Editing Buffer solution (section)

==Applications of buffers==
The pH of a solution containing a buffering agent can only vary within a narrow range, regardless of what else may be present in the solution. In biological systems this is an essential condition for [[enzyme]]s to function correctly. For example, in [[Blood|human blood]] a mixture of [[carbonic acid]] (H{{su|b=2}}CO{{su|b=3}}) and [[bicarbonate]] (HCO{{su|b=3|p=−}}) is present in the [[blood plasma|plasma]] fraction;  this constitutes the major mechanism for maintaining the pH of blood between 7.35 and 7.45. Outside this narrow range (7.40 ± 0.05 pH unit), [[acidosis]] and [[alkalosis]] metabolic conditions rapidly develop, ultimately leading to death if the correct buffering capacity is not rapidly restored. 

If the pH value of a solution rises or falls too much, the effectiveness of an enzyme decreases in a process, known as [[denaturation (biochemistry)|denaturation]], which is usually irreversible.<ref name="Scorpio 2000">{{cite book |title=Fundamentals of Acids, Bases, Buffers & Their Application to Biochemical Systems |last=Scorpio |first=R. |year=2000 |publisher=Kendall/Hunt Publishing Company |isbn=978-0-7872-7374-3}}</ref> The majority of biological samples that are used in research are kept in a buffer solution, often [[phosphate buffered saline]] (PBS) at pH&nbsp;7.4.

In industry, buffering agents are used in [[fermentation (biochemistry)|fermentation]] processes and in setting the correct conditions for dyes used in colouring fabrics. They are also used in chemical analysis<ref name=Hulanicki>{{cite book |last= Hulanicki |first= A. |title= Reactions of acids and bases in analytical chemistry |publisher= Horwood |year= 1987 |isbn=978-0-85312-330-9 |translator-first= Mary R.|translator-last= Masson}}</ref> and calibration of [[pH&nbsp;meter]]s.

===Simple buffering agents===
:{| class="wikitable"
! Buffering agent !! p''K''<sub>a</sub> !! Useful pH range
|-
| [[Citric acid]] || 3.13, 4.76, 6.40 || 2.1–7.4
|-
| [[Acetic acid]] || 4.8 || 3.8–5.8
|-
| [[potassium dihydrogenphosphate|KH<sub>2</sub>PO<sub>4</sub>]] || 7.2 || 6.2–8.2
|-
| [[N-Cyclohexyl-2-aminoethanesulfonic acid|CHES]] || 9.3 || 8.3–10.3
|-
| [[Borate]] || 9.24 || 8.25–10.25
|}
For buffers in acid regions, the pH may be adjusted to a desired value by adding a strong acid such as [[hydrochloric acid]] to the particular buffering agent. For alkaline buffers, a strong base such as [[sodium hydroxide]] may be added. Alternatively, a buffer mixture can be made from a mixture of an acid and its conjugate base. For example, an acetate buffer can be made from a mixture of acetic acid and [[sodium acetate]]. Similarly, an alkaline buffer can be made from a mixture of the base and its conjugate acid.

==="Universal" buffer mixtures===
By combining substances with p''K''<sub>a</sub> values differing by only two or less and adjusting the pH, a wide range of buffers can be obtained. [[Citric acid]] is a useful component of a buffer mixture because it has three p''K''<sub>a</sub> values, separated by less than two. The buffer range can be extended by adding other buffering agents. The following mixtures ([[McIlvaine's buffer]] solutions) have a buffer range of pH&nbsp;3 to 8.<ref>{{cite journal |last=McIlvaine |first=T. C. |year=1921 |title=A buffer solution for colorimetric comparaison |journal=J. Biol. Chem. |volume=49 |pages=183–186 |url=http://www.jbc.org/content/49/1/183.full.pdf |issue=1 |doi=10.1016/S0021-9258(18)86000-8 |url-status=live |archive-url=https://web.archive.org/web/20150226111238/http://www.jbc.org/content/49/1/183.full.pdf |archive-date=2015-02-26|doi-access=free }}</ref>
:{| class="wikitable"
! 0.2&nbsp;M [[Disodium phosphate|Na<sub>2</sub>HPO<sub>4</sub>]] (mL)
! 0.1&nbsp;M [[citric acid]] (mL)
! pH
|-
| 20.55
| 79.45
| style="background:#ff0000; color:white" | 3.0
|-
| 38.55
| 61.45
| style="background:#ff7777; color:white" |4.0
|-
| 51.50
| 48.50
| style="background:#ff7700;" | 5.0
|-
| 63.15
| 36.85
| style="background:#ffff00;" |6.0
|-
| 82.35
| 17.65
| style="background:#007777; color:white" | 7.0
|-
| 97.25
| 2.75
|style="background:#0077ff; color:white" | 8.0
|}

A mixture containing [[citric acid]], [[monopotassium phosphate]], [[boric acid]], and [[Barbital|diethyl barbituric acid]] can be made to cover the pH range 2.6 to 12.<ref>{{cite book |title=Vogel's textbook of quantitative chemical analysis |last1=Mendham |first1=J. |last2=Denny|first2= R. C. |last3=Barnes |first3=J. D. |last4=Thomas|first4= M. |edition=5th|year=2000 |publisher=Pearson Education |location=Harlow |isbn=978-0-582-22628-9|chapter= Appendix 5}}</ref>

Other universal buffers are the Carmody buffer<ref name=carmody>{{cite journal |last=Carmody |first=Walter R. |title=Easily prepared wide range buffer series |journal=J. Chem. Educ. |year=1961 |volume=38 |issue=11 |pages=559–560 |doi=10.1021/ed038p559 |bibcode = 1961JChEd..38..559C }}</ref> and the [[Britton–Robinson buffer]], developed in 1931.

===Common buffer compounds used in biology===
For effective range see [[#buffer capacity|Buffer capacity]], above. Also see [[Good's buffers]] for the historic design principles and favourable properties of these buffer substances in biochemical applications. 
{| class="wikitable"
|-
! Common name (chemical name)
! Structure
! [[Acid dissociation constant|p''K''<sub>a</sub>]], <br/>25&nbsp;°C
! {{abbr|Temp.|Temperature}} effect, <br/>{{sfrac|dpH|d''T''}} (K<sup>−1</sup>)<ref>{{Cite web |title=Buffer Reference Center |url=http://www.sigmaaldrich.com/life-science/core-bioreagents/biological-buffers/learning-center/buffer-reference-center.html |publisher=Sigma-Aldrich |access-date=2009-04-17 |url-status=live |archive-url=https://web.archive.org/web/20090417003507/http://www.sigmaaldrich.com/life-science/core-bioreagents/biological-buffers/learning-center/buffer-reference-center.html |archive-date=2009-04-17 }}</ref>
! [[Molecular weight|Mol. <br/>weight]] 
|-
| [[TAPS (buffer)|TAPS]], <br/>([tris(hydroxymethyl)methylamino]propanesulfonic acid)
| [[file:TAPS.svg|200px]] || 8.43 || −0.018 || 243.3
|-
| [[Bicine]], <br/>(2-(bis(2-hydroxyethyl)amino)acetic acid)
| [[file:Bicine.png|150px]] || 8.35 || −0.018 || 163.2
|-
| [[Tris]], <br/>(tris(hydroxymethyl)aminomethane, or <br/>2-amino-2-(hydroxymethyl)propane-1,3-diol)
| [[file:tris.png|100px]] || 8.07{{efn|text=Tris is a base, the p''K''<sub>a</sub> = 8.07 refers to its conjugate acid.}} || −0.028 || 121.14
|-
| [[Tricine]], <br/>(N-[tris(hydroxymethyl)methyl]glycine)
||[[file:Tricine.svg|150px]] || 8.05 || −0.021 || 179.2
|-
| [[TAPSO (buffer)|TAPSO]], <br/>(3-[N-tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid)
||[[file:TAPSO.svg|200px]] || 7.635 ||  || 259.3
|-
| [[HEPES]], <br/>(4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)
| [[file: HEPES.png|200px]] || 7.48 || −0.014 || 238.3
|-
| [[TES (buffer)|TES]], <br/><nowiki>(2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid)</nowiki>
| [[file:TES free acid.svg|200px]] || 7.40 || −0.020 || 229.20
|-
| [[MOPS]], <br/>(3-(N-morpholino)propanesulfonic acid)
| [[file:Mops is.svg|150px]] || 7.20 || −0.015 || 209.3
|-
| [[PIPES]], <br/>(piperazine-N,N′-bis(2-ethanesulfonic acid))
| [[file:PIPES.svg|200px]] || 6.76 || −0.008 || 302.4
|-
| [[Cacodylate]], <br/>(dimethylarsenic acid)
| [[file:Cacodylic acid.svg|100px]] || 6.27 ||  || 138.0
|-
| [[MES (buffer)|MES]], <br/>(2-(N-morpholino)ethanesulfonic acid)
| [[file:MES.svg|150px]] || 6.15 || −0.011 || 195.2
|}
{{noteslist}}