In the Laboratory
Chromatography
J. Chem. Educ. 1999.76:83. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/29/19. For personal use only.
Celestyn M. Bro´ zek Science Department, University of New Mexico–Valencia Campus, 280 La Entrada, Los Lunas, NM 87031
Paper chromatography of food dyes and inks is a standard experiment in my freshman chemistry laboratories. This activity introduces a powerful and common technique used in analytical chemistry. In addition, it demonstrates the difference between mixtures and pure substances. Finally, it is a very colorful and attractive experiment that is popular with students. The traditional explanation of the mechanism of chromatography includes the concept of stationary phase and moving phase. The separation of the components of a mixture is based on the difference in affinity that each component has for the stationary phase and for the moving phase. This explanation is quite often difficult to understand for my students. Therefore, I use the following analogy to describe the mechanism of chromatography. Chromatography is used to separate components of a
mixture. For example, imagine a mixture of wood pieces, pebbles, and large rocks to be separated and the chromatography setup as a stream. Flowing water is then the moving phase, and the bottom of the stream is the stationary phase. If our mixture is thrown into the stream, the wood pieces will move freely with the flowing water. The wood has, therefore, a high affinity (or attraction) for the moving phase. The large rocks will stay at the bottom and, thus, have a high affinity for the stationary phase. Finally, the pebbles will roll slowly at the bottom and have an intermediate affinity for the moving phase and for the stationary phase. This analogy has proven to be an excellent way of presenting chromatographic analysis to students. In addition to paper chromatography, other kinds of chromatography can be explained by this analogy, as well, since most include stationary and moving phases.
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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