[MarkPawelek (OP)]

Ocean Acidification
by The Ocean Portal Team; Reviewed by Jennifer Bennett (NOAA)

Even though the ocean is immense, enough carbon dioxide can have a major impact. In the past 200 years alone, ocean water has become 30 percent more acidic

<-- This is a massive blunder by NOAA. It shatters my confidence in "climate scientists".

I think she meant to say that there is 30% more CO2 in the ocean than pre-industrial times.

The statement "30% more acidic" is wrong for 2 reasons:
1) When CO2 (carbon dioxide) dissolves in water only 0.3% of it becomes carbonic acid according to the equation:

CO2(aq)  +  H2O(aq)  <=>  H2CO3(aq)

Most of the CO2 simply dissolves in water without making carbonic acid.

2) Carbonic acid is a weak acid with a Ka = 4.2E-04. Meaning less than 1% of that dissociates according to the formula:

H2CO3(aq) + H2O(aq) <=> H3O+(aq) + HCO3-(aq)

[chiralSPO]

Using percentages seems an odd way to discuss acidity, but I don't think that it is being misleading or alarmist. There are two possible interpretations that make sense to me:

(1) Acidity is a measure of the concentration of H⁺ ions in solution. A 30% increase in the concentration of H⁺ ions is the same as a 0.11 unit decrease in the pH. The 2nd graphic on the page you linked to suggests that the pH has fallen by about 0.04 units in the last 30 years, so a drop of about 0.1 units in the last 200 years doesn't seem that unreasonable at first glance. Given that the statement including the 30% figure is in a paragraph discussing historic pH, and a later paragraph claims a 0.1 pH unit fall since the Industrial Revolution, this seems most likely to me.

(2) The effective concentration of dissolved CO₂ has increased by 30%. Again, in the 2nd graphic it looks like the dissolved CO₂ has increased by 12.5% in the last 30 years. Again, it doesn't seem absurd for this to be in line with a 30% increase over 200 years.

Either way, the ambiguity of the claim doesn't change the alarming trend that the data indicate, and bringing attention to an alarming trend is not alarmist in my book.

I don't know the specifics of the methods used to calculate these historic pH's and cannot personally vouch for the soundness of the science, but I caution against dismissing the results without understanding them fully.

[Bored chemist]

Ocean Acidification
by The Ocean Portal Team; Reviewed by Jennifer Bennett (NOAA)

Even though the ocean is immense, enough carbon dioxide can have a major impact. In the past 200 years alone, ocean water has become 30 percent more acidic

<-- This is a massive blunder by NOAA. It shatters my confidence in "climate scientists".

I think she meant to say that there is 30% more CO2 in the ocean than pre-industrial times.

The statement "30% more acidic" is wrong for 2 reasons:
1) When CO2 (carbon dioxide) dissolves in water only 0.3% of it becomes carbonic acid according to the equation:

CO2(aq)  +  H2O(aq)  <=>  H2CO3(aq)

Most of the CO2 simply dissolves in water without making carbonic acid.

2) Carbonic acid is a weak acid with a Ka = 4.2E-04. Meaning less than 1% of that dissociates according to the formula:

H2CO3(aq) + H2O(aq) <=> H3O+(aq) + HCO3-(aq)

I'd need to check, but those equilibria are linear functions.
If you put 30% more CO2 in the water then, as you say,  only 0.3% actually reacts to form carbonic acid.
But only 0.3% of the stuff that was there before was in the form of carbonic acid.
So the increase in carbonic acid is 30%

Similarly, if there's an equilibrium where only a small fraction actually dissociates then that same equilibrium means that most of the carbonic acid that was present pre-industrially was dissociated.
and again, that factor affects both the "before" and the "after" so the simplistic expectation is that the acidity rises by 30%

What stops it is the buffering effect of the carbonate ions present. There simply isn't enough data presented to answer the question properly.
But that capacity is finite.
It's also not an equilibrium system- we are adding CO2 faster than the calcite deposits can react with it.

Once you have this  scenario "the shells of some animals are already dissolving in the more acidic seawater" you actually have a problem.
Trying to blame poor reporting  isn't going to solve that.

[MarkPawelek (OP)]

I did back of the envelope calculations, and found that NOAA exaggerate the acidity increase by more than 3 orders of magnitude. They exaggerate 5560 times.  Or 556000%. Now that's a big number, but NOAA like big numbers.

[MarkPawelek (OP)]

The current ocean pH is due to many ions: anions and cations. One of my critics (Bored Chemist) seems to assume ocean pH is all down to carbonic acid! I don't see how he works that out. Oceans are a soup of ions. For example: Salt is 35000ppm. Carbon is 27ppm. 1300 times more salt than carbon! I prefer my method. Calculate the number [H+]. Calculate the addition of [H+] due to more CO2. Compare the two. I blogged it.

[chiralSPO]

Ocean pH is due to only one thing [H⁺]. Yes, there are many other ions in solution, each of which might play a role in determining [H⁺], though not as many as you seem to be indicating (it doesn't matter how much salt there is: Na⁺, K⁺, Cl^–, and Br^– will have NO effect on pH in the range of plausible pH values, as the pK_a values of Cl^–, and Br^– are < 0).

There is a complex "buffer" related to concentrations of CO₂, HCO₃^–, CO₃^2–, H₂PO₄^–, HPO₄^2–, B(OH)₃, B(OH)₄^2–, Mg²⁺, Ca²⁺, etc.

As you say in your blog:

I will do "very back of the envelope" calculations here. If I make a mistake, please laugh at me. Then correct me.

I will skip the first step, and focus on the second:

Assuming the pH of oceans = 8.1, as National Geographic claim, lets do the back of envelope calculations to figure out how much pH may have fallen in the last 200 years.

The mass of oceans is usually given = 1.4E24 grams. How much acid is that?

A pH = 8.1 says that the [H3O++] = 10-8.1 = 7.9433E-09.

Multiply the two. Grams of H+ = 1E16 grams (very approximate!). A lot of hydrogen ions, but then, our oceans are very big.

How many hydrogen ions may we expect from 500Gt of carbon (dissolving as CO2)? The Atomic Mass of C = 12. 500Gt of carbon = 500 × 1000,000,000 t = 500 × 1000,000,000,000,000 g = 50E16 g of Carbon = 50/12 × 1016 moles, or 4E+16.

4E+16 × 0.3% × 0.5% = 6E+11 hydrogen ions. The ratio of hydrogen ions added is 18600:1 We increased the acidity from 18600 to 18601. Big deal! By adding all that CO2 we made the oceans more acidic by 1 extra hydrogen ion in 18600. An addition of 0.0054%, not 30% as NOAA said.

The above approach is fundamentally flawed (I have stricken the primary mistake through). As you point out, the system is buffered, by the equilibrium of H⁺ CO₂, H₂CO₃, HCO₃^–, and CO₃^2–. This complex buffering means that the linear relationship assumed in the above calculation is not valid. Instead, we need to consider the relationship between all of the species. You have also not considered how much of each of the carbon-related species are already in solution, and what their relationships are to each other and to pH.

I recommend studying up on this more before posting:

https://en.wikipedia.org/wiki/Bicarbonate_buffer_system#Henderson%E2%80%93Hasselbalch_equation
http://www.atmos.umd.edu/~russ/620_04OceanChem.pptx (downloadable lecture slides)

It is also worth emphasizing (and you have noted it, but I think you should consider the ramifications more carefully), that the system is NOT IN EQUILIBRIUM. The pH is not the same everywhere because the ocean is not mixed perfectly well. This means that at the very surface, where the ocean is in contact with the acidifying CO₂, the buffer can be temporarily over-burdened, before more bicarbonate comes up from the depths to restore equilibrium. This is bad news for marine life in shallow waters (read: everything you can see while snorkeling), which is damaged by the imbalance, even if it is temporary.

Essentially, the calcium carbonate in the marine organisms serves as the base in the equilibrium.

[chiralSPO]

Also, please note that I have changed the title of the thread to a QUESTION, to be in line with the policies of this forum.

[Bored chemist]

One of my critics (Bored Chemist) seems to assume ocean pH is all down to carbonic acid!

No.
I said it's essentially the carbonate/ bicarbonate/ carbonic acid buffer system that regulates the  pH.

There's a reason for that: it's true.
There are only vanishingly small concentrations of other ions like phosphate or borate that would act as pH buffers near the pH of sea water

https://web.stanford.edu/group/Urchin/mineral.html

I'm allowed to assume things that are actually true.

You seem not to have noticed that a solution of sodium chloride is practically neutral.

[MarkPawelek (OP)]

Thankyou for your reply chiralSPO. Consider this:

Feynman was a truly great teacher. He prided himself on being able to devise ways to explain even the most profound ideas to beginning students. Once, I said to him, "Dick, explain to me, so that I can understand it, why spin one-half particles obey Fermi-Dirac statistics." Sizing up his audience perfectly, Feynman said, "I'll prepare a freshman lecture on it." But he came back a few days later to say, "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it."

-- Goodstein and Goodstein

Feynman understood that if he could not explain something simply, he did not understand it. That bothered him. I think it should bother us all.

If you can't give me an alternative, simplified, explanation how ocean pH could've become "30% more acidic", then I'm not going to take you seriously. Everything I know and wrote about the behaviour of CO2 dissolving in ocean refutes this 30% claim.

When one gets right down to it, all is obfuscation: NOAA, National Geographic, you. None of you even attempt to justify your beliefs.

I'll stick with my back-of-envelope calculation until I read a better one.

[puppypower]

The pH scale is logarithmic. If we go from a pH 8 to a neutral pH of 7, for example, we will increase the acid level; H3O+,  by 10 times or from 10-8 to 10-7 moles/liter. This represents a 1000%  increase in acid concentration, based on starting with 10-8. The 30% estimate is connected to going from pH 8.2 to about 8.1

The pH is important to plant growth, since it will determine mineral and nutrient solubility. Most plants like the water/soil slightly acidic pH <7.  The higher acid concentration will make more nutrients available to drive plant growth.

Different types of algae like different pH's. In salt water aquariums, low pH, more acidic, tends to promote algae growth requiring more cleaning of the glass. In freshwater ponds, high pH tends to promote algae. The CO2 and acid level increase will help the ocean unlock nutrients and thereby dial in the algae it needs to take advantage of the CO2 food.  The ocean has done this before so it is not that big of a deal.

[Bored chemist]

The only difficult "calculation" we need to do is a conversion of units.
The graph as a best line fit on it which says the pH is falling by 0.00186 pH units per year.
It's hard to know what "the past" was like but lets make a guess.
They talk of acidification since "before industrial times"- call it 200 years.
But almost all the change has been in the last 50 years or so.
So lets extrapolate the 200 year change by multiplying the rate of change by 50
(I recognise I'm doing a great demonstration of "spurious accuracy" here- I will round it off later)
50* 0.00186 = 0.093 pH units.
The current value is 8.08
So the pH was about 0.093 units higher.
8.173

Now convert those to what is actually being talked about- the acidity
OK the concentration of hydrogen ions was 10^-8.173
6.7143 nM
and it's now  10^ -8.08
8.318 nM
So, the change is
1.603nM
Now we need to express that change as a percentage of the original
 that's 100* 1.603/6.714
Which is 24% or so.
Given the assumptions and approximations involved, referring to a figure of about 23.87%  as being "30%" when reporting it in a newspaper seems legitimate enough to me.


Fundamentally, you seem to have forgotten that the factors )of about 0.3% for dissolution vs hydration and about 1% for dissociation to hydrogen ions) apply to the carbon dioxide which is already present naturally, as well as to the added carbon dioxide.

You also seem to have overlooked the fact that the acidity of the oceans was measured directly- that's what a pH is.
So we know how much it has changed (something like 20 or 30%).
Your calculation says it's only of the order of  a thousandth of that.

When your calculation doesn't agree with reality, it is not because reality has made a mistake.

[Bored chemist]

None of you even attempt to justify your beliefs.

If you try actually reading, rather than ranting, you will find that I did justify my opinions.
Twice before you wrote that silly  assertion, and once more, in detail.

[chiralSPO]

Thankyou for your reply chiralSPO. Consider this:

Feynman was a truly great teacher. He prided himself on being able to devise ways to explain even the most profound ideas to beginning students. Once, I said to him, "Dick, explain to me, so that I can understand it, why spin one-half particles obey Fermi-Dirac statistics." Sizing up his audience perfectly, Feynman said, "I'll prepare a freshman lecture on it." But he came back a few days later to say, "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it."

-- Goodstein and Goodstein

Feynman understood that if he could not explain something simply, he did not understand it. That bothered him. I think it should bother us all.

If you can't give me an alternative, simplified, explanation how ocean pH could've become "30% more acidic", then I'm not going to take you seriously. Everything I know and wrote about the behaviour of CO2 dissolving in ocean refutes this 30% claim.

When one gets right down to it, all is obfuscation: NOAA, National Geographic, you. None of you even attempt to justify your beliefs.

I'll stick with my back-of-envelope calculation until I read a better one.

That you do not accept my answer does not necessarily imply that I am wrong--that is one of four possibilities:

1) I am wrong.
2) I am correct but did not explain it properly.
3) I am correct and explained it properly, but you did not understand.
4) I am correct and explained it properly, you did understand, but refuse to accept that you were wrong.

[MarkPawelek (OP)]

1)

The graph as a best line fit on it which says the pH is falling by 0.00186 pH units per year.

Where did that come from?  Out of the blue. No graph. No way to make such a graph when one considers the variation in ocean pH and ocean currents.

2) Read what?  Lecture slides written using a program I don't have on my computer. Talk about 'obfuscation', you are second to none.  Accusing you of obfuscation is certainly not 'ranting'.

[Bored chemist]

Where did that come from?

From the graph which you
cited here and
quoted on your blog..

Read what?

This thread.

[MarkPawelek (OP)]

That you do not accept my answer

What answer?  You gave no alternative explanation beyond: 1) this is too complex to discuss therefore 2) acidification must be 30% just like NOAA said. I do not think that's an explanation.

[Bored chemist]

Hi
Here's a copy of the graph from your blog if I can get this to work


Graph.png (538.97 kB . 1228x841 - viewed 9542 times)

[Bored chemist]

That you do not accept my answer does not necessarily imply that I am wrong--that is one of four possibilities:

1) I am wrong.
2) I am correct but did not explain it properly.
3) I am correct and explained it properly, but you did not understand.
4) I am correct and explained it properly, you did understand, but refuse to accept that you were wrong.

I'm leaning towards option 4

[Bored chemist]

What answer?  You gave no alternative explanation beyond: 1) this is too complex to discuss therefore 2) acidification must be 30% just like NOAA said. I do not think that's an explanation.

He gave an answer which explained WHY YOU ARE WRONG, which is sufficient to show that you are wrong.
It doesn't say anyone else is right.

[chiralSPO]

That you do not accept my answer

What answer?  You gave no alternative explanation beyond: 1) this is too complex to discuss therefore 2) acidification must be 30% just like NOAA said. I do not think that's an explanation.

No... I provided links that show the equations you need to use rather than the simplistic and wrong equation you showed. Perhaps this one will be easier to understand: https://en.wikipedia.org/wiki/Carbonic_acid#Acidity_of_carbonic_acid