23 Jan 2014

Electronic Absorption Spectroscopy of Aquated Transition Metal Ions

Lab Experiment

Submitted by Zachary Tonzetich, University of Texas at San Antonio

I developed this laboratory experiment for our instrumental analysis class. The course is taken by junior and senior chemistry majors, who for the most part have had one inorganic chemistry course and some physical chemistry. The laboratory is operationally very simple and has students record the UV-vis spectra of transition metal sulfate salts in water using volumetric technique. They record the molar absorptivities for each peak and use this data to determine the number of waters of hydration for each salt by comparing with literature absorptivity values. We withhold the number of waters of hydration from the commercial bottles and have the students proceed using only the ionic formulas. They thus caluclate values that are less than the literature values and must use the ratio of lit/calc to determine the true molecular weights and hence number of waters of hydration. This part of the experiment is more analytical in nature and introduces students to the UV-vis instrument while reinforcing the practice of preparing solutions of precise concentration.

As an additional aspect of the laboratory, the students use their data to calculate the crystal field splitting parameter for each of the hexaqua complexes using Tanabe-Sugano diagrams. I also include the spectra of two tetrachlorometallate salts, [NiCl4]2- and [CuCl4]2-, so students can compare band energies and intensities between octahedral and tetrahedral complexes. The classic hexaqua spectra are usually shown in textbooks, but it is neat to have the students generate the data themselves. Unfortunately, the experiment requires a UV-vis spectrometer with near-IR capabilities. We are fortunate enough to have one, but not all universities may. 

PDF icon Lab manual for experiment3.1 MB
Learning Goals: 

The goals of this experiment are to have students 

Equipment needs: 

UV-vis spectrophotometer with near IR capabilities (250 - 2000 nm).

Implementation Notes: 

See the Experiment_notes.docx file under the faculty-only files for implementation notes and example spectra.

Time Required: 
3-hour laboratory period
Evaluation Methods: 

The students prepare a laboratory report, usually at the discretion of the teaching assistants, reporting their data, analysis, and conclusions. Several questions are also provided in the lab manual that can be used for assessment. 

Evaluation Results: 

We have conducted this experiment twice over the last two semesters. It runs quite smoothly with class sizes of 25 - 30, where students typically work in groups of 2 or 3. The results obtained by the students are generally quite good, and match the true numbers of waters of hydration for each metal salt, save iron(II) sulfate (see implementation notes). I cover the inorganic chemistry concepts during lecture so that the students who haven't had too much cyrstal field theory are prepared to answer the questions in the lab manual.

Creative Commons License: 
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