Visually Tracking Acid–Base Extractions Using Colorful Compounds

Abstract

Acid–base extraction is an important concept in organic chemistry that beginning students often find challenging to comprehend. Herein, we describe the use of a simple demonstration suitable for lecture that allows students to visually track the chemical events that take place in a separatory funnel. The demonstration involves the use of two safe and available dyes with different acid–base properties, phenolphthalein and neutral red. The dyes present distinct colors in their ionic and nonionic forms. With a change in the pH of the media in the separatory funnel, the colors of the solutions change according to the compounds’ ionic states and their solubilities in either the aqueous or the organic layers. This provides a visual record of chemical concepts, which helps students to better understand the partition of different molecules between the organic and aqueous layers in an extraction process. In addition, we provide resources describing how to use this demonstration in class aiming to enhance students’ engagement and comprehension of the topic.

Student Discovery of the Relationship between Molecular Structure, Solubility, and Intermolecular Forces

“…The main reason that inquiry-based solubility experiments have not been used in the general chemistry laboratory is that students find it difficult to assess the solubility of colorless liquids, such as alcohols, in colorless solvents, , such as water and hexane, due to the low visual contrast between the materials (Figure A). From a solubility standpoint, amphiphilic molecules, such as alcohols, are interesting because they have polar and nonpolar components, and the relative proportion of each of these components determines the overall solubility of the molecules.…”

“…We overcame this challenge when we ran this experiment for the second time in Fall 2019 by tinting the alcohols using black Sharpie pigment; the clear color difference between solvent and alcohol allowed students to more easily visually assess the relative solubility of the alcohols (Figure B). Dyes have been used to help students visualize solubility and chemical partitioning in a variety of organic and analytical chemistry experiments. − Sharpie pigment is insoluble in both hexane and water, yet soluble in alcohols, with solubility declining with increasing length of the nonpolar hydrocarbon group ( i.e ., the pigment was more soluble in methanol than pentanol). Therefore, the pigment only dissolves in the solvents used in this experiment to the extent that the alcohols dissolve in them.…”

Separation of Food Colorings via Liquid–Liquid Extraction: An At-Home Organic Chemistry Lab

“…Given the successful completion of this lab by all 30 students, the retention of conceptual knowledge by 76% of the class in a later assessment, the safety, and the simplicity of this experiment, we consider the outcome generally successful. Students were more engaged because of the visual aspects 8 with food coloring and the use of materials from daily life. On the basis of a qualitative comparison of the students with cohorts from the past 11 years that did in-person experiments, this experiment provided equivalent or superior conceptual development without the need for lab space/glassware/organic solvents.…”

“…Alternately, a component that readily protonates may be extracted into an aqueous phase using acid, and then base to return the component to the organic phase. Many adaptations have been made to the standard liquid–liquid extraction procedure, including the use of vibrant dyes − to aid in visualization of the acid–base chemistry taking place as well as approaches that do not rely on wet lab experiments to help students understand specific concept areas (acid–base chemistry, density, etc. ).…”

Colorful Approach to Teaching Extraction Using Azo Dyes and Comparison of Hands-On vs Distance Learning Assessment

“…Colored indicator materials have long been used to help students visualize acid/base theory during titrations, − but instructional use of the dye structures themselves have only recently been published. − Food dyes, natural pigments, and biological stains have also been explored with TLC, , paper chromatography, , column chromatography, − and liquid–liquid extraction, − though these have focused on learning laboratory technique, without relating reactivity to structure. We have chosen conjugated azo dyes (Table ) to demonstrate their acid/base properties using extraction, as they are inexpensive, stable solids that come in a variety of colors.…”