[KGpte]

Hello there!

I am trying to get my head around the chemistry of Ocean Acidification. So far, I have managed to understand this equation:

CO₂ (aq) + H₂O + CO₃^2- → 2 HCO₃^- (aq)

This shows that the more CO₂ that's added to the ocean, the more carbonate ions are 'consumed' and the amount of bicarbonate ions increases.

However, I have also come across this one (see this image: http://newsimg.bbc.co.uk/media/images/45552000/gif/_45552631_ocean_acidification02_466in.gif)

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^- ⇌ 2 H⁺ + CO₃^2-

I don't quite understand how the formation of carbonic acid is 'taking away' carbonate ions from the ocean, impeding calcification and aiding ocean acidification. If anything, it seems that this process is adding carbonate ions to the ocean!

And finally, a calcification equation:

Ca²⁺ (aq) + 2 HCO₃^- (aq) ⇌ CaCO₃ (s) + H₂O + CO₂

Doesn't this show that calcification can occur with bicarbonate ions?

Please inform me if any of the equations are actually wrong. I'd be grateful for any *delete me* Thanks in advance!

[Borek]

CO₂ (aq) + H₂O + CO₃^2- → 2 HCO₃^- (aq)

What is happening is that carbon dioxide reacts with water creating carbonic acid, which dissociates producing H⁺. Now, if the solution already contained CO₃^2- this H⁺ protonates CO₃^2- producing HCO₃^-.

I guess you are missing the fact that in one case we are talking about solution that was initially CO₃^2- rich, and concentration of CO₃^2- went down when the solution was acidified, while in the other case we are talking about solution that didn't contain initially any CO₃^2- - and it is addition of CO₂ that (through two dissociation steps) introduced CO₃^2- to the solution.