Quote:
Originally Posted by Nick Theodorakis
I'm not sure how low you can get the pH with just atmospheric CO2; the lower pKa for carbonate is about 6.4. Note that culture media for cells grown in vitro is often buffered with carbonate/bicarbonate in equilibrium with a 5% CO2 atmosphere (much much higher than earth's atmospheric level) and the pH of the media is about 7-ish.
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A solution of NaHCO
3 or Na
2CO
3 is equivalent to adding a stoichiometric amount of NaOH to a pure solution of CO
2. So I'm not surprised your carbonate/bicarbonate buffer isn't as acid as a pure solution of carbonic acid.
But as you say, the ability of CO
2 to release hydrogen ions in sea water ought to be modest. I've just googled, and the pH of the surface areas of the ocean is about 8.1, which, er, isn't very acid at all. The article I quote below says that the pH of the ocean has reduced by about 0.075 over 250 years, but is projected to reduce much more rapidly over the next 100 years, perhaps by as much as 0.3 if we continue pouring the stuff out.
It seems there is another wikipedia article on this which is fully sourced, which I didn't find first time.
http://en.wikipedia.org/wiki/Ocean_acidification This article says there is experimental evidence for reduced calcification of some important species at increased CO
2 concentration, and attributes it to the fact that less of the CO
2 is in the form of carbonate ion at higher concentrations of CO
2, because carbonate forms a stable supersaturate at lower pH. This seems to be saying, yes, this is counterintuitive, but there is a kinetic effect at work (supersaturation of carbonate ions). Although presumably there must be a lower concentration of CO
2 at which these species eventually reduce calcification, and it would be interesting to understand what causes the changes in direction and why and where.
But this article also says (sourced)
"Recent work examining a sediment core from the North Atlantic found that while the species composition of coccolithophorids has remained unchanged for the industrial period 1780 to 2004, the calcification of coccoliths has increased by up to 40% during the same time."
So I'm not sure on what basis they can come to such a firm conclusion at the bottom of the article that the net direction of calcification resulting from increased acidity is downwards. It seems that what I am asking really is at the edge of science and not easily answered on the basis of established knowledge.