Simultaneous Determination of the Acid-Dissociation Constant and Partition Coefficient of Benzoic Acid in Heptane and Water-A Gas Chromatographic Analysis

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Chemistry | Physical Sciences and Mathematics


Richard White, Chemistry; Rober Fulton; Michael Ross


The concepts of acid-dissociation and partitioning are used extensively in undergraduate chemistry classrooms and laboratories. The acid-dissociation constant is generally first presented in introductory courses when teaching about acid-base chemistry and is used as a measure of acid strength and for buffer formulation. It isn’t until Organic Chemistry that the concept of the acid-dissociation is paired with partitioning to a significant degree. In the organic laboratory an acid or base is often added to an aqueous solution to aid in extraction when purifying and isolating a product. It is the coupling of equilibria that is employed in this experiment to experimentally determine the acid-dissociation constant and partition coefficient for benzoic acid using water and heptane as the aqueous and organic phases, respectively. When benzoic acid, heptane and water are mixed, the benzoic acid partitions itself between the heptane and aqueous layer. By determining the amount of benzoic acid that partitioned into the heptane using gas chromatography and measuring the pH of the aqueous layer, the acid-dissociation constant and partition coefficient were simultaneously determined. This experiment was designed to be used in undergraduate laboratories to familiarize students with gas chromatographic analysis, give a deeper understanding of the two equilibria, and to investigate the effects of ionic strength on equilibrium constants.