Modified potentiometric titration method to distinguish and quantify oxygenated functional groups on carbon materials by pKa and chemical reactivity
2020
Zhang, Zhongyao | Flaherty, David W.
The presence and form of oxygenated functions on carbons lead to significant differences in material properties that impact surface chemistry, catalysis, electronic, and mechanical properties. Titration methodologies, specifically potentiometric and Boehm titrations, classify the types of acidic functions present on carbon materials into broad categories described as carboxylic, lactonic, and phenolic functions based upon the measured or estimated acid dissociation constants (pKₐ). The accuracy of these methods depends, however, on the estimates for pKₐ values and suffers when multiple possible species possess similar pKₐ values. Here, we improve upon past methods by combining potentiometric and chemical titrations, which involves identifying acidic functions by pKₐ values but also examining how functional groups react with alkaline titrants. Specifically, Brønsted acids like carboxylic and phenolic groups deprotonate reversibly when contacted with alkaline titrant solutions; however, lactonic functions undergo hydrolysis to form carboxylic acids and hydroxyls during identical treatments. Consequently, comparisons between sequential potentiometric titrations can be used to discriminate between functional groups by examining their pKₐ values but also their reactivity. Tests on probe compounds and functionalized carbon materials show our method successfully distinguishes lactones from other acidic functions on carbon surfaces and suggests the methodology complements other characterization techniques for carbon materials.
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