Editing Carbonic acid (section)

== Relationship to bicarbonate and carbonate ==
[[File:Weak acid speciation.svg|thumb|right|[[Bjerrum plot]] of speciation for a hypothetical monoprotic acid: AH concentration as a function of the difference between {{Math|''p''K}} and {{Math|''p''H}}]]
Carbonic acid is the formal [[Brønsted–Lowry]] [[conjugate acid]] of the [[bicarbonate]] anion, stable in [[Base (chemistry)|alkaline solution]]. The protonation constants have been measured to great precision, but depend on overall [[ionic strength]] {{Mvar|I}}. The two equilibria most easily measured are as follows: <math chem display="block">\begin{align}
\ce{CO3^{2-}{} + H+{} <=> HCO3^-} && \beta_1 = \frac{[\ce{HCO3^-}]}{[\ce{H+}][\ce{CO3^{2-}}]} \\
\ce{CO3^{2-}{} + 2H+{} <=> H2CO3} && \beta_2 = \frac{[\ce{H2CO3}]}{[\ce{H+}]^2[\ce{CO3^{2-}}]}
\end{align}</math> where brackets indicate the [[concentration]] of [[Species (chemistry)|species]]. At 25&nbsp;°C, these equilibria empirically satisfy<ref>[[IUPAC]] (2006). "[https://old.iupac.org/publications/scdb/index.html Stability constants]" (database). </ref><math display="block">\begin{alignat}{6}
\log(\beta_1) =&& 0&.54&I^2 - 0&.96&I +&& 9&.93 \\
\log(\beta_2) =&& -2&.5&I^2 - 0&.043&I +&& 16&.07
\end{alignat}</math>{{Math|log(''&beta;''<sub>1</sub>)}} decreases with increasing {{Mvar|I}}, as does {{Math|log(''&beta;''<sub>2</sub>)}}. In a solution absent other ions (e.g. {{Math|''I'' {{=}} 0}}), these curves imply the following [[Dissociation constant|stepwise dissociation constants]]:<math display="block">\begin{alignat}{3}
p\text{K}_1 &= \log(\beta_2) - \log(\beta_1) &= 6.77 \\
p\text{K}_2 &= \log(\beta_1) &= 9.93
\end{alignat}</math> Direct values for these constants in the literature include {{math|''p''K<sub>1</sub> {{=}} 6.35}} and {{math|''p''K<sub>2</sub> - ''p''K<sub>1</sub> {{=}} 3.49}}.<ref>{{cite journal |last1=Pines |first1=Dina |last2=Ditkovich |first2=Julia |last3=Mukra |first3=Tzach |last4=Miller |first4=Yifat |last5=Kiefer |first5=Philip M. |last6=Daschakraborty |first6=Snehasis |last7=Hynes |first7=James T. |last8=Pines |first8=Ehud |date=2016 |title=How Acidic Is Carbonic Acid? |journal=J Phys Chem B |volume=120 |issue=9 |pages=2440–51 |doi=10.1021/acs.jpcb.5b12428 |pmc=5747581 |pmid=26862781}}</ref>

To interpret these numbers, note that two chemical species in an acid equilibrium are [[Equinumerosity|equiconcentrated]] when {{Math|''p''K {{=}} ''p''H}}. In particular, the [[extracellular fluid]] ([[cytosol]]) in biological systems exhibits {{Math|''p''H &approx; 7.2}}, so that carbonic acid will be almost 50%-dissociated at equilibrium.

=== Ocean acidification ===
[[File:Carbonate system of seawater.svg|thumb|upright=1.4|Carbonate speciation in seawater (ionic strength 0.7&nbsp;mol/dm<sup>3</sup>). The expected change shown is due to the [[Greenhouse gas emissions|current anthropogenic increase]] in [[atmospheric carbon dioxide]] concentration.|left]]

The [[Bjerrum plot]] shows typical equilibrium concentrations, in solution, in [[seawater]], of carbon dioxide and the various species derived from it, as a function of [[pH]].<ref name="peroxide">{{cite journal |last1=Pangotra |first1=Dhananjai |last2=Csepei |first2=Lénárd-István |last3=Roth |first3=Arne |last4=Ponce de León |first4=Carlos |last5=Sieber |first5=Volker |last6=Vieira |first6=Luciana |year=2022 |title=Anodic production of hydrogen peroxide using commercial carbon materials |journal=Applied Catalysis B: Environmental |volume=303 |page=120848 |doi=10.1016/j.apcatb.2021.120848 |s2cid=240250750}}</ref><ref name="Andersen">{{cite journal |last=Andersen |first=C. B. |year=2002 |title=Understanding carbonate equilibria by measuring alkalinity in experimental and natural systems |journal=Journal of Geoscience Education |volume=50 |issue=4 |pages=389–403 |doi=10.5408/1089-9995-50.4.389 |bibcode=2002JGeEd..50..389A |s2cid=17094010}}</ref> As human industrialization has [[Greenhouse gas emissions|increased the proportion]] of [[carbon dioxide in Earth's atmosphere]], the proportion of carbon dioxide dissolved in sea- and freshwater as carbonic acid is also expected to increase. This rise in dissolved acid is also expected to [[Acidifying agent|acidify]] those waters, generating a decrease in pH.<ref name="cald03">{{cite journal |last=Caldeira |first=K. |author2=Wickett, M. E. |title=Anthropogenic carbon and ocean pH |journal=[[Nature (journal) |Nature]] |volume=425 |issue=6956 |pages=365 |year=2003 |s2cid=4417880 |pmid=14508477 |bibcode=2001AGUFMOS11C0385C |doi=10.1038/425365a |doi-access=free |url=https://zenodo.org/record/1233227}}</ref><ref name="sabine">{{cite journal |last=Sabine |first=C. L. |year=2004 |title=The Oceanic Sink for Anthropogenic CO<sub>2</sub> |journal=Science |volume=305 |issue=5682 |pages=367–371 |pmid=15256665 |bibcode=2004Sci...305..367S |hdl=10261/52596 |hdl-access=free |s2cid=5607281 |doi=10.1126/science.1097403 |url=https://www.science.org/doi/abs/10.1126/science.1097403 |access-date=22 June 2021 |url-status=live |archive-url=https://web.archive.org/web/20080706143710/http://www.sciencemag.org/cgi/content/short/305/5682/367 |archive-date=6 July 2008}}</ref> It has been estimated that the increase in dissolved carbon dioxide has already caused the ocean's average surface pH to decrease by about 0.1 from pre-industrial levels.