Determination of dissociation constants of weak acids and bases using indicators

Ghosh, Saptarshi ; Chattopadhyay, Nitin (2013) Determination of dissociation constants of weak acids and bases using indicators The Chemical Educator, 18 . pp. 80-84. ISSN 1430-4171

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The conventional use of acid-base indicator is extended for the determination of dissociation constants of weak acids and bases through the measurement of the solution pH. In aqueous solution, depending on the pH of the solution, the indicator exhibits dual absorption bands characteristic to the two conjugate forms of the molecule. The ratiometric absorbance of an indicator at the two absorption bands is taken as a function of the solution pH to prepare a calibration plot. To determine the dissociation constant of a weak acid or base, one has to simply make a solution of it with known concentration, add the proper indicator, take the absorption spectrum and hence the ratiometric absorbance. Interpolation of this value on the calibration plot gives directly the pH of the solution from where the pK value can be determined from the known concentration. To encompass a wide range of pHs, a number of indicators covering different pH ranges are used. The strategy, based on the ratiometric principle, overcomes many of the instrumental as well as experimental artifacts. Once the literature is enriched with such calibration plots with different indicators for various pH ranges, a student will not need a pH meter for the purpose.

Item Type:Article
Source:Copyright of this article belongs to The Chemical Educator.
Keywords:Laboratories and Demonstrations; General Chemistry; Acid-base Chemistry; Dissociation Constants of Weak Acids and Bases; Strategic Study; Ratiometric Absorption
ID Code:98109
Deposited On:26 Mar 2014 12:01
Last Modified:26 Mar 2014 12:01

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