The instability of carbonic acid

[freeztar]

This wiki article...

 

Carbonic acid - Wikipedia, the free encyclopedia

 

...states that:

 

It has since been shown, by theoretical calculations, that the presence of even a single molecule of water causes carbonic acid to revert to carbon dioxide and water fairly quickly. Pure carbonic acid is predicted to be stable in the gas phase, in the absence of water, with a calculated half-life of 180,000 years.

 

My understanding of ocean sequestration of CO2 is that it froms carbonic acid in the ocean. How can I reconcile these seemingly different understandings?

 

Why is carbonic acid most stable in a gaseous state, yet is supposedly formed in ocean/CO2 interactions?

 

When CO2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species : dissolved free carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-). The ratio of these species depends on factors such as seawater temperature and alkalinity (see the article on the ocean's solubility pump for more detail).

 

Ocean acidification - Wikipedia, the free encyclopedia

 

Following the suggested link I come to this page:

Solubility pump - Wikipedia, the free encyclopedia

 

Carbon dioxide, like other gases, is soluble in water. However, unlike many other gases (oxygen for instance), it reacts with water and forms a balance of several ionic and non-ionic species (collectively known as dissolved inorganic carbon, or DIC). These are dissolved free carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-), and they interact with water as follows :

CO2 (aq) + H2O <-> H2CO3 <-> HCO3- + H+ <-> CO32- + 2 H+

 

Does this mean that the "ionic species" are constantly undergoing change towards homogenization? :)

[Mercedes Benzene]

The species [ce]CO2[/ce] and [ce]H2CO3[/ce] oscillate between those configurations in solution and are in equilibrium.

[ce]CO2[/ce] forms carbonic acid in water.

[ce]H2CO3[/ce] dissociates into carbon dioxide (or splits into [ce]H+[/ce] and [ce]HCO3-[/ce].

 

Certain factors push equilibrium one direction or another. I'm not quite sure what it is about the ocean that affects equilibrium but it seems that a certain amount of carbon dioxdie is able to stay dissolved.

 

The same concept applies in respiration. Most of the carbon dioxide in our bodies is actually in the form of biocarbonate ion. This is enzymatically converted to carbon dioxide gas when it gets to the alveoli in our lungs (or to the surface of our skin).

 

I hope this helps you understand the concepts you presented a little bit better. :)

[freeztar]

I hope this helps you understand the concepts you presented a little bit better. :)

 

Absolutely! Thanks! :)

 

As far as factors pushing the equilibrium one way or the other...

 

From the ocean acidification wiki link posted above...

The ratio of these species depends on factors such as seawater temperature and alkalinity

 

Unfortunately, their is no discussion at that link about HOW the temperature and alkalinity affect the species ratios. :)

[Mercedes Benzene]

A higher temperature would cause more [ce]CO2[/ce] to leave solution. As for pH, using Le Chatelier's principle, we can show that a lower pH (more [ce]H+[/ce]) would cause the equilibrium to shift to the left, causing more carbon dioxide to leave solution, whereas a higher pH would cause the equilibrium to shift more towards the products.

[freeztar]

Ok, I think I get it now.

 

Both increased temperature and H+ ion concentrations directly affect the ability of ocean water to sequester CO2. The CO2 that is sequestered is disassociated into the various species previously mentioned. The equilibrium of these ionic species is thusly a locally determinable ratio based upon the variables mentioned previously.

 

One final question...

Does it matter if the CO2 is in liquid or gaseous form?

 

Thanks for your help MB! :)

[Mercedes Benzene]

One final question...

Does it matter if the CO2 is in liquid or gaseous form?

 

Thanks for your help MB!

 

Well unless the oceans or our blood reach -77 C, I don't think we have to worry about it being liquid! :)

 

And, you're welcome.

[freeztar]

Well unless the oceans or our blood reach -77 C, I don't think we have to worry about it being liquid! :)

 

I certainly hope neither of those scenarios happen. Nonetheless, apparently CO2 can be in liquid state under increased pressure, 5.1 atm to be precise.

 

Carbon dioxide has no liquid state at pressures below 5.1 atm, but is a solid at temperatures below -78 °C.

 

Carbon dioxide - Wikipedia, the free encyclopedia

 

So an underwater volcano that is spewing CO2 in liquid form from 1600m under the surface is able to do so because of the immense pressures (much greater than 5.1atm at that depth) combined with the low temperatures in the deep sea water.

 

Surely this would influence the species ratios significantly...

[Turtle]

 

So an underwater volcano that is spewing CO2 in liquid form from 1600m under the surface is able to do so because of the immense pressures (much greater than 5.1atm at that depth) combined with the low temperatures in the deep sea water.

 

Surely this would influence the species ratios significantly...

 

Good topic Freezy! :) It is not just underwater volcanoes, but underwater hydrothermal vents mixing up the CO2 and other chemicals down at the bottom of the deep blue seas. This abstract is the tenth down the page at this link: Seamount Hydrothermal Systems: Volcanology, Biology, Geochemistry, and Oceanography II Posters - Volcanology, Geochemistry, Petrology [V]

 

The bolding is mine; clearly this group doesn't know about the liquid CO2 jets at Vailulu'u. :doh: ;)

 

V51C-1498

 

Carbon Fluxes from Submarine Arc Volcanoes - examples from the Mariana and Kermadec Arcs

 

Recent investigations of volcanic arcs have revealed unusually high fluxes of CO2 from several submarine arc volcanoes. In 2004 the ROPOS ROV was used to map and sample ~10 active volcanoes along the Mariana arc, and in 2005 a similar study of volcanoes along the Kermadec arc was conducted using the HURL Pisces submersible. Of particular interest are 3 volcanoes that, in addition to discharging hot vent fluid, were found to be venting a separate CO2-rich phase in the form of gas bubbles or, in one case, droplets of liquid CO2. The Champagne hydrothermal site situated at ~1600-m depth near the summit of NW Eifuku volcano (21.49°N, 144.04°E) in the northern Mariana Arc, was discovered in 2004 during NOAAs Submarine Ring of Fire (SROF) project. This unusual site was discharging two distinct fluids from the same vent field: a 103°C gas-rich hydrothermal fluid, and cold (4°C) droplets of liquid CO2. The hot fluid contained ~2.2 moles/kg CO2, the highest ever reported for submarine hydrothermal fluids and about twice the saturation value at that p,T. The carbon flux from this site was estimated to be ~23 moles CO2/sec, about 0.1% of the global MOR carbon flux. Two similar but much shallower CO2-rich systems were discovered on the Kermadec arc. Pisces dives on Giggenbach volcano (30.04°S, 178.71°W) in the Kermadec arc discovered a mixture of gas bubbles and 203°C fluid discharging at 164-m depth. The fluid contained 250 - 500 mM/kg total gas. At Volcano 1 (21.15°S, 175.75°W), Pisces found streams of gas bubbles rising from the seafloor at ~100 m depth. This vent area had areas of diffuse discharge (30 to 150°C) with gas contents up to 130 mM/kg. Although analyses are still in progress for these two sites, the gas bubbles are assumed to be mainly CO2. It is notable that discharges of pure CO2 have never been reported for MOR hydrothermal systems, and only one other submarine occurrence of liquid CO2 has been reported (in the Okinawa Trough, a back-arc system). This suggests that such CO2-rich systems occur much more frequently in subduction zone systems compared to MOR systems, probably due to the supply of subducted marine carbonates and organic matter. It seems likely that the high CO2 levels arise from direct degassing from a magma chamber and/or de-volatilization of the subducting slab. The apparent high carbon flux from these sites suggests that submarine arc volcanoes may play a larger role in oceanic carbon cycling than previous realized.