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In chemistry, diprotic acid is a class of Arrhenius acids which are capable of donating two protons or hydrogen cations per molecule when dissociating in aqueous solution.[1][2] The most important chemical feature for diprotic acid molecule is its ability to deprotonate two protons in two steps during dissociation. There are nearly kinds of diprotic acid, % of the all known acids, in the world. Most diprotic acids are common acids which are used everyday in many different areas. Also, they present everywhere in the nature, such as human bodies.

General Structure

Sulfuric acid is a strong acid that is completely dissociated in aqueous solution in its first deprotonation process.[3][4]

There are both organic (which is called dicarboxylic acids) and inorganic diprotic acids. Chromic acid (H2CrO4) and sulfuric acid (H2SO4) are common and widely-used inorganic acids. They have similar structures with two -OH groups which are able to donate two H+ ions linked to the center atom.[5] Some other inorganic diprotic acids such as hydrosulfuric acid (H2S) usually have two hydrogen atoms linked to an electronegative center.[6] dicarbonxylic acids have general molecular formula HOOC-R-COOH.[7]

Dissociation and acid-dissociation equilibrium constants

Dissociation in water

See also: acid-dissociation constant

The dissociation equation of a diprotic acid in water can be written as:

H2A (aq) + H2O(l)↔ H3O+ (aq) + HA- (aq) (1)[8]

HA- (aq) + H2O(l)↔ H3O+ (aq) + A2- (aq) (2)[8]

The dissociation of a diprotic acid does not happen all at once due to the two stages of dissociation having different Ka values. Hence, each kind of diprtotic acid has two different acid-dissociation equilibrium constants, Ka1 and Ka2. The corresponding conjugate base of each conjugate acid state also has a different base-dissociation equilibrium constant Kb1 and Kb2, respectively.[9]

The constants Ka1 and Ka2 are defined 25°C in water[10] and can be written as:

[11]

[12]

Ka values for common diprotic aicds

For most Diprotic acids, the value of Ka1 is at least one hundred times larger than the value of Ka2.[13] This is mainly because that more energy is required for a positively charged proton to be removed from HA- with a negative charge in the above equation (2) than from H2A which is electric neutral in the above equation (1). It is also illustrated by Pauling's first rule.[14]

See also: Pauling's rules

Acid-Dissociation Equilibrium Constants for Common diprotic Acids[15][16]
Acid Ka1 Ka2
sulfuric acid (H2SO4) 1.0 x 103 1.2 x 10-2
chromic acid (H2CrO4) 9.6 3.2 x 10-7
oxalic acid (H2C2O4) 5.4 x 10-2 5.4 x 10-5
sulfurous acid (H2SO3) 1.7 x 10-2 6.4 x 10-8
glycine (C2H6NO2) 4.5 x 10-3 2.5 x 10-10
carbonic acid (H2CO3) 4.5 x 10-7 4.7 x 10-11
hydrogen sulfide (H2S) 1.0 x 10-7 1.3 x 10-13
malonic acid (H2C3H2O4) 1.5 x 10-3 2.0 x 10-6

pH calculation

Solution of H2A

For a solution prepared by adding dipotic acid with initial concentration [H2A]=F dissociating in water,the pH value at equilibrium can be calculated by simply treating as a monoprotic acid if Ka1 is greater than Ka2 by a factor of 103 or larger.[17]Then we can apply the equation (1):

(1）

For [H+]=[HA-] at equilibrium, we can assume that [H+]=[HA-]=x and then the equation (1) can be written as:

Note if the diprotic acid is a weak acid which has a Ka1 value ≤10-3, the equation above can be further simplified as:

By solving for x, we obtain the final concentration of the hydrogen ion of the solution. Finally, we can calculate the pH value by substituting x=[H+] into pH definition[18]:

Diprotic Buffers

see also: Henderson–Hasselbalch equation

A buffer made by a diprotic acid and its conjugate base can be treated in the same way as a buffer made by a monoprotic acid.[19] We can write two Henderson–Hasselbalch equations:

This is an ideal titration curve for alanine, a diprotic amino acid[20]. Point 2 is the first equivalent point where the amount of NaOH added equals the amount of alanine in the original solution.

Depending on the quantity we know, we can either substituting [HA-] and [H2A] or [HA-] and [A-] to calculate the pH value of the buffer.[21]

Titration

To determine the concentration of a diprotic acid in an aqueous solution, an acid-base titration is commonly performed. A strong base, usually NaOH, with known concentration is added to neutralize the acid solution. [22]

Titration curves

A titration curve of a diprotic acid titrated by a base has two axis, with the base volume on the x-axis and solution pH on the y-axis.

Equivalence points

Due to the successive dissociation processes, there are two equivalence points in the titration curve of a diprotic acid.[23] The first equivalence point appears at , at either equivalence point, a drop of base added will cause a steep rise of the pH value in the system.

Buffer regions and mid points

Diprotic acids in daily life

In human bodies

In industry

Sulfuric acid is the most widely used acid in industry, which is also the most-produced industrial chemical in the world. It is mainly used in producing fertilizes, detergent, batteries and dyes, as well as used in processing many products such like removing impurities.[24] According to the statistics data in 2011, the annual production of sulfuric acid was around 200 million tonnes in the world.[25]

In food

Carnonated water(H2CO3 aqueous solution) is one of the main ingredients listed the ingredient sheet of a can of Coca-Cola.

Many diprotic acids can be found in various kinds of food which is considered to be slightly acidic (pH<7).Some of the acids naturally existed in food and others are artificial addictive. Carbonic acid is one of the most common diprotic acids addictive that is widely added in soft drinks, such as Coca-cola. During the manufacturing process of soft drinks, CO2 is usually pressurized to dissolve in these drinks to generate carbonic acid. Carbonic acid is very unstable and tend to decompose into water and CO2 in normal temperature and pressure. Therefore, when we open the bottles or cans of the soft drinks, CO2 bubbles come out and thus we feel 'sparks'.[26]

See also

References

  1. ^ "Diprotic Acid Definition". About.com Education. Retrieved 2016-01-22.
  2. ^ "What is a Diprotic Acid?". wiseGEEK. Retrieved 2016-01-23.
  3. ^ Choe, Yoong-Kee; Tsuchida, Eiji; Ikeshoji, Tamio (2007-04-21). "First-principles molecular dynamics study on aqueous sulfuric acid solutions". The Journal of Chemical Physics. 126 (15): 154510. doi:10.1063/1.2718526. ISSN 0021-9606.
  4. ^ Atkins, Peter; Jones, Loretta (2007-08-01). Chemical Principles: The Quest for Insight. Macmillan. ISBN 9781429209656.
  5. ^ Kuczkowski, Robert L.; Suenram, R. D.; Lovas, Frank J. "Microwave spectrum, structure, and dipole moment of sulfuric acid". Journal of the American Chemical Society. 103 (10): 2561–2566. doi:10.1021/ja00400a013.
  6. ^ "CHEMINFO: Hydrogen sulfide". www.ccohs.ca. Retrieved 2016-01-23.
  7. ^ L., Leiserowitz,; IUCr (1976-03-15). "Molecular packing modes. Carboxylic acids". scripts.iucr.org. doi:10.1107/S0567740876003968. Retrieved 2016-01-23.{{cite web}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  8. ^ a b "Chemistry Tutorial : Polyprotic Weak Acid Concepts". www.ausetute.com.au. Retrieved 2016-01-23.
  9. ^ "Acids/Bases - MCAT Review". mcat-review.org. Retrieved 2016-01-23.
  10. ^ hriver, D.F; Atkins, P.W. (1999). Inorganic Chemistry (3rd ed.). Oxford: Oxford University Press. ISBN 0-19-850331-8. Chapter 5: Acids and Bases.
  11. ^ "pH of polyprotic acid/base solution". www.chembuddy.com. Retrieved 2016-01-23.
  12. ^ "Polyprotic Acids & Bases - Chemwiki". chemwiki.ucdavis.edu. Retrieved 2016-01-23.
  13. ^ Rhee, Jae Seong; Dasgupta, Purnendu K. (2002-05-01). "The second dissociation constant of sulfur dioxide.water". The Journal of Physical Chemistry. 89 (9): 1799–1804. doi:10.1021/j100255a052.
  14. ^ Greenwood, N.N. (1997). Chemistry of the Elements. Oxford: Butterworth-Heinemann. p. 50. ISBN 0-7506-3365-4.
  15. ^ "Polyprotic Acids And Bases - Chemwiki". chemwiki.ucdavis.edu. Retrieved 2016-01-23.
  16. ^ "Diprotic and Triprotic Acids and Bases". chemed.chem.purdue.edu. Retrieved 2016-01-23.
  17. ^ King, D. Whitney; Kester, Dana R. "A general approach for calculating polyprotic acid speciation and buffer capacity". Journal of Chemical Education. 67 (11). doi:10.1021/ed067p932.
  18. ^ Covington, A. K.; Bates; Durst (1985). Definitions of pH scales, standard reference values, measurement of pH, and related terminology (PDF). Vol. 57. http://www.iupac.org/publications/pac/1985/pdf/5703x0531.pdf. pp. 531–542. doi:10.1351/pac198557030531. {{cite book}}: External link in |location= (help)CS1 maint: location (link) CS1 maint: location missing publisher (link)
  19. ^ Harris, Daniel C. (2010-04-30). Quantitative Chemical Analysis. Macmillan. p. 193. ISBN 9781429218153.
  20. ^ Jameson, Reginald F. "Assignment of the proton-association constants for 3-(3,4-dihydroxyphenyl)alanine (L-dopa)". pubs.rsc.org. doi:10.1039/DT9780000043. Retrieved 2016-01-29.
  21. ^ Harris, Daniel C. (2003-01-01). Quantitative Chemical Analysis, Sixth Edition. Macmillan. p. 212. ISBN 9780716744641.
  22. ^ Robert De, Levie (1999). Aqueous Acid-Base Equilibria and Titrations. New York: Oxford University Press.
  23. ^ "Titration of Diprotic Acid". dwb.unl.edu. Retrieved 2016-01-24.
  24. ^ "The Top 10 Industrial Chemicals - For Dummies". www.dummies.com. Retrieved 2016-02-05.
  25. ^ "Sulfuric acid". www.essentialchemicalindustry.org. Retrieved 2016-02-06.
  26. ^ McMillin, John R.; Tracy, Gene A.; Harvill, William A.; Jr, William S. Credle (Dec 8, 1981), Method of and apparatus for making and dispensing a carbonated beverage utilizing propellant carbon dioxide gas for carbonating, retrieved 2016-02-06

•Bodner Research Web:Diprotic and Triprotic Acids and Bases

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