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.