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pH Paradoxes: Demonstrating That It Is Not True That pH ≡ -log[H+]
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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:
First-Year Undergraduate / GeneralKeywords:
DemonstrationsKeywords:
JCE DigiDemos: Tested DemonstrationsKeywords:
Textbooks / Reference BooksKeywords:
Acids / BasesCiting Articles
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This article has been cited by 2 ACS Journal articles (2 most recent appear below).
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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