The Separation and Identification of Metal Ions Using Paper Chromatography
Background:
Different substances have different solubilities in a given solvent, depending on the polarity of the
solute and the solvent. Because of the differences in solubility, mixtures of substances can be
separated using various laboratory techniques. One of the most widely used resolution techniques for
separating mixtures based upon solubility is chromatography. In this experiment, you will use paper
chromatography to separate and identify the metal ion components of an unknown.
A mixture of metal ions will be deposited upon a strip of filter paper acting as an absorbing substance.
The filter paper will then be placed in contact with a solvent which will flow through the paper by
capillary action. As the solvent moves through the paper, the ions will be carried along with the
solvent at various rates dependent upon the two opposing forces of attraction: the attraction of the ions
for the solvent molecules and the attraction of the ions for the absorbing substance (the paper). The
more soluble the substance in the solvent, the faster it will travel up the paper. The greater the affinity
of the ions for the cellulose in the paper, the slower it will travel up the paper. The interaction of these
two forces will cause the ions to move at various rates and become separated on the paper. When the
solvent has neared the edge of the paper, the paper will be removed from the solvent tank. The
relative position of the ions will be determined by treating the paper with a staining agent which will
produce a colored product as it reacts with the metal ions on the paper. By observing the position and
the color of each spot and comparing it to with the positions and colors produced by an unknown
mixture of some of these ions, you can readily determine the composition of an unknown.
Chemists frequently use a quantity called Rf value to describe the position of the spots after the
chromatography has developed. The Rf value can be calculated by dividing the distance the individual
ion moves by the distance the solvent moves over a specified period of time.
Distance component moves
Rf value =
Distance solvent moves
The Rf value is a characteristic property of a given component in a chromatography experiment
conducted in a particular solvent under specific conditions. It does not depend on concentration or
upon other components present. Therefore, it can be reported in literature and used by other
researchers.
The chief component of filter paper is cellulose, a fibrous material which gives trees their structural
strength. Cellulose is a carbohydrate that can be represented by formula (C6H10O5) n, which indicates that
cellulose molecules contain the C6H10O5 unit, repeats some large number, n times. The structure of cellulose places
–OH groups near the surface of the molecules and thus enables filter paper to weakly bond to solutes
and water through the oxygen atoms or through hydrogen bonds. In addition, the surface of the filter
paper tends to have a negative charge that results in an additional strong attraction for positive ions,
such as the hydrated metal cations.
The solvent that is used in this experiment is 2-butone mixed with hydrochloric acid and is much less
polar than water. If the polarities of the solvent and of the aqueous filter paper (water is absorbed
form the hydrochloric acid component of the system) where the only factors involved, the hydrated
metal cations, would not separate much, since the attraction for the solvent would be much less than
the attraction for the aqueous filter paper. However, the solubilities of the metal cations are increased
by the presence of the hydrochloric acid in the solvent. Many metal cations tend to form the chlorocomplexes in the presence of the hydrochloric acid in the solvent and their solubility is directly related
to the ease of formation of such complexes. The more stable the complex, the greater the solubility of
the ion in the solvent.
The four metal ions used in this experiment form chloro-complexes to differing extents in the solvent
system:
1. Fe(OH2)63+ + 4Cl-  FeCl4- + 6H2O
3. Co(OH2)62+ + 4Cl-  CoCl42- + 6H2O
2+
22. Cu(OH2)6 + 4Cl  CuCl4 + 6H2O
4. Ni(OH2)62+ + 4Cl-  NiCl42- + 6H2O
The colors of the chloro-complexes are more intense than the colors of the hydrated metal ions and
therefore will be visible during the separation. The hydrated metal ions that are colored will be present on
the paper in such dilute concentrations that they will appear colorless.
Objective:
To use paper chromatography to separate and identify the metal ion components of an unknown
Safety:
1. Do not touch the paper
2. Use pencil to mark on the paper, NEVER use a pen
3. place the spot ON the line
4. Once paper is in the solvent do not move the beaker
5. Watch carefully – do not let the solvent go too far
6. Trace the solvent line immediately after you remove the paper from the solution with a pencil
7. Dry paper completely or chlorine gas will form when you stain it
Materials:
Filter paper, nitrate solutions, unknown, capillary tubes, beakers, 6.0M HCl, Acetone, 400mL beaker,
plastic wrap, 1% potassium ferrocyanide, pencil, ruler
Procedure:
Day 1:
1. Obtain a piece of filter paper or chromatography paper 17cm long by 11 cm wide. Along the
17cm edge of the paper, draw a pencil line approximately 2cm from that edge. Along the line,
approximately 3cm apart, mark and x and write the formulas of the cations to be studied: Fe, Co,
Cu, Ni and UN (for your unknown). See figure.
2 cm
Roll and staple
X
X
X
X
X
Fe
Co
Cu
Ni
Un
Day 2:
1. Apply the known metal ions and your unknown to the appropriate spots on the filter paper
sequentially using a fine capillary tube as an applicator. Dip the tip of the capillary tube into small
portions of the available 0.1M solution of the nitrate salt of the metal with which your are
working. Then briefly touch the tip of the capillary tube to the appropriate mark on the filter paper
to form a spot about 5mm in diameter. (you can practice on a separate sheet of paper) Allow the
spot to dry
2. Repeat the procedure for the remaining solutions and the unknown.
3. Freshly prepare 16mL of eluting solution by mixing 4.0mL of 6.0M HCl with 12.0mL of acetone
in a 400mL beaker. Cover the experiment while the chromatography experiment is in progress.
4. Allow the solvent to rise up the paper until the solvent front is approximately 1cm from the upper
edge (about 30 min). Do not allow the solvent front to reach the upper edge. When the solvent
front has reached the appropriate distance remove the paper from the beaker, replace the cover on
the paper, draw a pencil line along the solvent front, and allow the paper to dry. CAUTION: THE
PAPER MUST BE TOTALLY DRY IN ORDER TO AVOID THE FORMATION OF ANY
HAZARDOUS GAS IN THE NEXT STEP.
Day 3:
5. When the paper is dry, spray lightly with the staining agent, 1% potassium ferrocyanide. In order
to see the stained spots, you can rinse the paper free of staining solution by holding it under tap
water. The spots will not readily wash out of the filter paper, and their colors are more apparent
against the white background.
6. Use a pencil to draw a circle around each spot and measure the distance from the center of the
circle to the original mark on the pencil line where the ions were spotted. Calculate the Rf value
for each cation. Then calculate the Rf value for each spot on the unknown. Compare the Rf values
and colors of each spot of the unknown to identify the components of your unknown.
Pre Lab Questions:
1. Name at least three types of chromatography.
2. What are the two phases of chromatography and how do they work to separate molecules?
3. Name two uses for chromatography
4. List the cautions and dangers of the lab exercise
Data:
Metal ion
Cu2+
Distance Ion Moved
Distance Solvent Moved
Rf Value
Color after staining
Ni2+
Fe3+
Co2+
UN
Composition of Unknown: ________________________________________________
Calculations:
Show Rf calculations. (All calculations must be shown to get credit!)
Post Lab Questions:
1. Why do you write on the chromatography paper in pencil instead of pen?
2. What is an Rf value used for?
Conclusion: