Coral pH Limits?

[J1a]

Good to know. Forgive me for asking so many likely ignorant questions here (most chemistry info I find in my personal research is very technical jargon heavy and - as I don’t have a chemistry background - rather difficult to understand as a result), but does raising Alkalinity always raise pH? I ask because, to this point, I’ve just heard that pH is essentially the ”important” number while Alkalinity is basically just important because it determines how capable something is of resisting pH change (specifically pH lowering). I know that’s a gross-oversimplification of things, but that’s basically how I’ve seen it explained.

Also, to be able to understand the actual relationship between pH and Alkalinity better, would you happen to either be willing to explain it simply here or to point me in the direction of a source or two that might explain things simply (preferably in layman’s terms or nearly so) but in some detail?

https://www.nature.com/scitable/knowledge/library/ocean-acidification-25822734/
This is a good reading for bicarbonate buffer system.

 

[ISpeakForTheSeas]

Thank you both for your help! This has been rather enlightening to me. I do have two last questions for now out of curiosity, though: given the inaccuracy of the Smithsonian’s description, what actually causes the coral skeletons to dissolve at low pH (i.e. by what mechanism is the dissolution brought about), and what does the H+ actually do?

 

[Randy Holmes-Farley Verified Community Expert]

Thank you both for your help! This has been rather enlightening to me. I do have two last questions for now out of curiosity, though: given the inaccuracy of the Smithsonian’s description, what actually causes the coral skeletons to dissolve at low pH (i.e. by what mechanism is the dissolution brought about), and what does the H+ actually do?

The increased H+ reduces the carbonate concentration in the water.

OK, here's a more realistic primer on solubility of anything.

Let's take a moderately soluble and simple material such as sodium chloride.

If you put a solid chunk of that salt into any water solution, sodium and chloride ions individually pop off off the surface and into the water. The rate of this process is largely independent of the concentration of sodium and chloride in the water. It always happens, and happens amazingly fast on a human scale. Likely millions per salt chunk per second.

If there is any sodium and chloride in the water, there is a simultaneous process of sodium and chloride ions each popping onto the surface from the water. The rate that this happens is directly proportional to the amount of sodium and chloride ions in the water.

There is some point in sodium and chloride concentration where these two processes exactly cancel each other out, and there is no net dissolution or precipitation, even though both are happening at a huge rate on a microscopic scale. The point is called the solubility limit, and no more of the solid chunk will disappear into the water, even if individual ions are flying back and forth.

Skipping now to calcium carbonate, all of the same processes exist, plus some others. The most important is that the concentration of carbonate changes with pH. Once below pH 8 in seawater, each drop of 1 pH unit drops the concentration of carbonate in the water by a factor of about 10 by converting more and more of it to bicarbonate:

CO3-- + H+ --> HCO3-

Bicarbonate plays no direct role in the solubility of calcium carbonate, so at lower pH, the carbonate concentration in the water declines, the rate at which carbonate pops back onto the solid surface declines, and thus the net solubility increases because the rate that calcium and carbonate pop off the surface was unchanged.

I hope that makes sense. :)

 

[ISpeakForTheSeas]

The increased H+ reduces the carbonate concentration in the water.

OK, here's a more realistic primer on solubility of anything.

Let's take a moderately soluble and simple material such as sodium chloride.

If you put a solid chunk of that salt into any water solution, sodium and chloride ions individually pop off off the surface and into the water. The rate of this process is largely independent of the concentration of sodium and chloride in the water. It always happens, and happens amazingly fast on a human scale. Likely millions per salt chunk per second.

If there is any sodium and chloride in the water, there is a simultaneous process of sodium and chloride ions each popping onto the surface from the water. The rate that this happens is directly proportional to the amount of sodium and chloride ions in the water.

There is some point in sodium and chloride concentration where these two processes exactly cancel each other out, and there is no net dissolution or precipitation, even though both are happening at a huge rate on a microscopic scale. The point is called the solubility limit, and no more of the solid chunk will disappear into the water, even if individual ions are flying back and forth.

Skipping now to calcium carbonate, all of the same processes exist, plus some others. The most important is that the concentration of carbonate changes with pH. Once below pH 8 in seawater, each drop of 1 pH unit drops the concentration of carbonate in the water by a factor of about 10 by converting more and more of it to bicarbonate:

CO3-- + H+ --> HCO3-

Bicarbonate plays no direct role in the solubility of calcium carbonate, so at lower pH, the carbonate concentration in the water declines, the rate at which carbonate pops back onto the solid surface declines, and thus the net solubility increases because the rate that calcium and carbonate pop off the surface was unchanged.

I hope that makes sense. :)

That actually does, thank you!