H2CO3

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carbonic acid

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/02/20 06:34:40」(JST)

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炭酸
IUPAC名 Carbonic acid 炭酸
別称 二酸化炭素溶液 Dihydrogen carbonate acid of air Aerial acid Hydroxymethanoic acid
識別情報
CAS登録番号	463-79-6
ChemSpider	747
KEGG	C01353
ChEMBL	CHEMBL1161632
SMILES O=C(O)O
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
特性
化学式	H2CO3
モル質量	62.03 g/mol
密度	1.0 g/cm3 (希薄溶液)
融点	n/a
水への溶解度	溶液中にのみ存在
酸解離定数 pKa	6.352 (pKa1)
特記なき場合、データは常温 (25 °C)・常圧 (100 kPa) におけるものである。

炭酸（たんさん、英: carbonic acid）は、化学式 H₂CO₃ で表される炭素のオキソ酸であり弱酸の一種である。

性質

普通は水溶液（炭酸水）中のみに存在し、水に二酸化炭素を溶解（炭酸飽和）することで生じる。

水に溶解した二酸化炭素の一部は水分子の付加により炭酸となる。

この反応の平衡定数 (K_h) は 25 °Cで 1.7 × 10⁻³ であり、著しく左に偏っているため水溶液中の二酸化炭素の大部分は CO₂ 分子として存在する。触媒が存在しない場合、二酸化炭素と炭酸の間の反応が平衡に達する速度は低く、正反応の速度定数は 0.039 s⁻¹、逆反応の速度定数は 23 s⁻¹ である。

二酸化炭素と炭酸の平衡は体液の酸性度を調節する上で非常に重要であり、ほとんどの生物はこれら2つの化合物を変換させるための炭酸脱水酵素を持っている。この酵素は反応速度をおよそ10億倍にする。

炭酸は水溶液中で2段階の解離を起こす。25 ℃における酸解離定数は1段階目が pK_a1 = 3.60、2段階目が pK_a2 = 10.25 であり、炭酸は真の解離定数において酢酸よりも強い酸であるが、上記の二酸化炭素との平衡が存在するために、見かけ上の pKa* が高い非常に弱い酸である。このため炭酸塩は相応の塩基性を示し、灰汁として古代より日常生活のアルカリとして洗浄などに活用されてきた。

酸解離に関する標準エンタルピー変化、ギブス自由エネルギー変化、エントロピー変化の値が報告されており、解離に伴いエントロピーの減少がおこるのは、電荷の増加に伴いイオンの水和の程度が増加し、電縮が起こり水分子の水素結合による秩序化の度合いが増加するからである。この値は以下の平衡に対するものでpK_a1*は見かけの酸解離定数である。

水酸化ナトリウム水溶液による中和滴定曲線

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,　　　

第一解離	7.64 kJ mol−1	36.34 kJ mol−1	−96.3 J mol−1K−1	−377 J mol−1K−1
第二解離	14.85 kJ mol−1	58.96 kJ mol−1	−148.1 J mol−1K−1	−272 J mol−1K−1

炭酸の不安定性

長い間、炭酸そのものを室温で単離することは不可能だと考えられていた。しかし、1991年にNASA・ゴダード宇宙飛行センターの科学者が初めて純粋な H₂CO₃ を作り出すことに成功した。彼らは凍結させた水と二酸化炭素に高エネルギーの放射線を照射したのち、加温して余分な水を取り除くことにより単離を行った。得られた炭酸の構造は赤外分光法によって検討された。宇宙空間には水や二酸化炭素の氷が普通に存在することから、この実験結果は宇宙線や紫外線によってそれらが反応することで生成した炭酸も宇宙空間には存在する可能性があることを示唆している。

理論計算によって、水が1分子でも存在すると炭酸はすぐに二酸化炭素と水に戻ってしまうが、水を含まない純粋な炭酸は気体状態で安定であることが示されており、その半減期はおよそ18万年であると見積もられている。

炭酸と雨水

大気中の二酸化炭素 (0.033 %) が溶け込んだ水の pH は 5.6 である。通常の雨水は二酸化炭素で飽和状態になってはいないため、大気汚染物質がなければその pH は 6 前後である。これは二酸化硫黄などの工業廃棄物によって雨水の pH が激しく低下する酸性雨現象とは異なる。しかし、雨の酸性度はチョークや石灰岩などの炭酸塩鉱物に関する重要な地質学的問題である。岩石に含まれる炭酸カルシウムと炭酸水素カルシウムの間には、以下のような溶液中での平衡が成り立っている。

これにより、水が入りこんだ断層線付近の地下洞窟が浸食されることがある。カルシウムを多く含んだ水が蒸発すると炭酸カルシウムが沈殿し、しばしば鍾乳石や石筍を形成する。チョークからなる帯水層からくみ上げられた水は多量の炭酸カルシウムが溶解しており、「硬水」と呼ばれる。

脚注

参考文献

関連項目

• オルト炭酸
• 炭酸塩

外部リンク

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Not to be confused with Carbolic acid, an antiquated name for phenol.

Carbonic acid is also an archaic name for carbon dioxide.

Carbonic acid

Names
IUPAC name Carbonic acid
Other names Carbon dioxide solution; Dihydrogen carbonate; acid of air; Aerial acid; Hydroxymethanoic acid
Identifiers
CAS number	463-79-6 Y
ChEBI	CHEBI:28976 Y
ChEMBL	ChEMBL1161632 Y
ChemSpider	747 Y
InChI InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Y Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N Y InChI=1/CH2O3/c2-1(3)4/h(H2,2,3,4) Key: BVKZGUZCCUSVTD-UHFFFAOYAU
Jmol-3D images	Image
KEGG	C01353 Y
SMILES O=C(O)O
Properties
Molecular formula	H2CO3
Molar mass	62.03 g/mol
Density	1.668 g/cm3
Solubility in water	Exists only in solution
Acidity (pKa)	3.6 (pKa1 for H2CO3 only), 6.3 (pKa1 including CO2(aq)), 10.32 (pKa2)
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Y verify (what is: Y/N?)
Infobox references

Carbonic acid is the chemical compound with the chemical formula H₂CO₃ (equivalently OC(OH)₂). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H₂CO₃. 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.

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.

Chemical equilibrium

When carbon dioxide dissolves in water it exists in chemical equilibrium producing carbonic acid:

CO₂ + H₂O H₂CO₃

The hydration equilibrium constant at 25°C is called K_h, which in the case of carbonic acid is [H₂CO₃]/[CO₂] ≈ 1.7×10⁻³ in pure water and ≈ 1.2×10⁻³ in seawater. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO₂ molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s⁻¹ for the forward reaction (CO₂ + H₂O → H₂CO₃) and 23 s⁻¹ for the reverse reaction (H₂CO₃ → CO₂ + H₂O). Carbonic acid is used in the making of soft drinks, inexpensive and artificially carbonated sparkling wines, and other bubbly drinks. The addition of two molecules of water to CO₂ would give orthocarbonic acid, C(OH)₄, which exists only in minute amounts in aqueous solution.

Addition of base to an excess of carbonic acid gives bicarbonate. With excess base, carbonic acid reacts to give carbonate salts.

Role of carbonic acid in blood

Carbonic acid is an intermediate step in the transport of CO₂ out of the body via respiratory gas exchange. The hydration reaction of CO₂ is generally very slow in the absence of a catalyst, but red blood cells contain carbonic anhydrase, which both increases the reaction rate and dissociates a hydrogen ion (H⁺) from the resulting carbonic acid, leaving bicarbonate (HCO₃⁻) dissolved in the blood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO₂ and allows it to be expelled. This equilibration plays an important role as a buffer in mammalian blood.

Role of carbonic acid in ocean chemistry

The oceans of the world have absorbed almost half of the CO₂ emitted by humans from the burning of fossil fuels.  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 process is known as ocean acidification.

Acidity of carbonic acid

Carbonic acid is one of the polyprotic acids: It is diprotic - it has two protons, which may dissociate from the parent molecule. Thus, there are two dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion HCO₃⁻:

H₂CO₃ HCO₃⁻ + H⁺

K_a1 = 2.5×10⁻⁴; pK_a1 = 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 H₂CO₃ is much lower than the concentration of CO₂. In many analyses, H₂CO₃ includes dissolved CO₂ (referred to as CO₂(aq)), H₂CO₃* is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows:

H₂CO₃* HCO₃⁻ + H⁺

K_a(app) = 4.6×10⁻⁷; pK(app) = 6.3 at 25 °C and ionic strength = 0.0

Whereas this apparent pK_a 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 CO₂-containing solutions. A similar situation applies to sulfurous acid (H₂SO₃), 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 CO₃²⁻:

HCO₃⁻ CO₃²⁻ + H⁺

K_a2 = 4.69×10⁻¹¹; pK_a2 = 10.329 at 25 °C and ionic strength = 0.0

The three acidity constants are defined as follows:

pH and composition of carbonic acid solutions

At a given temperature, the composition of a pure carbonic acid solution (or of a pure CO₂ solution) is completely determined by the partial pressure of carbon dioxide above the solution. To calculate this composition, account must be taken of the above equilibria between the three different carbonate forms (H₂CO₃, HCO₃⁻ and CO₃²⁻) as well as of the hydration equilibrium between dissolved CO₂ and H₂CO₃ with constant (see above) and of the following equilibrium between the dissolved CO₂ and the gaseous CO₂ above the solution:

CO₂(gas) CO₂(dissolved) with where k_H=29.76 atm/(mol/L) at 25 °C (Henry constant)

The corresponding equilibrium equations together with the relation and the charge neutrality condition result in six equations for the six unknowns [CO₂], [H₂CO₃], [H⁺], [OH⁻], [HCO₃⁻] and [CO₃²⁻], showing that the composition of the solution is fully determined by . The equation obtained for [H⁺] is a cubic whose numerical solution yields the following values for the pH and the different species concentrations:

(atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3−] (mol/L)	[CO32−] (mol/L)
1.0 × 10−8	7.00	3.36 × 10−10	5.71 × 10−13	1.42 × 10−09	7.90 × 10−13
1.0 × 10−7	6.94	3.36 × 10−09	5.71 × 10−12	5.90 × 10−09	1.90 × 10−12
1.0 × 10−6	6.81	3.36 × 10−08	5.71 × 10−11	9.16 × 10−08	3.30 × 10−11
1.0 × 10−5	6.42	3.36 × 10−07	5.71 × 10−10	3.78 × 10−07	4.53 × 10−11
1.0 × 10−4	5.92	3.36 × 10−06	5.71 × 10−09	1.19 × 10−06	5.57 × 10−11
3.5 × 10−4	5.65	1.18 × 10−05	2.00 × 10−08	2.23 × 10−06	5.60 × 10−11
1.0 × 10−3	5.42	3.36 × 10−05	5.71 × 10−08	3.78 × 10−06	5.61 × 10−11
1.0 × 10−2	4.92	3.36 × 10−04	5.71 × 10−07	1.19 × 10−05	5.61 × 10−11
1.0 × 10−1	4.42	3.36 × 10−03	5.71 × 10−06	3.78 × 10−05	5.61 × 10−11
1.0 × 10+0	3.92	3.36 × 10−02	5.71 × 10−05	1.20 × 10−04	5.61 × 10−11
2.5 × 10+0	3.72	8.40 × 10−02	1.43 × 10−04	1.89 × 10−04	5.61 × 10−11
1.0 × 10+1	3.42	3.36 × 10−01	5.71 × 10−04	3.78 × 10−04	5.61 × 10−11

• We see that in the total range of pressure, the pH is always largely lower than pKa₂ so that the CO₃²⁻ concentration is always negligible with respect to HCO₃⁻ concentration. In fact CO₃²⁻ plays no quantitative role in the present calculation (see remark below).
• For vanishing , the pH is close to the one of pure water (pH = 7) and the dissolved carbon is essentially in the HCO₃⁻ form.
• For normal atmospheric conditions ( atm), we get a slightly acid solution (pH = 5.7) and the dissolved carbon is now essentially in the CO₂ 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 HCO₃⁻.
• For a CO₂ pressure typical of the one in soda drink bottles ( ~ 2.5 atm), we get a relatively acid medium (pH = 3.7) with a high concentration of dissolved CO₂. These features contribute to the sour and sparkling taste of these drinks.
• Between 2.5 and 10 atm, the pH crosses the pKa₁ value (3.60) giving a dominant H₂CO₃ concentration (with respect to HCO₃⁻) 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, [CO₃²⁻] may be neglected for this specific problem, resulting in the following very precise analytical expression for [H⁺]:

Spectroscopic studies of carbonic acid

Theoretical calculations show that the presence of even a single molecule of water causes carbonic acid to revert to carbon dioxide and water. In the absence of water, the dissociation of gaseous carbonic acid is predicted to be very slow, with a half-life of 180,000 years. This may only apply to isolated carbonic acid molecules however, as it has been predicted to catalyze its own decomposition

It has long been recognized that pure carbonic acid cannot be obtained at room temperatures (about 20 °C or about 70 °F). It can be generated by exposing a frozen mixture of water and carbon dioxide to high-energy 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 H₂O + CO₂ mixture may suggest that H₂CO₃ might be found in outer space, where frozen ices of H₂O and CO₂ are common, as are cosmic rays and ultraviolet light, to help them react. The same carbonic acid polymorph (denoted beta-carbonic acid) was prepared by heating alternating layers of glassy aqueous solutions of bicarbonate and acid in vacuo, which causes protonation of bicarbonate, followed by removal of the solvent. The previously suggested alpha-carbonic acid, which was prepared by the same technique using methanol rather than water as a solvent was shown to be a monomethyl ester CH₃OCOOH.

See also

• Carbonated water
• Ocean acidification
• Carbon dioxide
• Dihydroxymethylidene (carbonous acid)
• Nonvolatile acid

References

Further reading

• 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. (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. 
• 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

UpToDate Contents

全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.

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• 2. Chapter 11A：腎臓での水排泄 chapter 11a renal hydrogen excretion
• 3. 乳酸アシドーシスにおける重炭酸療法 bicarbonate therapy in lactic acidosis
• 4. 胎児の酸塩基の生理学 fetal acid base physiology
• 5. Chapter 10A：酸と塩基 chapter 10a acids and bases

English Journal

• Spectroscopic detection of the most stable carbonic acid, cis-cis H2CO3.

• Mori T, Suma K, Sumiyoshi Y, Endo Y.SourceDepartment of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan.
• The Journal of chemical physics.J Chem Phys.2011 Jan 28;134(4):044319.
• Carbonic acid had not been detected by any spectroscopic means for a long period. Recently, we have reported the detection of its second most stable conformer, cis-trans H(2)CO(3), as the first spectroscopic detection of the isolated carbonic acid molecule. In the present work, the most stable confo
• PMID 21280737

• H2CO3 forms via HCO3- in water.

• Stirling A, Papai I.SourceChemical Research Center of the Hungarian Academy of Sciences, Budapest, Hungary.
• The journal of physical chemistry. B.J Phys Chem B.2010 Dec 23;114(50):16854-9. Epub 2010 Nov 29.
• According to the generally accepted picture of CO(2) dissolution in water, the formation of H(2)CO(3) proceeds in a single step that involves the attack of a water oxygen on the CO(2) carbon in concert with a proton transfer to a CO(2) oxygen. In the present work, a series of ab initio molecular dyn
• PMID 21114307

Japanese Journal

• Asymmetric Borane Reduction of Prochiral Ketone Using Chiral Bis(α,α-diphenyl-2-pyrrolidinemethanol) Carbonate

• YANAGI Takashi,KIKUCHI Ken,TAKEUCHI Hideki,ISHIKAWA Takehiro,NISHIMURA Toshihiro,KUBOTA Minoru,YAMAMOTO Iwao
• Chemical & pharmaceutical bulletin 51(2), 221-223, 2003-02-01
• Chiral bis(α,α-diphenyl-2-phyrrolidinemethanol) carbonate (DPP_2・H_2CO_3) is a useful asymmetric auxiliary for the asymmetric borane reduction of prochiral ketones. Chiral DPP_2・H_2CO_3 is recoverable …
• NAID 110003615270

• Change of Structure of Porous Hydrated Calcium Silicate by Dissolution in Paddy Soil

• Saigusa Masahiko,Yamamoto Akiko,Shibuya Kyoichi
• Soil science and plant nutrition 46(1), 89-95, 2000-03
• Change of the structure of porous hydrated calcium silicate material (PS) by dissolution both in paddy soil and under laboratory conditions was studied using a X-ray diffractometer and a scanning elec …
• NAID 110001717839

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