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Easy Derivation of pH Is Approximately Equal To (pKa1 + pKa2) / 2 Using Autoprotolysis of HA-: Doubtful Value of the Supposedly More Rigorous Equation

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Stephen J. Hawkes

Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003

J. Chem. Educ., 2000, 77 (9), p 1183

DOI: 10.1021/ed077p1183

Publication Date (Web): September 1, 2000

Section:

History, Education, and Documentation

Abstract

[H⁺]² K_a1K_a2 can be derived in only two lines by recognizing that the principal reaction in a solution of HA^- is its autoprotolysis. The supposedly rigorous equation derived in many texts gives pH values that are no more reliable.

Keywords (Audience):

Second-Year Undergraduate

Keywords (Domain):

Biochemistry

Keywords (Subject):

Acids / Bases

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Citing Articles

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This article has been cited by 2 ACS Journal articles (2 most recent appear below).

Unified Approximations: A New Approach for Monoprotic Weak Acid-Base Equilibria
Harry L. Pardue , Ihab N. Odeh and Teweldemedhin M. Tesfai
Journal of Chemical Education2004 81 (9), 1367
- Unified Approximations: A New Approach for Monoprotic Weak Acid-Base Equilibria
  Harry L. Pardue , Ihab N. Odeh and Teweldemedhin M. Tesfai
  Journal of Chemical Education2004 81 (9), 1367
  This article describes a new approach to approximate calculations for monoprotic acid–base equilibria in otherwise pure water. The new approach, identified herein as unified approximations, uses a simple decision criterion to select between situations ...
  Abstract | Hi-Res PDF | PDF w/ Links
The pH at the First Equivalence Point in the Titration of a Diprotic Acid
Addison Ault
Journal of Chemical Education2003 80 (12), 1382
- The pH at the First Equivalence Point in the Titration of a Diprotic Acid
  Addison Ault
  Journal of Chemical Education2003 80 (12), 1382
  The author argues that the validity of the simple result achieved by an earlier paper can be seen even more easily than its author indicates.
  Abstract | Hi-Res PDF | PDF w/ Links