Reef Chemistry Question of the Day #125 Coral Skeleton Dissolution

Randy Holmes-Farley

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Reef Chemistry Question of the Day [HASHTAG]#125[/HASHTAG]

Under which of the following conditions is an exposed coral skeleton most likely to begin to dissolve (all other conditions equal natural seawater)?

A. Calcium 210 ppm, alkalinity 2.5 meq/L (7.0 dKH), pH 8.2
B. Calcium 420 ppm, alkalinity 1.0 meq/L (4.2 dKH), pH 8.2
C. Calcium 210 ppm, alkalinity 1.5 meq/L (4.2 dKH), pH 8.5
D. Calcium 300 ppm, alkalinity 2.0 meq/L (5.6 dKH), pH 8.2
E. Calcium 500 ppm, alkalinity 3.0 meq/L (8.4 dKH), pH 8.2
F. Calcium 420 ppm, alkalinity 2.0 meq/L (5.6 dKH), pH 7.6

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AquamanE

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considered F cause of Ph, but it doesnt seem as out of whack as my guess:

C- Ca, and Alk way too low. Those numbers seem crazy low.
 
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Randy Holmes-Farley

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And the answer is...F. Calcium 420 ppm, alkalinity 2.0 meq/L (5.6 dKH), pH 7.6

The potential for dissolving calcium carbonate is determined by the solubility product constant:

Ksp = [Ca++] x [CO3--]

which means the concentration of calcium times the concentration of carbonate.

If that value falls below a certain number (which we know but do not need for this question), calcium carbonate will dissolve.

So if calcium drops by a factor of two (500 to 250 ppm, for example) the Ksp drops by a factor of 2.

Since carbonate is directly related to the alkalinity, dropping the alkalinity by a factor of 2 drops carbonate and hence drops the Ksp by a factor of 2.

pH is more complicated, but as pH drops in seawater, there is less carbonate and more bicarbonate. In the range we are talking about here, a drop of 0.3 pH units is about a factor of 2 drop in carbonate, even with alkalinity unchanged. So a pH drop from 8.2 to 7.9 means a drop of a factor of 2 in Ksp.

To answer the question, we determine the drop in Ksp for each scenario relative to normal seawater.

We'll do the just for F, with the others being similar calculations but smaller effects.

Calcium is the same in F as NSW, so that isn't a factor.
Alkalinity is slightly lower, 5.6 dKH/7 dKH is a drop of a factor of 0.8 in the Ksp.
The pH in F is the big driver. At pH 7.6 there is a 0.6 pH unit difference between F and NSW at pH 8.2. That corresponds to a factor of 4 in carbonate concentration.

So the Ksp in F is 0.8 x 0.25 = 0.2 times as much as in NSW.

All of the other values are higher and closer to NSW, and hence calcium carbonate is less likely to dissolve in all of the other answers. :)

FWIW, B is the second lowest.
 

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