Lesson plan
Acidity function
An acidity function is a measure of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to (or accept protons from) a solute (Brønsted acidity). The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions. Other acidity functions have been proposed for different environments, most notably the Hammett acidity function, H 0 , for superacid media and its modified version H − for superbasic media. The term acidity function is also used for measurements made on basic systems, and the term basicity function is uncommon. Hammett-type acidity functions are defined in terms of a buffered medium containing a weak base B and its conjugate acid BH + : :: H_0 = {\rm p}K_{\rm a} + \log {{c_{\rm B}}\over{c_{\rm BH^+}}} where pK a is the dissociation constant of BH + . They were originally measured by using nitroanilines as weak bases or acid-base indicators and by measuring the concentrations of the protonated and unprotonated forms with UV-visible spectroscopy. Other spectroscopic methods, such as NMR, may also be used. The function H − is defined similarly for strong bases: :: H_- = {\rm p}K_{\rm a} + \log {{c_{\rm B^-}}\over{c_{\rm BH}}} Here BH is a weak acid used as an acid-base indicator, and B − is its conjugate base.
Article summary
An acidity function is a measure of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to (or accept protons from) a solute (Brønsted acidity). The pH scale is by far the most commonly used acidity function, and is ideal for dilute aqueous solutions. Other acidity functions have been proposed for different environments, most notably the Hammett acidity function, H 0 , for superacid media and its modified version H − for superbasic media. The term acidity function is also used for measurements made on basic systems, and the term basicity function is uncommon. Hammett-type acidity functions are defined in terms of a buffered medium containing a weak base B and its conjugate acid BH + : :: H_0 = {\rm p}K_{\rm a} + \log {{c_{\rm B}}\over{c_{\rm BH^+}}} where pK a is the dissociation constant of BH + . They were originally measured by using nitroanilines as weak bases or acid-base indicators and by measuring the concentrations of the protonated and unprotonated forms with UV-visible spectroscopy. Other spectroscopic methods, such as NMR, may also be used. The function H − is defined similarly for strong bases: :: H_- = {\rm p}K_{\rm a} + \log {{c_{\rm B^-}}\over{c_{\rm BH}}} Here BH is a weak acid used as an acid-base indicator, and B − is its conjugate base.
Key claims
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.