When CO2 disolves in sea water it does some interesting chemistry,
the same chemistry that goes on in our cell plasma,
and in our blood.
The CO2 reacts with water, which is what it did for photosynthesis,
but that was a totally different reaction. This is just a an a-biological chemical
reaction, where you join those two
molecules together and make a carbonic acid, H2CO3.
If you figure out the oxidation state of the
carbon here, it's the same as the oxidation state was here.
Because adding an H20, you know you get plus, you're minus two
for this and plus two for that, and so it balances,
It's the same,
we're not changing the oxidation state.
It doesn't take energy to do this reaction, it just happens all by itself.
Carbonic acid is called an acid, a chemical is
called an acid, if it can release H+ to the water.
A water solution that has
a high concentration of H+, of protons,
is what they call an acidic solution.
The reason why we mention it
at all is because H+ is a very aggressive chemical.
if you spill acid on your skin you'll burn yourself, because the
H+ can react with the the molecules in your skin.
It can dissolve the threads in your pants.
Carbonic acid
is what they call a diprotic acid,
because it can release first one and then a second H+.
This is bicarbonate, like baking soda is sodium bicarbonate.
And this is carbonate ion. Has a charge of minus two.
How this chemistry works is kind of complicated and it can be best understood
by writing the chemical reactions in a way that don't just vaguely
sluff off these H+'s, because there aren't that many H+'s in solution.
It's not like some infinite slush fund that you can
just throw stuff into as much as you want to.
Actually the best approximation is to write
these reactions where it's not changing the
H+ at all, because there's so much more carbon than there is H+.
Writing the reaction in that way,
we have CO2 plus this carbonate ion ( CO3(2-) ) and
a water on one side and on the other side, we have two of these bicarbonates ( HCO3(-) ).
This is an equilibrium chemical reaction, to which we can
apply an idea known as the Le Chatelier's principle.
Le Chatelier's principle says that if a system
is in equilibrium, and then you perturb it by adding some chemical...
on one side of the reaction, it will push the reaction in
the other direction to try to minimize the imbalance that you've created.
If you add CO2 to this, what happens is it consumes this carbonate ion,
and shifts more toward the bicarbonate.
It's as though the carbonate ion here is sharing one of its two minus charges
with the CO2. The importance of that is that these
guys that have electrical charges, They're called ions.
An ion can't evaporate into the air, there are no ions in the air.
because they're just not stable.
You can find them in water because the water
allows it to form, but not in the air.
By lending one of its negative charges to the CO2 to make a bicarbonate
out of it, the carbonate is essentially hiding that CO2 from the atmosphere.
The existence of this chemical reaction allows
the water to take up more CO2 than it would if
there were no such chemistry.
It turns out there's ten times more of this carbonate ion
than there is dissolved CO2 in water.
That means that the buffer strength, the ability of the water to take
up CO2, is ten times stronger than it would be if this chemistry didn't exist.
Oxygen, for example, or nitrogen are gases that can dissolve in water.
But they don't do any of this wondrous pH chemistry.
And so they don't have this buffer.
They don't have much ability to take up those gases as CO2.
The depletion of the carbonate ion by this
buffer chemistry is what's known as ocean acidification.
When you acidify the water you have
less of this carbonate ion, and this is
the part of the acidification that has the strongest impact
on the biota of the ocean,
especially life forms that make calcium carbonate shells like corals,
or there are little plankton that have shells of calcium carbonate.
Looking at this chemical reaction as an equilibrium again
you could imagine that if some process takes out carbonate ion,
this calcium carbonate is going to dissolve, to replenish that carbonate ion.
It's paradoxical.
You look a coral reef, and it's a big chunk of stuff made out of carbon,
And you would think that putting more carbon
into the system would make that chunk grow bigger.
But actually because of this PH chemistry,
it makes the makes corals dissolve.