Exploring the pH Dependent Aqueous Speciation of Metal Complexes through UV-Vis Spectroscopy

Coordination chemistry is a major component of the undergraduate inorganic chemistry curriculum, and yet, the presentation of the material can be cumbersome due to the limitations of the course typically being taught in one semester. Also, because of the large scope of this branch of chemistry encompassing all of the elements, the course design has not been standardized. These factors result in some important coordination chemistry themes being given insufficient development. Herein, we propose a novel activity to formally introduce metal complex aqueous speciation in a holistic active-learning manner that includes a lecture component and a hands-on experience. This topic has real-world relevance and contextualizes many important coordination concepts. It would extend student comprehension about the intricate factors that affect metal complexation in an aqueous solution environment by focusing on the influence of pH. The activity explores the pH dependent speciation of the well-characterized interaction between Fe(III) and 2,3-dihydroxynapthalene-6-sulfonate and reveals the colorful changes in species throughout the pH range 0-13. Students learn how to generate speciation plots and to understand the ultraviolet-visible (UV-Vis) electronic absorption spectroscopy of transition metal compounds to be able to analyze the source of color that they observe. Assessment of the activity was conducted with 24 students who completed a Likert scale survey and responded to open-ended questions. The activity was then applied in actual course settings in which student comprehension was quantitatively evaluated. The activity can be easily adapted to students of different stages of academic development from elementary to college students.