Spectrophotometric Determination of the Dissociation Constant of an Acid-Base Indicator Using a Mathematical Deconvolution Technique

• Published in: Journal of chemical education 2005-11, Vol.82 (11), p.1682
• Main Authors: Alter, Krystyn P, Molloy, John L, Niemeyer, Emily D
• Format: Article
• Language: English
• Subjects: Acids; Analytical chemistry; Chemical speciation; Chemistry; Curricula; Data analysis; Experiments; Instrumentation; Laboratory Experiments; Mathematical Models; Methods Research; Organic chemistry; Science education; Scientific Methodology; Scientific Research; Skill Development; Spectrophotometry; Spectroscopy; Spectrum analysis; Standard 17; Student Development
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/ed082p1682

A laboratory experiment that uses a spectrophotometric method to determine the apparent acid-dissociation constant (K a') of neutral red, a common acid–base indicator, has been developed. The experiment incorporates a unique data analysis technique using a mathematical deconvolution program that allows students to resolve distinct spectral components within complex absorbance spectra. After measuring the neutral red absorbance over a range of pH values, students quantify the acidic and basic forms of the indicator present in each solution, correct for differences in the molar absorptivities of each species, and determine the K a′ of the indicator. This laboratory exercise gives students an opportunity to use advanced data analysis techniques and to observe directly how the spectral characteristics of an acid–base indicator are affected by the solution pH.