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pH Paradoxes: Demonstrating That It Is Not True That pH ≡ -log[H+]
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Abstract
Six demonstrations highlighting paradoxes that arise if pH is incorrectly defined as -log[H+] are presented as justification for the recommendation that pH should be correctly defined as pH = -log aH+ in textbooks. For example, when acid with pH ~1 is diluted with an equal volume of 5 M MgCl2, one would expect the pH calculated as -log[H+] to increase as the concentration of acid is halved; surprisingly, it decreases to values below zero, as demonstrated with a pH meter or methyl green indicator. If a sample of the acid at pH ~2, and a second sample, diluted with salt solution so that it has pH ~0.5 are titrated with NaOH solution, equal volumes of base solution are required; but [H+] = antilog(-pH) is 0.01 in the first case and 0.32 in the second, leading to predictions of much different volumes of titrant. We could tolerate an approach to pH calculations that sacrificed reasonable practical answers for a sound theoretical foundation, but our current pedagogy seems to provide neither!
Keywords (Audience):
First-Year Undergraduate / GeneralKeywords (Domain):
DemonstrationsKeywords (Feature):
JCE DigiDemos: Tested DemonstrationsKeywords (Pedagogy):
Textbooks / Reference BooksKeywords (Subject):
Acids / BasesCiting Articles
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This article has been cited by 5 ACS Journal articles (5 most recent appear below).
In Situ Measurement of Reaction Volume and Calculation of pH of Weak Acid Buffer Solutions Under High Pressure
Stephen K. Min, Chaminda P. Samaranayake, and Sudhir K. SastryThe Journal of Physical Chemistry B2011 Article ASAPIn Situ Measurement of Reaction Volume and Calculation of pH of Weak Acid Buffer Solutions Under High Pressure
Stephen K. Min, Chaminda P. Samaranayake, and Sudhir K. SastryThe Journal of Physical Chemistry B2011 Article ASAPDirect measurements of reaction volume, so far, have been limited to atmospheric pressure. This study describes a method for in situ reaction volume measurements under pressure using a variable volume piezometer. Reaction volumes for protonic ionization ...
Potentiometric pH Measurements of Acidity Are Approximations, Some More Useful than Others
Robert de LevieJournal of Chemical Education2010 87 (11), 1188-1194Potentiometric pH Measurements of Acidity Are Approximations, Some More Useful than Others
Robert de LevieJournal of Chemical Education2010 87 (11), 1188-1194A recent article by McCarty and Vitz “demonstrating that it is not true that pH = −log[H+]” is examined critically. Then, the focus shifts to underlying problems with the IUPAC definition of pH. It is shown how the potentiometric method can provide ...
In Situ Measurement of pH Under High Pressure
Chaminda P. Samaranayake and Sudhir K. SastryThe Journal of Physical Chemistry B2010 114 (42), 13326-13332In Situ Measurement of pH Under High Pressure
Chaminda P. Samaranayake and Sudhir K. SastryThe Journal of Physical Chemistry B2010 114 (42), 13326-13332In situ measurement of pH under high pressure is complicated by the need to remain functional under extreme pressures, electrically noisy environments, and the difficulty in inferring chemical equilibrium conditions under pressure. We describe a pH sensor ...
pH Basics
Francesco Scagnolari , Bruno LunelliJournal of Chemical Education2009 86 (2), 246pH Basics
Francesco Scagnolari , Bruno LunelliJournal of Chemical Education2009 86 (2), 246The exposition of the pervasive concept of pH, of its foundations and implementation as a meaningful quantitative measurement, in nonspecialist university texts is often not easy to follow because too many of its theoretical and operative underpinnings ...
Defining and Teaching pH
Richard F. BurtonJournal of Chemical Education2007 84 (7), 1129Defining and Teaching pH
Richard F. BurtonJournal of Chemical Education2007 84 (7), 1129The 1909 definition of pH given in most general chemistry textbooks conflicts with the modern, operationally-defined pH scale that underlies laboratory measurement and relates to activities. At an elementary level, pH and the algebra of equilibria can be ...
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- Received: August 03, 2009
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