Name:
Date:
Analysis of Metal Cations Using Paper Chromatography
Objective: To use paper chromatography to separate a mixture of metal cations.
Materials:
Acetone, hydrochloric acid, ammonium hydroxide, and various metallic cation solutions
Equipment:
600 mL beaker, chromatography or filter paper, pencil, glass cover, scissors, dropper, capillary tube,
graduate cylinder
Abstract:
There are different methods for separating and identifying metal ions in solution. In this experiment,
paper chromatography is used to identify metal cations in aqueous solution. In chromatography, solute
species are exchanged between a mobile phase and a stationary phase. In paper chromatography, the
stationary phase is filter paper and the mobile phase is a liquid solvent which is also called the eluting
solution. The sample mixture to be analyzed is adsorbed onto the stationary phase which is then
contacted with the mobile phase. The mobile phase travels along the stationary phase creating a
solvent front. When the solvent front reaches the sample mixture, an equilibrium is established. Each
ion in the mixture elutes with a characteristic retention factor (Rf) which is measured relative to the
movement of the solvent front. Rf can be defined as follows:
Rf = (distance ion travels on filter paper) / (distance solvent front travels on filter paper)
Typically, Rf values are less than one, since the ion generally moves less rapidly than the solvent front
(Figure 1).
Figure 1. On the left is the filter paper strip before chromatography with the sample shown by the
shaded spot. The diagram on the right hand side represents the chromatogram at the end of the
experiment. The distances that define Rf, the retention factor, are indicated by arrows.
In this Experiment, paper chromatography is used to measure Rf values for a series of metal ions: Fe,
Co, Cu, and Mn. Then, you will be given an unknown solution containing one or more of these ions.
Using paper chromatography, you will identify the metal ions in your solution.
Procedure:
:
Prepare your mobile phase by filling a graduated cylinder with approximately 20 mL of a 90%
acetone/10% hydrochloric acid solution. Keep the graduated cylinder covered to prevent evaporation.
Pour some of the acetone/HCl mixture into a 400 mL beaker to a level of about 1 cm and cover the
beaker with a watch glass. Cut the chromatography strips into 14 cm lengths. Using a pencil, draw a
line across the paper about 1 cm from one end of the paper (Figure 1). Obtain small amounts of each
of the standard solutions that contain either Fe, Co, Cu, and Mn. Select an unknown for the last
chromatogram. Dip a capillary tube into one of the solutions and then dab it several times onto the
line that you just drew on your filter paper strip. Let the filter paper dry for a few minutes and then
place the filter paper strip into your beaker of acetone/HCl so that the paper just touches the solvent
(Figure 2). Paper clip the papers onto the side of the beaker. Keep the beaker covered. When the
solvent front has moved to within about 1 cm of the top of the paper, remove the paper from the
beaker and immediately mark the top of the solvent front with a pencil.
Figure 2. Experimental setup for paper chromatography. Filter paper strip is placed in eluting solution
of acetone/HCl.
Let the paper dry and then develop the chromatogram by using the paper clips to attach them to the
sides of a 400 mL beaker with approximately 1 cm of ammonium hydroxide (NH4OH) in it. Keep the
beaker covered and do not let the paper touch the aqueous ammonia solution. By exposing the metal
ions to ammonia vapor, complex compounds with characteristic colors should form.
Repeat this procedure for solutions containing each metal ion: Fe, Co, Cu, and Mn as well as the
unknown. Be sure to record the colors of your initial solution, and your developed, as well as
undeveloped chromatograms. Measure the distance from the starting line to the top of the solvent front
and from the starting line to the top of the ion front, so that you can calculate Rf values (Figure 1).
Mount the chromatograms in your lab notebook.
Data:
Make a table as follows:
Metal cation
Undeveloped color(s)
Developed
color(s)
Distance developed
color (cm)
Solvent front
distance (cm)
Analysis/Calculations:
Calculate Rf values for all of the metal cations.
Conclusion:
Based on the Rf values and observational data, determine the component(s) of the unknown. Include
the letter/number of the unknown solution and as much experimental evidence as is necessary to
support your conclusion.