I have a query.....Do you mean oxygen as a molecule (O2) or as an  ion (O--)  in the atmosphere which combines with H+ ions of water to make oceans alkaline. I think this is in context of a green house gas or global warming. So increase in CO2 would create carbonic acid making the pH fall below 8. The oceans are alkaline because of atmospheric CO2 along with dissolved ammonia and dissolved carbonate rocks. CO2 dissolved in water may cause fall in basicity.

Dear Alistair,

no, the answer why the ocean has a pH higher than 7 has nothing to do with oxygen; and your idea would not work out since producing OH- from H+ and O2 would violate charge conservation. But charge conservation can lead you to the right answer: Seawater contains a number of dissolved ions since it is salty, but being electroneutral it contains as many positive as negative charges. So the sum of all positive charges must equal the sum of all negative charges, or

[Na+] + 2 [Mg2+] + ... = [Cl-] + [HCO3-] + ...

where the brackets stand for concentrations.

Most of these ions, like Mg2+ or Cl- do not change their concentration when you change pH. Others, like HCO3- or CO2 do, e.g. because they combine with H+. If you put all those that don't change their concentration on one side of the equation, and all the others on the other side, you arrive at

[Na+] + 2 [Mg2+] - [Cl-] + ... = [HCO3-] + 2 [CO32-] + [B(OH)4-] + ...

The quantity on the right hand side is called alkalinity (I am simplifying a bit here, because alkalinity is defined 'relative to a specific pH') and, by its derivation, should not change when you change pH. The alkalinity of seawater is on average about 2400 micromol/kg soln., because seawater is so salty.

Now, for a given concentration of CO2 (e.g. from equilibrating the ocean with an atmosphere), and a given alkalinity, you can calculate pH. This is most easily seen when you approximate alkalinity with its two largest contributors Alk ~ [HCO3-] + 2 [CO32-].

From the equilibrium conditions of the carbonate system you know that

[HCO3-] = K1 [CO2] / [H+]

[CO32-] = K2 [HCO3-] / [H+] = K1 K2 [CO2] / [H+]2

where K1 and K2 are te first and second acidity constant of the carbonate system, which are functions of temperature and salinity.

Inserting this in the expression for Alk above, and setting [CO2] to a constant value gives you a quadratic equation for [H+] alone. It becomes a bit more complicated if you include the other terms in alkalinity as well, but the general idea remains, that with Alk and [CO2] given, the pH follows.

So, the short answer to your question is, I would say (and maybe this is too simple): Because the ocean is salty.

Does that help?

Best regards, Christoph

Hi Viniti and Christoph,

Thanks for your replies but you have not really answered my question. You have told me why adding CO2 to the the ocean makes it less alkaline. But I want to know why it is alkaline before you add the CO2? I am not asking what alkalinity is. I am asking why the oceans are alkaline even if they have no dissolved CO2.

The cause I am suggesting is that some of the O2 molecules disassociate when oxygen is dissolved in water. e.g.

O2 -> 2O2-

then

O2- + H+ -> OH-

So adding oxygen increases the OH- ions without changing the H+ ions. There are plenty of explanations of alkalinity in the oceans but what I have not seen is why are the oceans alkaline. Is there an accepted answer?

Dear Alistair,

I think I see where you are coming from; in my words you want to know why the alkalinity of the ocean is so high at first place. I have to think about that, and my expectation would be that it has the same reason as why the ocean is salty, namely that the rivers carry the products of chemical rock weathering into the ocean, and that the ions get enriched in the oceans, because the salts have a longer residence time in the ocean than the water, which continuously evaporates and leaves the dissolved salts behind. But I have to think this through a bit more, since weathering also produces bicarbonate, which can be transformed into CO2 and exchange with the atmosphere.

But anyway, I still don't understand your idea from your last message: Where does the negative charge in your first reaction come from?

Best regards, Christoph

Hi Christoph,

Yes, what i want to know is why the oceans are so alkaline in the first place.

But if I am correct, then before the atmosphere became oxygenated, then the ocean would have been acid in the first place from the high CO2 levels which existed during the Hadean and Archean eons :-)

Another possibility is that chlorine from volcanoes  is responsible.

Or water vapour in the upper atmosphere has been ionised and the H+ ions escaped to space leaving the OH- ions to return to the ocean. Four billion years of that may have been the cause.

I will be interested in what you find out.

I am with Christoph. Alkalinity in oceans stems from dissolution of basic minerals. Na, K, Ca, Mg etc. form minerals that dissolve as basic reacting species. With time these basic species have reacted with carbonic acid to produce hydrogen carbonate which is the main contributor to alkalinity in the oceans. 

Henrik,

See my reply to Christof. You are both answering the wrong question. Your answers tmake me think that no one has thought about this question before. It is not what causes alkalinity? It is what causes the oceans to be alkaline? Adding CO2 to the oceans makes them more acid, because "hydrogen carbonate" is an acid. If that were the only player then the oceans should be acidic not alkaline.

Dear Alistair,

The decomposition of aquatic flora and fauna...and release of ammonia is also responsible for alkalinity.

Also degradation of rocks...and their conversion to carbonates and bicarbonates of calcium and magnesium...might also be the cause...

Just speculating on the second thought..☺

Alastair, I believe I have read your question and thought about the source of alkalinity before you posed the question.

Your proposed reaction in the answer 5 days ago is not possible because there are 4 electrons to much on the product side. Where do you propose the electrons come from then you make the "dissociated" oxygen: O2 -> 2O2- ?

Viniti and Henrik,

Sorry to be so late in thanking you for your answers. I have been studying the ocean carbonate system and only now do I feel that I am getting a grip on it.

My idea that oxygen involved was silly, as you pointed out Henrik.

I have discovered there are things called soda lakes which are even more alkaline than the oceans. Their high pHs are due to their high levels of sodium carbonate, washing soda, and other bases. So that confirms what you wrote Henrik, except that it seems that the main source of sodium and calcium to the oceans may be from black and white smokers in the oceans rather than terrestrial volcanoes.