevated parameters is most likely to be dropped back to exactly the natural levels stated above thrugh calcification by these organisms?

A.
500 mg/L calcium
19.4 dKH
1286 mg/L magnesium

B.
500 mg/L calcium
11.4 dKH
1301 mg/L magnesium

C.
580 mg/L calcium
19.4 dKH
1307 mg/L magnesium

D.
470 mg/L calcium
20 dKH
1320 mg/L magnesium

 

[Tommy's Reef]

Is it B?

 

[scardall]

Yes I do agree "B" because ALK and magnesium are in the acceptable range.

 

[Fishfinder]

A

Cal and alk consumption is roughly 20ppm to 2.5dkh. Mag consumption is minimal.

 

[Martin Kuhn]

i also guess "A"
consumption Ca 500-420 = 80 mg/l
consuption ALk 19,4 - 7 = 12,4 dKH
-> Ratio consumed Ca/Alk = 6,45 (typical)

consumption Mg 1286-1280 = 6 mg/l ... typically low

 

[JimWelsh]

A.

 

[Salty1962]

A

 

[Alfrareef]

A

 

[CNDReef]

A

 

[sghera64]

Case A:
(19.4 dKH - 7 dKH) * (20 ppmCa / 2.8 dKH) = 88.6 ppmCa -----> [Ref. 1]

Subtract 88.6 ppmCa from the initial case A condition of 500 ppmCa = 411 ppmCa [Calcium level lower than NSW]


Case B:
(11.4 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) = 31.4 ppm Ca

Subtract 31.4 ppmCa from the initial Case B condition of 500 ppmCa = 468 ppm Ca [Calcium level higher than NSW]


Case C:
(19.4 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) =88.6 ppm Ca

Subtract 88.6 ppmCa from the initial Case C condition of 580 ppmCa = 491 ppm Ca [Calcium level higher than NSW]

Case D:
(20 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) =92.9 ppm Ca

Subtract 92.9 ppmCa from the initial Case C condition of 470 ppmCa = 377 ppm Ca [Calcium level lower than NSW]


None of these cases will return to exact NSW levels if Ca and CO2-- are consumed at the ratio of 20 ppmCa : 2.8 dKH. Additionally, some of the initial conditions would undergo abiotic precipitation at reef pH's.

Ref. 1: http://reefkeeping.com/issues/2006-06/rhf/

 

[Randy Holmes-Farley]

None of these cases will return to exact NSW levels if Ca and CO2-- are consumed at the ratio of 20 ppmCa : 2.8 dKH. Additionally, some of the initial conditions would undergo abiotic precipitation at reef pH's.

Ref. 1: http://reefkeeping.com/issues/2006-06/rhf/

Hint:

Don't forget that magnesium gets incorporated into calcium carbonate in place of calcium, and that abiotic precipitation has similar alk to calcium ratios.

 

[sghera64]

Hint:

Don't forget that magnesium gets incorporated into calcium carbonate in place of calcium, and that abiotic precipitation has similar alk to calcium ratios.

Assuming Mg+2 substitutes for Ca+2 at the same ratio, then a simple substitution in Case A of the excess of Mg above NSW (6 ppm)**- would result in Ca levels in case A to be, not 411, but 417 ppm. Assuming other elements like Sr+2 might also jump into the matrix, that might get the system even closer to 420 ppm Ca.

[*- I'm still looking for a reference to the natural ratio of Mg uptake relative to Ca in stony corals.]

 

[Randy Holmes-Farley]

Assuming Mg+2 substitutes for Ca+2 at the same ratio, then a simple substitution in Case A of the excess of Mg above NSW (6 ppm)**- would result in Ca levels in case A to be, not 411, but 417 ppm.
]

It doesn't, since magnesium weighs much less than calcium. It replaces 1:1 on an ion (mole) basis, not based on weight.

I show ratio data for many corals and coralline algae here. It ranges from almost none to at least about 4.4% of the calcium carbonate by weight.

Aquarium Chemistry: Magnesium In Reef Aquaria ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/oct2003/chem.htm

FWIW, a tank with higher than natural levels of magnesium may incorporate a higher percentage. That's certainly true of strontium, which seems to get incorporated largely by accident in place of calcium.

 

[sghera64]

It doesn't, since magnesium weighs much less than calcium. It replaces 1:1 on an ion (mole) basis, not based on weight.

I show ratio data for many corals and coralline algae here. It ranges from almost none to at least about 4.4% of the calcium carbonate by weight.

Aquarium Chemistry: Magnesium In Reef Aquaria ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/oct2003/chem.htm

FWIW, a tank with higher than natural levels of magnesium may incorporate a higher percentage. That's certainly true of strontium, which seems to get incorporated largely by accident in place of calcium.

Ah yes. In my rush to make pancakes for the family and do chemistry in my head I forgot that "ppm" is on a mass basis not moles. Well, at least I did not burn the pancakes (got something right).

Ca is about 1.66 times heavier than Mg. So, on a rough-rough basis, those 6 ppm of Mg being incorporated in my scenario A above would equate to replacing about 10 ppm Ca. Now, my 411 ppm Ca should be adjusted to 421 ppm Ca; and that is pretty close to NSW.

 

[iemsparticus]

I would have thought A, but it sounds like that's incorrect.

Following.

 

[sghera64]

I would have thought A, but it sounds like that's incorrect.

Following.

Oh, I think you got it right. After Randy helped my fine-tune my chemistry a bit, Case A falls almost exactly on NSW.

 

[Randy Holmes-Farley]

Any more takers before I give the answer?

 

[coral-boss]

D?

 

[Keiffer the reefer]

Case A:
(19.4 dKH - 7 dKH) * (20 ppmCa / 2.8 dKH) = 88.6 ppmCa -----> [Ref. 1]

Subtract 88.6 ppmCa from the initial case A condition of 500 ppmCa = 411 ppmCa [Calcium level lower than NSW]


Case B:
(11.4 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) = 31.4 ppm Ca

Subtract 31.4 ppmCa from the initial Case B condition of 500 ppmCa = 468 ppm Ca [Calcium level higher than NSW]


Case C:
(19.4 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) =88.6 ppm Ca

Subtract 88.6 ppmCa from the initial Case C condition of 580 ppmCa = 491 ppm Ca [Calcium level higher than NSW]

Case D:
(20 kKH - 7 dKH) * (20 ppmCa / 2.8 dKH) =92.9 ppm Ca

Subtract 92.9 ppmCa from the initial Case C condition of 470 ppmCa = 377 ppm Ca [Calcium level lower than NSW]


None of these cases will return to exact NSW levels if Ca and CO2-- are consumed at the ratio of 20 ppmCa : 2.8 dKH. Additionally, some of the initial conditions would undergo abiotic precipitation at reef pH's.

Ref. 1: http://reefkeeping.com/issues/2006-06/rhf/

Based on your math, I'll say A since strontium and maybe some other minerals will displaces some of the Ca in the deposits.

 

[drawman]

I'm going B all the way.