Liquid-liquid equilibrium (LLE) extractions are used in the separation of penicillin from the broth, the separation of aromatic hydrocarbons (e.g., toluene, benzene) from paraffin in petroleum processing, and many other processes. LLE extractions are common in industry and academic laboratories. In the teaching labs specifically, LLE extractions are often the go-to operation to isolate compounds from a reaction mixture, yet the exploration of the experimental variables involved in the extraction are often superficially studied by students. Students then develop an attitude that LLE extractions are procedures to follow without much attention to their intrinsic characteristics. The work presented herein designs laboratory experiences that foster a deeper understanding of the complexity and ambiguity of empirical work in relation to LLE extractions. The lab experiences focus on the study of the equilibrium constant (Kd) and its dependence on the composition of the feed solution (i.e., carrier solution with the analyte) and the amount of extracting solvent added during the extraction step(s). That is, students know that equilibrium constants depend on the temperature, as it is often explained to them; however, in the case of LLE systems, Kd also varies with the composition of the mixtures and the amount of the materials utilized in the extraction process. Thus, two ternary systems were studied, each with different carrier solutions and extracting solvent. The systems were water/acetone (carrier/analyte) solutions with hexane (extracting solvent); and K2CO3/water (carrier/analyte) with isopropanol (extracting solvent). The selection of these chemicals is based on their easiness of analysis, low toxicity, wide availability, good chemical stability, low environmental impact overall, among other properties. The variability of Kd was determined for different mass percent compositions of feed solutions. Findings indicate that Kd varies with the composition of the overall mixture of the feed solution and extracting solvent amounts. Also, these variations in Kd can be understood by plotting equilibrium data in ternary diagrams to then make predictions on other mixture compositions. All laboratory procedures and techniques are planned for introductory chemistry level laboratories. Examples of the procedures, data, and results will be presented and discussed in light of their design and implementation for introductory chemistry laboratory programs.

Understanding the complexity and ambiguity of empirical laboratory work using liquid-liquid equilibrium extractions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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