Quantitative Determination of Iron in Limonite Using Spectroscopic Methods with Senior and General Chemistry Students: Geology-Inspired Chemistry Lab Explorations

Abstract

Limonite is the field term for a mixed assemblage of ferric oxyhydroxides, often containing nonferric silicate impurities. It is abundant on Earth's surface, possesses variable iron content, and is easily recognized by distinctive yellow and ochre hues. Limonite is a unique centerpiece for undergraduate chemistry laboratories because each sample represents a true unknown to faculty and students alike, and because limonite does not digest readily with common methods. Senior students were guided through a primary literature review to assess and establish an appropriate digestion method. Students then constructed the procedure for determining the iron content in limonite using atomic absorption spectroscopy. A final report, produced in the style of this Journal, completed the start-to-finish process used by scientists, helping students learn how novel problems are solved in the laboratory. General chemistry students were provided with limonite extracts and used UV−vis spectroscopy to determine their iron contents, gaining proficiency in wet-lab, data collection, and analysis skills. Lab reports included an interdisciplinary discussion/interpretation of results from chemistry and geologic perspectives. The method used by students for digestion and iron extraction was validated using goethite and iron(III) oxide as standards (<3% error). The two spectrometric methods provided comparable results for the iron content in limonite. Despite challenging all students, these experiments were rated favorably in written evaluations, and students self-rated their learning gains as very high. Limonite experiments promote curiosity, discussion, and departure from laboratory exercises with predetermined results. Students become vested in analyzing a geologically important material that is inherently complex and heterogeneous.

An Interdisciplinary, Project-Based Inquiry into the Chemistry and Geology of Alkaline Surface Lake Waters in the General Chemistry Laboratory

“…■ DATA AND ANALYSIS Alkalinity Students' familiarity with spreadsheet use from previous experiments 98,100,101,111,115,116 was instrumental in tackling this involved data analysis. Only a handful of students had taken calculus previously.…”

“…Integrating this chemical analysis with the geological insight of the lakes a more holistic understanding of the lakes. 98,100,101,115,116 All of these were applied by students to successfully understand how the geologic events of the past contributed to the unique chemistry of the lake waters. Using an analytical instrument (atomic absorption (AA) spectrometer) and derivative curves for end point determination were added bonuses.…”

“…The administrative structure at The Evergreen State College (TESC), enabling faculty to integrate disciplines without hurdles, made it possible for this investigation to be truly interdisciplinary. Other laboratory exercises created in this environment have been published. − The value of interdisciplinary education is gaining recognition. − …”

“…It included a visit to Dry Falls and a fossil rhinoceros mold (see field trip handout in Supporting Information). This PJBL was envisioned by the instructors as the culminating, interdisciplinary, hands-on learning experience in a course where interdisciplinary exercises were commonly used. ,,,− …”

“…Chemical equilibria, weak acids/bases, pH, and titration curves were covered in chemistry lectures before the field trip. Students learned titration skills and used pH probes and Microsoft Excel for graphing and data analysis in prior experiments. ,,,, All of these were applied by students to successfully understand how the geologic events of the past contributed to the unique chemistry of the lake waters. Using an analytical instrument (atomic absorption (AA) spectrometer) and derivative curves for end point determination were added bonuses. Students saw chemistry and geology employed together to provide a more complete analysis to answer questions about the lake system that would not have been possible otherwise, making the PJBL truly interdisciplinary . …”