Section: Science

156) Paper chromatography can be used to identify metal ions in wastewater. A drop of sample solution is placed on filter paper. The bottom of the paper is set in a solvent that travels up the paper (see Figure 1).



The solvent carries the ions up the paper. Some ions move faster, and therefore farther than others, resulting in a separation as they move up the paper. The paper is dried, then stained, causing the ions to appear as colored spots. Rf values are calculated for each spot:



Table 1 shows Rf values for 5 ions. Table 2 shows Rf values from 3 samples of wastewater. The same solvent was used for all ions and samples.





Table 1 adapted from Thomas McCullough, CSC, and Marissa Curlee, “Qualitative Analysis of Cations Using Paper Chromatography.” ©1993 by the American Chemical Society.

Note: Samples contain only the metal ions listed in Table 1.



Based on the information in Table 1, to best identify a metal ion using paper chromatography, one should know the:

1. spot color for the ion only
2. distance the solvent traveled only
3. Rf value and spot color for the ion only
4. distance the solvent traveled and spot color of the ion only.

Answer 3 is correct. Spot color can be used to restrict the range of options used to identify a metal ion. However, some metals, such as cobalt and mercury, have the same spot color (brown-black). To determine the identity of a brown-black spot, one also needs to know the Rf of the spot.