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Topic: Why can CaCO3 precipitate by utilizing bicarbonate?  (Read 421 times)

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Offline Uwe

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Why can CaCO3 precipitate by utilizing bicarbonate?
« on: October 13, 2020, 08:24:17 PM »
It is sometimes stated that CaCO3 precipitation in seawater can occur either following

(1) Ca2+ + Co32- -> CaCO3


(2) Ca2+ + HCO32- -> CaCO3 + H+

In an abiotic system, in which CaCO3 precipitation only follows thermodynamics, why would CaCO3 precipitation follow equation 2?

It is sometime stated that this would occur as DIC at a the pH range of seawater is mainly composed of HCO32-. But the CaCO3 lattice is composed of Ca2+ and carbonate, not bicarbonate ions, so ultimately the precipitation reaction will again be equation 1

Offline Borek

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Re: Why can CaCO3 precipitate by utilizing bicarbonate?
« Reply #1 on: October 14, 2020, 03:04:05 AM »
Think about it this way: first equation refers to the overall process consisting of a dissociation followed by the precipitation.
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