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Description
In this open-ended activity, students design crystallizations to can see who can grow the biggest crystals of their colorful products. This addition is something that I add to the standard M(acac)3 syntheses that many of us do as an introductory lab in an upper level course or as a final lab in an introductory type course. Syntheses of the M(acac)3 starting materials are available in most published inorganic laboratory manuals.
- Szafran, Z.; Pike, R. M.; and Singh, M. M. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience; Wiley & Sons: New York, 1991, p. 224.
- Girolami, G.S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic Chemistry: A Laboratory Manual, 3rd Ed.; University Science Books: Sausalito, CA, 1999, p. 117.
- Inorganic Experiments, 3rd Ed., Woollins, J. D., Ed.; Wiley/VCH: Weinheim, 2010, p. 109.
| Attachment | Size |
|---|---|
| RecrystallizationMacac.doc | 43 KB |
Learning Goals
- Students will learn how to do solubility tests on inorganic compounds
- Students will learn how to recrystallize compounds using layering, vapor diffusion, and slow evaporation techniques (different from those they typically use in organic lab)
- Students will understand the concept of miscibility and how it relates to crystallization techniques
- Students will also learn that crystallization is a trial and error process and not every method results in the production of good crystals!
Equipment needs
- M(acac)3 complexes such as those of Fe(III), Co(III), Cr(III), and Mn(III)
- Small glass scintillation vials, large capped glass jars for vapor diffusion
- A range of solvents of different polarities and densities: my version uses water, methanol, ethanol, acetone, acetonitrile, methylene chloride, ethyl ether, toluene, and pentane.
- Tables of miscibilities, densities, and polarities (available on the web and in catalogs such as the Sigma-Aldrich catalog, see weblinks below).
Implementation Notes
The narrower the vial, the easier it is to get good layers in the layering part of the experiment. Many scintillation vials take an extra twist to get them all the way shut, otherwise you lose volatile solvents such as methylene chloride or pentane.
Time Required
1 1/2 hours in lab
Web Resources
Evaluation
Evaluation Methods
I do this as a crystal growing contest and give a prize (usually lunch with me and the other winners). Students are judged in 3 categories: best crystals for each metal (one awarded per metal), largest number of nice crystals, and the greatest number of successful attempts! Everyone gets credit for their attempt as long as they precipitate something (even powder) and complete enough attempts.
Evaluation Results
Nearly everyone eventually grows some crystals though many are pretty small, showing them that it isn't easy to get big beautiful crystals! The Mn(acac)3 sometimes has particular difficulty, presumably because it is a bit contaminated with MnO2. The most frequent reason for not getting a precipitate is that students don't make their initial solutions concentrated enough or use enough of the solvent in which the compound is insoluble.
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I've just started the M(acac) lab this year, and it's gone pretty well. I'm looking forward to trying this idea next year!
We end our sophomore level inorganic lab with metal acac complexes synthesis and characterization: UV-vis, and magnetism by the Johnson Matthey magnetic susceptibility balance and by the Evans NMR method. Last spring, I offered Hilary's learning object as an "extra" for anyone that wanted to give recrystallizations a try. They could use their own metal acac complex or some of my "stock" material, built up from years of teaching this lab.
I had 3 students that were successful growing beautiful crystals, all by the solvent layering technique, although each discovered slightly different solvent pairings to use. Cr(acac)3 and Fe(acac)3 grew as somewhat irregular-shaped hexagonal plates, several mm across. One student grew beautiful thin needles of dark blue Cu(acac)2. The needles grew up to 2-4 mm long.
The students had a lot of fun "playing" in the lab and enjoyed the colorful contrast to organic recrystallizations! Their only prize was their pride, but that did not seem to matter.
We make several acac complexes in a second year prac (different students make different ones) and they recrystallise before characterisation.
I would suggest shortening the description of the process slightly - it is rather wordy at the moment and I doubt many students really follow it. Is this the first recrystallisation they have done?
Dr Madeleine Schultz Chemistry Queensland University of Technology Brisbane QLD 4001 Australia madeleine.schultz@qut.edu.au
In reply to nice colours by Madeleine Schultz / Queensland University of Technology
Colleagues,
A nice addition might be XRD powder patterns because a number of institutions have these available for students. Not sure if anyone has the JCPDS files to "share", however.
Donald Linn
I have general chemistry students prepare Fe(acac)3 and Inorganic students competatively grow crystals for single crystal x-ray diffraction. The structure make a nice poster in the hallway.
This fall my inorganic students will grow five different varieties of M(acac)3 cyrstals for single-crystal analysis. I'll offer up the powder XRD option as an extension for anyone interested. If someone bites, I'll report back.
My inorganic lab students this fall made the Co, Cr, Fe, Mn, and Cu acac complexes. The copper compound seemed to give us the most trouble - it was a very fluffly compound to work with and not very soluble in anything. The students were able to grow the long needle-like crystals of Cu(acac)2 that Maggie mentioned above, but when we tried to mount single crystals, we found the needles broke into multiple shards along the long axis. The other groups had no problems growing nice crystals and collecting XRD data. (I gave the copper group a .cif that I found online to use instead.) We also took UV-vis and measured magnetic susceptibility with our Johnson-Matthey balance. We skipped the Evans method this year due to time constraints.
Typically how long does it take for crystals of these complexes to begin to form? Days? Weeks?
days... he says years later...