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OA not OK part 3: Wherever I lay my shell, that's my home

Posted on 8 July 2011 by Doug Mackie

This is the 3rd post in our Ocean Acidification is not OK series. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.

In the last post we introduced thermodynamics and explained how it could predict a reaction direction. In this post we will describe why making calcium carbonate shells via Equation 1 is so easy.

Equation 1

Calcium carbonate, CaCO3, is a salt.  A salt is a substance containing an ion with a positive electric charge, (cation - like calcium Ca2+ or sodium Na+) and an ion with a negative electric charge (anion - like carbonate CO3 or chloride Cl).  Thus, sodium chloride or table salt, NaCl, contains Na+ and Cl.  While Equation 1 reminds us that while CaCO3 contains the ions calcium Ca2+ and carbonate CO32–, the actual reaction to make it is more complicated than calcium + carbonate = calcium carbonate.

In chemistry precipitation is the process when a solid (often a salt) forms from a solution. One familiar example is the way crystals of sodium chloride, NaCl, form as seawater evaporates.

Grassmere salt production

Figure 1. Salt harvesting at Lake Grassmere , New Zealand. The pink colour comes from a salt-loving (halophile) algae.

What is happening? Does the seawater contain particles of sodium chloride (NaCl)? No. Seawater contains sodium ions (Na+) and chloride ions (Cl), each surrounded by water molecules. The water molecules act as a shield, muffling the charges on the Na+ and Cl- ions so they don't attract each other.  As the seawater evaporates there is no longer enough water to keep the ions apart and their opposite electric charges allow them to get close and form solid sodium chloride.

What about the reverse process, dissolving sodium chloride in some water? It is obvious that we can't add salt indefinitely. Did you know that the amount that can dissolve differs from substance to substance? The maximum amount of a substance that can be dissolved in a given volume of water is called the solubility and is a fundamental property of the substance. For sodium chloride a surprising 360 g will dissolve per kg of freshwater at room temperature; for sodium fluoride, a close relative, the value is only 41 g. Calcium carbonate is even less soluble: only about 0.01 g (10 mg) will dissolve will dissolve under the same conditions. (Spoiler alert: Post 12 will discuss the different forms of calcium carbonate that have different solubilities and will go over the critical difference this will make under ocean acidification).

A solution that contains the maximum possible amount of a substance is described as being saturated. The corollary of this is that if you have a saturated solution and then add more of one or both of the component ions in the salt, (i.e. if you exceed the solubility and have too many ions in solution), then the substance will begin to precipitate until the excess has been removed and the solution is back to being saturated.

However, as well as being saturated, a solution can be supersaturated. This means the solution contains more ions that can combine to form a salt than is 'theoretically' possible. Plainly it is possible, since the surface seawater is supersaturated with CaCO3. Remember, the solubility of CaCO3 in freshwater is only 10 mg per kg. In comparison, the concentration of Ca2+ (the 3rd most abundant cation in seawater, after sodium and magnesium) is 412 mg (!) per kg of seawater and the surface concentration of bicarbonate HCO3 and carbonate CO3 (post 5 will explain how bicarbonate and carbonate can interconvert) is 114 mg per kg of seawater. These concentrations are much higher than we would predict from simple solubility calculations alone. This means it takes very little effort to precipitate CaCO3 from seawater and the making of shells should be easy.

In the next post we remind readers about pH and discuss the recent changes to the surface seawater.

Written by Doug Mackie, Christina McGraw , and Keith Hunter . This post is number 3 in a series about ocean acidification. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.




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Comments 1 to 18:

  1. I'm thoroughly enjoying this series. The explanation of such a complex topic beats the stuffy marine biochemistry textbook I had to use. With all this reference to saturation, do I detect a segue to a future post on Carbonate Compensation Depths? They were my favourite :-/
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  2. No better post to remind people: If you're not part of the solution, you're part of the precipitate. great series.
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  3. I came across the phrase “hydrogen carbonate (formerly bicarbonate)” on a blog I frequent. Can you say a word about the nomenclature you are choosing to use?
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  4. Tor B. What you see in this series is chemical nomenclature following the IUPAC nomenclature of inorganic chemistry. What you see on the page you linked to uses Nuclide naming nomenclature. Roughly if the superscript number comes before it's the isotope of the element (physics), if the number comes after it's the number of ions (+, 2+ etc; -, -2 etc for extra electrons), subscript after means the number of instances of that element (e.g. CO2 Carbon and two Oxygen) - thats all chemistry. It's a bit context dependant but as chemists and physicists don't communicate often, there's little confusion ;£
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  5. LOL on the super-subscripts (and nobody listens to geologists ... :) Les's IUPAC link to wikipedia offers: "The modern method specifically names the hydrogen atom. Thus, NaHCO3 would be pronounced sodium hydrogen carbonate." indicates and supports: Search for: bicarbonate IUPAC Name: hydrogen carbonate CHO3- I guess my question regarding the use of the term "bicarbonate" or "hydrogen carbonate" remains.
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    Moderator Response: (Rob P) Any further "Look. Squirrel!" comments will be deleted. Let's keep to the topic at hand.
  6. The really scandalous aspect of this hydrogen carbonate stuff is its close link to the thoroughly vile and evil substance Dihydrogen Monoxide!
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  7. @TorB I see no point in having a big discussion over the nomenclature. Most chemists are brought up using the official name Hydrogen Carbonate and also with the historical name Bicarbonate. At least during my chemical education in Holland this was common practice. The origin of the name Bicarbonate is described here (second answer) : @SteveBrown Very funny website, thanks.
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  8. Tor, your nomenclature comment feels like a "see the squirrel" in the same way earlier comments attempted a derail with questions about the meaning of "acidification". IUPAC does indeed recommend "hydrogen carbonate" instead of "bicarbonate". However, "bicarbonate" is still the commonly used name for HCO3- in chemistry, marine sciences, and general usage, so we are sticking to the familiar terminology. You knew what we meant with "bicarbonate". The readers knew what we meant. The readers would not have known what we meant if we used "hydrogen carbonate". You know this. Please try stay on topic. If you have issues with the actual science then by all means raise them here.
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  9. 5.- Tor B You are welcome, it's always a pleasure to help people understand science. Now, as Doug says, back to tho science issues at hand.
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  10. Maybe this will come up in future posts ... I was wondering about the pH effect (and related potential for CO2 uptake) that other cations have (Mg+2, K+, Na+, etc.) - and regarding the idea of mitigating pH changes and enhancing oceanic uptake of CO2, how would adding either carbonates or silicates with Mg, K, or Na compare to adding Ca (I know the difference between silicates and carbonates is that you can add the dissolved cations from the silicates and actually remove CO2 from the air and form a carbonate mineral (if conditions are right - this may be only common with Ca as I understand it) whereas adding carbonate to enhance CO2 uptake requires that additional substance to remain in solution).
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  11. I am sorry Patrick but I do not follow what you are saying. What do you mean by "the pH effect"? When you write about adding carbonates or silicates, do you mean humans adding these things over and above natural geochemical processes? I think, but please clarify, that you mean adding cations so that various carbonate minerals form. Is that what you mean? If so, how do you think that this would reduce CO2 in the ocean? If this did serve to reduce CO2 (and I suggest you write a few chemical equations to help decide about that), then in what form would you add the cations and how much do you think would need to be added to the ocean?
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  12. Re 11 Doug Mackie - thanks, first, I'm aware that dissolution of carbonate minerals will provide carbonate ions to react with CO2 to form bicarbonate and thus allow the water to take up more CO2; and also, that ultimately (when things are in equilibrium in the time average)geologic emissions from CO2 are balanced by geologic sequestration, the inorganic portion being the formation of carbonate minerals, which can occur without release of CO2 from the water to the air when cations are supplied by silicate weathering rather than carbonate dissolution. There are some ideas people have for accelerating either carbonate dissolution (using carbonates found on land, I think) or geologic sequestration to help mitigate climate change and ocean acidification. Some involve putting CO2 into rock formations to form carbonate minerals right where the silicates are. I was wondering about the idea of crushing either carbonates or silicates and distributing it in the ocean. And whether only Ca silicates would be effective. (Of course there could be other environmental effects depending on where and how this would be done, and I'm not one of those who would abandon efforts to reduce emissions just because this option might exist.) (I'm also curious about why MgCO3 (if it did form) or CaMg(CO3)2 formed more or less in different geologic times but that may be too far off-topic?)
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  13. ... I think a representative net reaction would be (I'm not sure but CaSiO3 might be wollastonite) CaSiO3(s) + CO2(aq) = SiO2(s) + CaCO3(aq) where adding CaCO3(aq) to the ocean, can react with CO2(aq) to form bicarbonate ions (well, actually I guess this would just as easily happen when CaCO3(aq) first forms in solution during chemical weathering), allowing the ocean (or whatever water is involved, which may head to the ocean) to hold more CO2; that CO2 uptake will be given back to the air when CaCO3 precipitates, but the net reaction would be as above except the CaCO3(aq) would be CaCO3(s). In reality I think there are a number of silicate minerals that can be in the reactant side and a number that can be on the product side.
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  14. Thanks Patrick, but my question to you was: How much carbonates do you think would need to be added and how would you do this? Since you were not able to do these calculations I shall point you to the first of several similar papers (that would have been easy to find)that presents such calculations: In 2008 Danny Harvey at U. Toronto published a paper ( abstract) that does the calculations. Harvey L.D.D. (2008) Mitigating the atmospheric CO2 increase and ocean acidification by adding limestone powder to upwelling regions. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C04028, 2008. The conclusion:
    Geographically optimal application of 4 billion t of CaCO3 a−1 (0.48 Gt C a−1) could induce absorption of atmospheric CO2 at a rate of 600 Mt CO2 a−1 after 50 years, 900 Mt CO2 a−1 after 100 years, and 1050 Mt CO2 a−1 after 200 years.
    That is, a fleet of tankers dumping 4 billion tons of powdered limestone per year could be sucking up 600 million tons of CO2 per year after 50 years. A slight problem is that current emissions are about 30,000 million tons of CO2. Yes, current emissions are 50x the amount that would be being sequestered by 2100 – and your guess is as good as mine for what emissions will be in 2100. Every little bit helps? Perhaps. But mining, crushing and transporting the limestone might be a little carbon intensive. Harvey is not seriously proposing this as a solution. He goes through those calculations too, as he says to show that:
    The calculations presented here serve to illustrate the enormity of the task of even partially reversing the acidification of the oceans that is yet to occur under even the most optimistic scenarios concerning reductions in CO2 emissions. The task is not only enormous but would need to continue for several 100 years. These calculations also underline the fact that in the absence of stringent reductions in CO2 emissions, efforts to reduce adverse impacts on ocean chemistry will be ineffective.
    Harvey goes on to say that if emissions have dropped to zero by 2100:
    then application of limestone at a rate of 4 Gt a_1 (0.48 Gt C a_1) beginning in 2020 serves to restore about 20% of the difference between the minimum pH and preindustrial pH by 2200 and restores about 40% of the difference by 2500, with the same benefits for the degree of supersaturation with respect to calcite.
    Yes quite. If emissions have dropped to zero by 2100.
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  15. Re Doug Mackie - okay, thank you.
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  16. second summary post

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  17. A small typo to correct on this page - search for: will dissolve will dissolve

    ... and remove two words ...


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  18. There seams to be several minor errors with wrong loading  carbonate ions as well?

    CO3- and not CO3--

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    Moderator Response:

    [Rob P] - Thanks for pointing out those typos. Hopefully someone who has access will correct them in due course.

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