Editing Buffer solution (section)

==="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.