2010•Creative EducationOpen access

Determination of the pKa Value of Phenolphthalein by Means of Absorbance Measurements

Manuel Alonso, Sebastián Chapela, María L. Cristaldo, Inés Nievas, Hilda I. Burgos Oliver Gamondi, Carlos A. Stella

Abstract

We here report a laboratory protocol for the determination of the pKa value of an acid by means of determinations obtained with a spectrophotometer. Students determine the acidity constant (Ka) and the pKa associated with phenolphthalein from the absorbance values obtained from phenolphthalein solutions at different pHs. The present protocol for the determination of the pKa takes a very short time and is useful when teaching in conditions with limited equipment.

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Key concepts: Phenolphthalein, Absorbance, Chemistry, Analytical Chemistry (journal), Chromatography, Organic chemistry