Section: Science

157) 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 Tables 1 and 2, one can conclude that Sample 2 contains:

1. 1 metal ion only
2. 2 metal ions only
3. either 1 or 2 metal ions
4. more than 2 metal ions

Answer 2 is correct. Sample 2 had 2 colored spots: a brown-black spot with an Rf = 0.35 and a brown-black spot with an Rf = 0.95. These 2 spots correspond with 2 spots from Table 1: the brown-black spot with an Rf = 0.35 for Co2+ and the brown-black spot with an Rf = 0.95 for Hg2+. Co2+ and Hg2+ are metal ions, so Sample 2 contains 2 metal ions.