Carbonic acid - wikipedia Carbonic acid Not to be confused with Carbolic acid, an antiquated name for phenol. Carbonic acid Names Preferred IUPAC name Carbonic acid Other names Carbon dioxide solution Dihydrogen carbonate Hydrogen bicarbonate Acid of air Aerial acid Hydroxymethanoic acid Identifiers CAS Number 463 - 79 - 6 3D model ( JSmol ) Interactive image ChEBI CHEBI : 28976 ChEMBL ChEMBL1161632 ChemSpider 747 ECHA InfoCard 100.133. 015 EC Number 610 - 295 - 3 KEGG C01353 PubChem CID 767 InChI InChI = 1S / CH2O3 / c2 - 1 ( 3 ) 4 / h ( H2, 2, 3, 4 ) Key : BVKZGUZCCUSVTD - UHFFFAOYSA - N InChI = 1 / CH2O3 / c2 - 1 ( 3 ) 4 / h ( H2, 2, 3, 4 ) Key : BVKZGUZCCUSVTD - UHFFFAOYAU SMILES O = C ( O ) O Properties Chemical formula H CO Molar mass 62.03 g / mol Density 1.668 g / cm Solubility in water Only stable in solution Acidity ( pK ) 3.6 ( pK for H CO only ), 6.3 ( pK including CO ( aq ) ), 10.32 ( pK ) Related compounds Related compounds Acetone Urea Carbonyl fluoride Except where otherwise noted, data are given for materials in their standard state ( at 25 ° C ( 77 ° F ), 100 kPa ). Y verify ( what is ? ) Infobox references Carbonic acid is a chemical compound with the chemical formula H CO ( equivalently OC ( OH ) ). It is also a name sometimes given to solutions of carbon dioxide in water ( carbonated water ), because such solutions contain small amounts of H CO. In physiology, carbonic acid is described as volatile acid or respiratory acid, because it is the only acid excreted as a gas by the lungs. It plays an important role in the bicarbonate buffer system to maintain acid -- base homeostasis. Carbonic acid, which is a weak acid, forms two kinds of salts : the carbonates and the bicarbonates. In geology, carbonic acid causes limestone to dissolve, producing calcium bicarbonate, which leads to many limestone features such as stalactites and stalagmites. It was long believed that carbonic acid could not exist as a pure compound. However, in 1991 it was reported that NASA scientists had succeeded in making solid H CO samples. Contents 1 Chemical equilibrium 2 Role of carbonic acid in blood 3 Role of carbonic acid in ocean chemistry 4 Acidity of carbonic acid 4.1 pH and composition of carbonic acid solutions 5 Pure carbonic acid 6 See also 7 References 8 Further reading 9 External links Chemical equilibrium When carbon dioxide dissolves in water it exists in chemical equilibrium with carbonic acid : CO 2 + H 2 O ↽ − − ⇀ H 2 CO 3 ( displaystyle ( ce ( CO2 + H2O < = > H2CO3 ) ) ) The hydration equilibrium constant at 25 ° C is called K, which in the case of carbonic acid is ( H CO ) / ( CO ) ≈ 1.7 × 10 in pure water and ≈ 1.2 × 10 in seawater. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s for the forward reaction ( CO + H O → H CO ) and 23 s for the reverse reaction ( H CO → CO + H O ). The addition of two molecules of water to CO would give orthocarbonic acid, C ( OH ), which exists only in minute amounts in aqueous solution. Addition of base to an excess of carbonic acid gives bicarbonate ( hydrogen carbonate ). With excess base, carbonic acid reacts to give carbonate salts. Role of carbonic acid in blood Bicarbonate is an intermediate in the transport of CO out of the body by respiratory gas exchange. The hydration reaction of CO is generally very slow in the absence of a catalyst, but red blood cells contain carbonic anhydrase, which increases the reaction rate, producing bicarbonate ( HCO ) dissolved in the blood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO and allows it to be expelled. This equilibration plays an important role as a buffer in mammalian blood. A 2016 theoretical report suggests that carbonic acid may play a pivotal role in protonating various nitrogen bases in blood serum. Role of carbonic acid in ocean Chemistry The oceans of the world have absorbed almost half of the CO emitted by humans from the burning of fossil fuels. It has been estimated that the extra dissolved carbon dioxide has caused the ocean 's average surface pH to shift by about − 0.1 unit from pre-industrial levels. This is known as ocean acidification, even though the ocean remains basic. Acidity of carbonic acid Carbonic acid is a polyprotic acid -- specifically it is diprotic, meaning that it has two protons that may dissociate from the parent molecule. Thus, there are two dissociation constants, first of which is for the dissociation into the bicarbonate ( also called hydrogen carbonate ) ion HCO : H 2 CO 3 ↽ − − ⇀ HCO 3 − + H + ( displaystyle ( ce ( H2CO3 < = > HCO3 ^ - + H+ ) ) ) K = 2.5 × 10 ; pK = 3.6 at 25 ° C. Care must be taken when quoting and using the first dissociation constant of carbonic acid. In aqueous solution, carbonic acid exists in equilibrium with carbon dioxide, and the concentration of H CO is much lower than the concentration of CO. In many analyses, H CO includes dissolved CO ( referred to as CO ( aq ) ), H CO * is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows : H CO * ⇌ HCO + H K ( app ) = 4.47 × 10 ; pK ( app ) = 6.35 at 25 ° C and ionic strength = 0.0. Whereas this apparent pK is quoted as the dissociation constant of carbonic acid, it is ambiguous : it might better be referred to as the acidity constant of dissolved carbon dioxide, as it is particularly useful for calculating the pH of CO-containing solutions. A similar situation applies to sulfurous acid ( H SO ), which exists in equilibrium with substantial amounts of unhydrated sulfur dioxide. The second constant is for the dissociation of the bicarbonate ion into the carbonate ion CO : HCO 3 − ↽ − − ⇀ CO 3 − 2 + H + ( displaystyle ( ce ( HCO3 ^ - < = > CO3 ^ ( - 2 ) ( ) + H+ ) ) ) K = 4.69 × 10 ; pK = 10.329 at 25 ° C and ionic strength = 0.0. The three acidity constants are defined as follows : K a 1 = ( H + ) ( HCO 3 − ) ( H 2 CO 3 ), ( displaystyle K_ ( a1 ) = ( \frac ( ( ( \text ( H ) ) ^ ( + ) ) ( ( \text ( HCO ) ) _ ( 3 ) ^ ( - ) ) ) ( ( ( \text ( H ) ) _ ( 2 ) ( \text ( CO ) ) _ ( 3 ) ) ) ), ) K a ( app ) = ( H + ) ( HCO 3 − ) ( H 2 CO 3 ) + ( CO 2 ( aq ) ), ( displaystyle K_ ( a ) ( \text ( ( app ) ) ) = ( \frac ( ( ( \text ( H ) ) ^ ( + ) ) ( ( \text ( HCO ) ) _ ( 3 ) ^ ( - ) ) ) ( ( ( \text ( H ) ) _ ( 2 ) ( \text ( CO ) ) _ ( 3 ) ) + ( ( \text ( CO ) ) _ ( 2 ) ( \text ( ( aq ) ) ) ) ) ), ) K a 2 = ( H + ) ( CO 3 2 − ) ( HCO 3 − ). ( displaystyle K_ ( a2 ) = ( \frac ( ( ( \text ( H ) ) ^ ( + ) ) ( ( \text ( CO ) ) _ ( 3 ) ^ ( 2 - ) ) ) ( ( ( \text ( HCO ) ) _ ( 3 ) ^ ( - ) ) ) ). ) PH and composition of carbonic acid solutions At a given temperature, the composition of a pure carbonic acid solution ( or of a pure CO solution ) is completely determined by the partial pressure p CO 2 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) ) of carbon dioxide above the solution. To calculate this composition, account must be taken of the above equilibria between the three different carbonate forms ( H CO, HCO and CO ), as well as of the hydration equilibrium between dissolved CO and H CO with constant K h = ( H 2 CO 3 ) ( CO 2 ) ( displaystyle K_ ( h ) = ( \tfrac ( ( ( \text ( H ) ) _ ( 2 ) ( \text ( CO ) ) _ ( 3 ) ) ) ( ( ( \text ( CO ) ) _ ( 2 ) ) ) ) ) ( see above ) and of the following equilibrium between the dissolved CO and the gaseous CO above the solution : CO ( gas ) ⇌ CO ( dissolved ) with ( CO 2 ) p CO 2 = 1 k H, ( displaystyle \textstyle ( \frac ( ( ( \text ( CO ) ) _ ( 2 ) ) ) ( p_ ( ( \text ( CO ) ) _ ( 2 ) ) ) ) = ( \frac ( 1 ) ( k_ ( \text ( H ) ) ) ), ) where k = 29.76 atm / ( mol / L ) ( Henry constant ) at 25 ° C. The corresponding equilibrium equations together with the ( H + ) ( OH − ) = 10 − 14 ( displaystyle ( ( \text ( H ) ) ^ ( + ) ) ( ( \text ( OH ) ) ^ ( - ) ) = 10 ^ ( - 14 ) ) relation and the charge neutrality condition ( H + ) = ( OH − ) + ( HCO 3 − ) + 2 ( CO 3 2 − ) ( displaystyle ( ( \text ( H ) ) ^ ( + ) ) = ( ( \text ( OH ) ) ^ ( - ) ) + ( ( \text ( HCO ) ) _ ( 3 ) ^ ( - ) ) + 2 ( ( \text ( CO ) ) _ ( 3 ) ^ ( 2 - ) ) ) result in six equations for the six unknowns ( CO ), ( H CO ), ( H ), ( OH ), ( HCO ) and ( CO ), showing that the composition of the solution is fully determined by p CO 2 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) ). The equation obtained for ( H ) is a cubic, whose numerical solution yields the following values for the pH and the different species concentrations : p CO 2 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) ) ( atm ) pH ( CO ) ( mol / L ) ( H CO ) ( mol / L ) ( HCO ) ( mol / L ) ( CO ) ( mol / L ) 1.0 × 10 7.00 3.36 × 10 5.71 × 10 1.42 × 10 7.90 × 10 1.0 × 10 6.94 3.36 × 10 5.71 × 10 5.90 × 10 1.90 × 10 1.0 × 10 6.81 3.36 × 10 5.71 × 10 9.16 × 10 3.30 × 10 1.0 × 10 6.42 3.36 × 10 5.71 × 10 3.78 × 10 4.53 × 10 1.0 × 10 5.92 3.36 × 10 5.71 × 10 1.19 × 10 5.57 × 10 3.5 × 10 5.65 1.18 × 10 2.00 × 10 2.23 × 10 5.60 × 10 1.0 × 10 5.42 3.36 × 10 5.71 × 10 3.78 × 10 5.61 × 10 1.0 × 10 4.92 3.36 × 10 5.71 × 10 1.19 × 10 5.61 × 10 1.0 × 10 4.42 3.36 × 10 5.71 × 10 3.78 × 10 5.61 × 10 1.0 × 10 3.92 3.36 × 10 5.71 × 10 1.20 × 10 5.61 × 10 2.5 × 10 3.72 8.40 × 10 1.43 × 10 1.89 × 10 5.61 × 10 1.0 × 10 3.42 3.36 × 10 5.71 × 10 3.78 × 10 5.61 × 10 We see that in the total range of pressure, the pH is always largely lower than pK so that the CO concentration is always negligible with respect to HCO concentration. In fact, CO plays no quantitative role in the present calculation ( see remark below ). For vanishing p CO 2 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) ), the pH is close to the one of pure water ( pH = 7 ), and the dissolved carbon is essentially in the HCO form. For normal atmospheric conditions ( p CO 2 = 3.5 × 10 − 4 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) = 3.5 \times 10 ^ ( - 4 ) ) atm ), we get a slightly acidic solution ( pH = 5.7 ), and the dissolved carbon is now essentially in the CO form. From this pressure on, ( OH ) becomes also negligible, so that the ionized part of the solution is now an equimolar mixture of H and HCO. For a CO pressure typical for bottled carbonated drinks ( p CO 2 ( displaystyle p_ ( ( \text ( CO ) ) _ ( 2 ) ) ) ~ 2.5 atm ), we get a relatively acidic medium ( pH = 3.7 ) with a high concentration of dissolved CO. These features contribute to the sour and sparkling taste of these drinks. Between 2.5 and 10 atm, the pH crosses the pK value ( 3.60 ), giving a dominant H CO concentration ( with respect to HCO ) at high pressures. A plot of the equilibrium concentrations of these different forms of dissolved inorganic carbon ( and which species is dominant ) as a function of the pH of the solution is known as a Bjerrum plot. Remark As noted above, ( CO ) may be neglected for this specific problem, resulting in the following very precise analytical expression for ( H ) : ( H + ) ≃ ( 10 − 14 + K h K a 1 k H p CO 2 ) 1 / 2. ( displaystyle ( ( \text ( H ) ) ^ ( + ) ) simeq left ( 10 ^ ( - 14 ) + ( \frac ( K_ ( h ) K_ ( a1 ) ) ( k_ ( \text ( H ) ) ) ) p_ ( ( \text ( CO ) ) _ ( 2 ) ) \right ) ^ ( 1 / 2 ). ) Pure carbonic acid It was long believed that carbonic acid could not exist as a pure compound. However, in 1991 scientists at NASA 's Goddard Space Flight Center ( USA ) succeeded in making solid H CO samples. They did so by exposing a frozen mixture of water and carbon dioxide to high - energy proton radiation, and then warming to remove the excess water. The carbonic acid that remained was characterized by infrared spectroscopy. The fact that the carbonic acid was prepared by irradiating a solid H O + CO mixture, or even by irradiation of dry ice alone, has given rise to suggestions that H CO might be found in outer space or on Mars, where frozen ices of H O and CO are found, as well as cosmic rays. It was announced in 1993 that solid carbonic acid had been created by a cryogenic reaction of potassium bicarbonate and HCl dissolved in methanol. Later work showed that in fact the methyl ester had been formed, but other methods were successful. Theoretical calculations showed that a single molecule of water can catalyze the decomposition of a gas - phase carbonic acid molecule to carbon dioxide and water. In the absence of water, the dissociation of gaseous carbonic acid has been predicted to be very slow, with a half - life of 180,000 years. This only applies if the molecules are few and far between, because it has also been predicted that gas - phase carbonic acid will catalyze its own decomposition by forming dimers, which then break apart into two molecules each of water and carbon dioxide. For a while it was thought that there were two polymorphs of solid carbonic acid, called α and β. The polymorph denoted β - carbonic acid was prepared by heating alternating layers of glassy aqueous solutions of bicarbonate and acid in vacuum, which causes protonation of the bicarbonate, followed by removal of the solvent. The previously suggested α - carbonic acid, which was prepared by the same technique using methanol rather than water as a solvent, was later shown to be a monomethyl ester CH OCOOH. See also Carbonated water ( soft drink ) Carbon dioxide Dihydroxymethylidene ( carbonous acid ) Nonvolatile acid Ocean acidification References Jump up ^ Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 ( Blue Book ). Cambridge : The Royal Society of Chemistry. 2014. pp. 414, 781. doi : 10.1039 / 9781849733069 - FP001. ISBN 978 - 0 - 85404 - 182 - 4. Jump up ^ Acid - Base Physiology 2.1 -- Acid - Base Balance by Kerry Brandis. ^ Jump up to : M.H. Moore ; R.K. Khanna ( 1990 ). `` Infrared and mass spectral studies of proton irradiated H O + CO ice : Evidence for carbonic acid ''. Spectrochimica Acta Part A. doi : 10.1016 / 0584 - 8539 ( 91 ) 80097 - 3. ^ Jump up to : Greenwood, Norman N. ; Earnshaw, Alan ( 1997 ). Chemistry of the Elements ( 2nd ed. ). Butterworth - Heinemann. p. 310. ISBN 0 - 08 - 037941 - 9. Jump up ^ Housecroft and Sharpe, Inorganic Chemistry, 2nd ed, Prentice - Pearson - Hall 2005, p. 368. Jump up ^ Soli, A.L. ; R.H. Byrne ( 2002 ). `` CO system hydration and dehydration kinetics and the equilibrium CO / H CO ratio in aqueous NaCl solution ''. Marine chemistry. 78 ( 2 -- 3 ) : 65 -- 73. doi : 10.1016 / S0304 - 4203 ( 02 ) 00010 - 5. Jump up ^ `` excretion ''. Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago : Encyclopædia Britannica, 2010. Jump up ^ `` Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I : Acid and Base Coordinate and Charge Dynamics '', S. Daschakraborty, P.M. Kiefer, Y. Miller, Y. Motro, D. Pines, E. Pines, and J.T. Hynes. J. Phys. Chem. B ( 2016 ), 120, 2271. Jump up ^ Sabine, C.L. ; et al. ( 2004 ). `` The Oceanic Sink for Anthropogenic CO ''. Science. 305 ( 5682 ) : 367 -- 371. doi : 10.1126 / science. 1097403. PMID 15256665. Archived from `` the original on July 6, 2008. Jump up ^ National Research Council. `` Summary ''. Ocean Acidification : A National Strategy to Meet the Challenges of a Changing Ocean. Washington, DC : The National Academies Press, 2010. 1. Print. ^ Jump up to : Loerting, T. ; Tautermann, C. ; Kroemer, R.T. ; Kohl, I. ; Hallbrucker, E. ; Mayer, A. ; Liedl, K.R. ( 2000 ). `` On the Surprising Kinetic Stability of Carbonic Acid ''. Angew. Chem. Int. Ed. 39 ( 5 ) : 891 -- 895. doi : 10.1002 / ( SICI ) 1521 - 3773 ( 20000303 ) 39 : 5 < 891 : : AID - ANIE891 > 3.0.CO ; 2 - E. PMID 10760883. ^ Jump up to : Reisenauer, H.P. ; Wagner, J.P. ; Schreiner, P.R. ( 2014 ). `` Gas - Phase Preparation of Carbonic Acid and Its Monomethyl Ester ''. Angew. Chem. Int. Ed. 53 ( 44 ) : 11766 -- 11771. doi : 10.1002 / anie. 201406969. Jump up ^ Hage, W. ; Hallbrucker, A. ; Mayer, E. ( 1993 ). `` Carbonic Acid : Synthesis by Protonation of Bicarbonate and FTIR Spectroscopic Characterization Via a New Cryogenic Technique ''. J. Am. Chem. Soc. 115 ( 18 ) : 8427 -- 8431. doi : 10.1021 / ja00071a061. Jump up ^ `` Press release : International First : Gas - phase Carbonic Acid Isolated ''. Technische Universität Wien. 11 January 2011. Jump up ^ de Marothy, S.A. ( 2013 ). `` Autocatalytic decomposition of carbonic acid ''. Int. J. Quantum Chem. 113 ( 20 ) : 2306 -- 2311. doi : 10.1002 / qua. 24452. Retrieved 19 January 2015. Further reading `` Climate and Carbonic Acid '' in Popular Science Monthly Volume 59, July 1901 Welch, M.J. ; Lifton, J.F. ; Seck, J.A. ( 1969 ). `` Tracer studies with radioactive oxygen - 15. Exchange between carbon dioxide and water ''. J. Phys. Chem. 73 ( 335 ) : 3351. doi : 10.1021 / j100844a033. Jolly, W.L. ( 1991 ). Modern Inorganic Chemistry ( 2nd Edn. ). New York : McGraw - Hill. ISBN 0 - 07 - 112651 - 1. Moore, M.H. ; Khanna, R. ( 1991 ). `` Infrared and Mass Spectral Studies of Proton Irradiated H2O + Co2 Ice : Evidence for Carbonic Acid Ice : Evidence for Carbonic Acid ''. Spectrochimica Acta. 47A ( 2 ) : 255 -- 262. doi : 10.1016 / 0584 - 8539 ( 91 ) 80097 - 3. W. Hage, K.R. Liedl ; Liedl, E. ; Hallbrucker, A ; Mayer, E ( 1998 ). `` Carbonic Acid in the Gas Phase and Its Astrophysical Relevance ''. Science. 279 ( 5355 ) : 1332 -- 1335. doi : 10.1126 / science. 279.5355. 1332. PMID 9478889. Hage, W. ; Hallbrucker, A. ; Mayer, E. ( 1995 ). `` A Polymorph of Carbonic Acid and Its Possible Astrophysical Relevance ''. J. Chem. Soc. Farad. Trans. 91 ( 17 ) : 2823 -- 2826. doi : 10.1039 / ft9959102823. External links Ask a Scientist : Carbonic Acid Decomposition Why was the existence of carbonic acid unfairly doubted for so long ? Carbonic acid / bicarbonate / carbonate equilibrium in water : pH of solutions, buffer capacity, titration and species distribution vs. pH computed with a free spreadsheet How to calculate concentration of Carbonic Acid in Water Hydrogen compounds H AsO H AsO HAt HSO F HBF HBr HBrO HBrO HBrO HBrO HCl HClO HClO HClO HClO HCN HCNO H CrO / H Cr O H CO H CS HF HFΟ HI HIO HIO HIO HIO HMnO H MoO HNC NaHCO HNCO HNO HNO HNO H N O HNO S H NSO H O H O H O H PO H PO H PO H P O H P O H PtCl H Se H SeO H SeO H SiO H SiF HSCN HNSC H SO H SO H SO H S O H S O H S O H S O CF SO H H Te H TeO H TeO H TiO H Po HCo ( CO ) Oxocarbons Common oxides CO CO Exotic oxides CO CO CO CO O O O O O O O O O O O O O O O O O O O O O O O Polymers Graphite oxide C O CO CO Compounds derived from oxides Metal carbonyls Carbonic acid Bicarbonates Carbonates Dicarbonates Tricarbonates Inorganic compounds of carbon and related ions Compounds CBr CCl CF CF CI CO CO CO COS CS CS CSe C O SiC Carbon ions Carbides ( : C ≡ C : ), ( : : C : : ), ( : C = C = C : ) Cyanides ( : C ≡ N : ) Cyanates ( : O-C ≡ N : ) Thiocyanates ( : S-C ≡ N : ) Fulminates ( : C ≡ N-O : ) Thiofulminates ( : C ≡ N-S : ) Oxides and related Oxides Metal carbonyls Carbonic acid Bicarbonates Carbonates GND : 4192064 - 8 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Carbonic_acid&oldid=828574700 '' Categories : Mineral acids Carbonates Hidden categories : Articles without UNII source ECHA InfoCard ID from Wikidata Articles containing unverified chemical infoboxes Wikipedia articles with GND identifiers Talk Contents About Wikipedia Afrikaans Azərbaycanca تۆرکجه বাংলা Bân - lâm - gú Беларуская ( тарашкевіца ) ‎ Български Brezhoneg Català Čeština Dansk Deutsch Eesti Ελληνικά Español Esperanto فارسی Français Gaeilge 한국어 Հայերեն हिन्दी Hornjoserbsce Hrvatski Bahasa Indonesia Íslenska Italiano עברית ქართული Latina Latviešu Lëtzebuergesch Lietuvių Magyar मराठी Bahasa Melayu Mìng - dĕ̤ng - ngṳ̄ မြန်မာဘာသာ Nederlands 日本 語 Norsk Norsk nynorsk Plattdüütsch Polski Português Română Русский Scots Slovenčina Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska ไทย Türkçe Українська اردو Vepsän kel ' Tiếng Việt 吴语 粵語 中文 55 more Edit links This page was last edited on 3 March 2018, at 12 : 19. 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Question: what is the pka value of carbonic acid
Answer: 3.6 (pKa1 for H2CO3 only), 6.3 (pKa1 including CO2(aq)), 10.32