Editing Acid (section)

=== Example: Diprotic acid ===
[[File:Titration alanine.jpg|thumb|This is an ideal titration curve for [[alanine]], a diprotic amino acid.<ref>{{Cite journal|title = Assignment of the proton-association constants for 3-(3,4-dihydroxyphenyl)alanine (L-dopa)|journal = Journal of the Chemical Society, Dalton Transactions|volume = |issue = 1|pages = 43–45|doi = 10.1039/DT9780000043|language = en|first = Reginald F.|last = Jameson|year = 1978}}</ref> Point 2 is the first equivalent point where the amount of NaOH added equals the amount of alanine in the original solution.]]
For each diprotic acid titration curve, from left to right, there are two midpoints, two equivalence points, and two buffer regions.<ref>{{Cite book|title = Ion Exchange|url = https://books.google.com/books?id=F9OQMEA88CAC|publisher = Courier Corporation|date = 1962-01-01|isbn = 9780486687841|language = en|first = Friedrich G.|last = Helfferich}}</ref>

==== Equivalence points ====
Due to the successive dissociation processes, there are two equivalence points in the titration curve of a diprotic acid.<ref>{{Cite web|title = Titration of Diprotic Acid|url = http://dwb.unl.edu/calculators/activities/diproticacid.html|website = dwb.unl.edu |access-date = 2016-01-24|archive-url = https://web.archive.org/web/20160207011433/http://dwb.unl.edu/calculators/activities/diproticacid.html|archive-date = 7 February 2016|url-status = dead}}</ref> The first equivalence point occurs when all first hydrogen ions from the first ionization are titrated.<ref name = learning>{{Cite book|title = Chemistry & Chemical Reactivity |url = https://books.google.com/books?id=i1g8AwAAQBAJ|publisher = Cengage Learning|date = 2014-01-24|isbn = 9781305176461|language = en|first1 = John C.|last1 = Kotz|first2 = Paul M.|last2 = Treichel|first3 = John|last3 = Townsend|first4 = David|last4 = Treichel}}</ref> In other words, the amount of OH<sup>−</sup> added equals the original amount of H<sub>2</sub>A at the first equivalence point. The second equivalence point occurs when all hydrogen ions are titrated. Therefore, the amount of OH<sup>−</sup> added equals twice the amount of H<sub>2</sub>A at this time. For a weak diprotic acid titrated by a strong base, the second equivalence point must occur at pH above 7 due to the hydrolysis of the resulted salts in the solution.<ref name = learning/> At either equivalence point, adding a drop of base will cause the steepest rise of the pH value in the system.

==== Buffer regions and midpoints ====
A titration curve for a diprotic acid contains two midpoints where pH=pK<sub>a</sub>. Since there are two different K<sub>a</sub> values, the first midpoint occurs at pH=pK<sub>a1</sub> and the second one occurs at pH=pK<sub>a2</sub>.<ref>{{Cite book|title = Lehninger Principles of Biochemistry|url = https://books.google.com/books?id=7chAN0UY0LYC|publisher = Macmillan|date = 2005-01-01|isbn = 9780716743392|language = en|first1 = Albert L.|last1 = Lehninger|first2 = David L.|last2 = Nelson|first3 = Michael M.|last3 = Cox}}</ref> Each segment of the curve that contains a midpoint at its center is called the buffer region. Because the buffer regions consist of the acid and its conjugate base, it can resist pH changes when base is added until the next equivalent points.<ref name="Ebbing">{{Cite book|title = General Chemistry|url = https://books.google.com/books?id=BnccCgAAQBAJ|publisher = Cengage Learning|date = 2016-01-01|isbn = 9781305887299|language = en|first1 = Darrell|last1 = Ebbing|first2 = Steven D.|last2 = Gammon|edition=11th}}</ref>