Paper Chromatography

A Multimedia Lesson and Laboratory Exercise

Outline of Contents:

1. Introduction
2. Goals: Cognitive Goal, Affective Goals, Strategies used for Instruction Universal Traits, Curricular Goals
3. Instructional Notes on Chromatography
4. Laboratory Exercise #1
5. Laboratory Exercise #2
6. Open Ended Problems for Students
7. Links to Related Sites

Introduction

The purpose of this site is to utilize the resources of the internet to teach a lesson not always taught in a first-year, general chemistry class at the secondary level. Teachers will be able to consult this site to construct a lesson and/or a laboratory exercise for their students to use in class. When used with standard laboratory equipment and materials and some other easily obtained food sources, this site contains a complete lesson which can be modified by the teacher, or done in its entirety by the student.

The cognitive and affective goals, instructional strategies and universal traits mentioned in the "Goals" section of this site are based on the work of Richard W. Paul, as explained in Critical Thinking; How to Prepare Students for a Rapidly Changing World, Foundation for Critical Thinking, Santa Rosa CA, 1995. One of the explicit objectives of this site is to increase the critical thinking skills of students while also teaching the content of a chemistry class.

 

Goals

Cognitive Goals:
Students will explicitly assess information provided to them.
Students will gather and organize information and data.

Affective Goals:
Students will start to develop intellectual perseverence
Students will develop confidence in their own reasoning.

Instructional Strategies:
Students will refine generalizations and avoid oversimplification.
Students will compare analogous situations and transfer insights to new concepts.
Students will generate and assess solutions to problems.
Students will note significant similarities and differences.

Universal Traits Emphasized:
Logic, Relevance

Curricular Goals:
Students will learn the conceptual theory of paper chromatography.
Students will attempt to use the process of paper chromatography to separate
various mixtures of substances in a qualitative way.
Students will use paper chromatography Rf values to quantify the identification
of various substances in a mixture.
Students will be introduced to the uses of related separation techniques such as thin-layer chromatography, gel
chromatography, and gas chromatography.

 

Instructional Notes on Chromatography

A. Definitions and uses of paper chromatography

1. Chromatography is a procedure used to separate substances in a
mixture based on the different mobilities of the components.

2. In paper chromatography, porous paper (like filter paper, coffee filters,
chromatography paper, paper towels, or even newspaper) is the
stationary phase and water or another solvent, like alcohol, is the
mobile phase. See slides which illustrate stationary and mobile
phases.

3. Mixtures that are colored, or form colored compounds can be
separated into component colors by paper chromatography.

B. Theory of paper chromatography

1. A small sample of a mixture is placed on porous paper which is in
contact with the solvent. The solvent moves through the paper
due to capillary action and dissolves the mixture spot. The
components of the sample start to move along the paper at the
same rate as the solvent.

2. Components of the mixture with a stronger attraction to the paper than
to the solvent will move more slowly that the components with a
strong attraction to the solvent. The differences in the rates with
which the components travel along the paper leads to their
separation.

3 Particular mixtures will have chromatographic patterns that are
consistent and reproducible as long as the paper, solvent, and
time are constant. This makes paper chromatography a
qualitative method for identifying some of the components in
a mixture.

4. Different solvents will change the patterns of component separation.

5. Colored spots on the finished and dried chromatogram can be cut out
of the paper with scissors and redissolved to obtain a pure sample
of one component of the mixture.

C. Quantitative analysis of chromatographic data

1. Retention factors, or retardation factors (Rf) are calculated to give a
quantitative measure of a componentís properties in a mixture.

2. Rf = Distance traveled by solute/Distance traveled by solvent

3. When chromatograms are made with the same solvent and developed
for the same amount of time, Rf values are identical for the same
components in the mixture.

D. Related separation techniques

1. Thin-layer Chromatography. The stationary phase is a thin layer of
silica gel or starch dried on a glass or metal plate. The mixture to
be separated is placed at the bottom of the plate and the plate is
placed in a closed chamber of solvent, which is the mobile phase.
The solvent moves across the gel or starch the same way that solvent moves through paper.

2. Gel Chromatography. A porous gel packed into a column of glass is
the stationary phase and the mixture to be separated is poured into
the top of the column. Various samples of liquids exiting from the
column at the bottom (eluents) are collected and further analyzed.
The time it takes for a certain molecule to travel through the
column is unique for that type of molecule. This technique is
often used to separate different sizes of large biological molecules
because the size of the pores in the gel can be controlled easily.

3. Gas Chromatography. In this technique, the mixture to be analyzed is in
gaseous form. The gases in the mixture react with a coating on the
inside of a glass tube (stationary phase) and are carried by inert
gases such as helium (mobile phase). Gas components exit the
tube at different times and are often further analyzed using a
spectrophotometer. This technique can be used to separate
complex mixtures of aromas and flavors, or to help analyze
pollutants in the air.

 

Laboratory Exercise #1 - Qualitative and Quantitative Separation of a Mixture of Colored Dyes

A. Objectives: Perform a paper chromatography procedure according to
information found on various sites on the internet. This paper
chromatography will involve separating a mixture of colored dyes.

B. Equipment and Materials, Procedure: Go to this site for instructions and
videos of the procedure.

Teacher note: These instructions are most suitable for separating dyes in
water soluble pens - such as Mr. Sketch felt tip pens. However, see

additional ideas in the section on "Open Ended problems for Students."

C. Data: Go to the following site for instructions on how to collect and
analyze your data.

D. Results: Answer the following questions related to your experiment:

1. How many components were in the dye mixture that you used?
If you did several chromatograms with the same mixture, were the
results exactly the same, and why or why not?

2. Were the Rf values of the components of your mixture exactly the same
each time you did the experiment? What could causes differences
in these numbers?

3. What is the purpose of double-spotting the chromatography paper at the
beginning of the experiment?

4. Name several practical uses for the type of chromatography you
investigated.

 

 

 

Laboratory Exercise #2 - Qualitative and Quantitative Separation of a Mixture of Metallic Ions by Complex Formation

 

A. Objectives: Perform a paper chromatography procedure to separate a standard
and an unknown mixture of metallic ions. Qualitative and quantitative
identification of the ions will be accomplished.

B. Equipment and Material, Procedure: Go to this site for lab tutorial.

C. Data: Go to the following site for instructions on how to collect and
analyze your data.

D. Results: Answer the following questions about your investigation.

1. What was the purpose of exposing your chromatogram to ammonia
vapor? How did you accomplish this task?

2. Was there an exact correspondence between the locations and Rf
values of the components in the known and unknown mixtures?
Why or why not?

3. What are the variables that must be controlled in performing this
procedure?

4. What are some practical uses of this procedure?

 

Open-Ended Problems for Students

1. Several different internet sites about paper chromatography describe the use of water
as the mobile phase in separating the dyes in water-soluble inks in pens. Alcohol
can also be used as the mobile phase in chromatography and will dissolve dyes that
are not water soluble. Devise an experiment that separates water insoluble dyes,
by using a solvent that is not water.

2. Food coloring is a mixture of dyes that are water soluble. Students can perform
paper chromatography on the four standard food colorings found in a
grocery store package, or they can mix the dyes in various combinations to
analyze.

3. Food coloring is used in many small candies that have hard sugar coatings. A small
amount of water on a cotton swab will remove enough dye from the surface of the
candy to perform paper chromatography. After creating a standard set of
chromatograms with Rf values for each component, students can compare to
the mixtures of dyes in another brand of candy.

4. Food coloring is used in many edible products and students can use their skills in paper
chromatography to devise experiments for identifying the components of dyes in
different kinds of foods. For example, students might try to extract dye
chromatograms from concentrated samples of soda (cola, grape, orange), or
concentrated samples of dry powdered drink mixes.

5. Students can further investigate the paper chromatography of metallic ions by varying
the solvent mixture or trying other combinations of ions.

 

 

Links to Multimedia Sites on Paper Chromatography

1. Paper Chromatography. Description and detailed instructions for doing paper
chromatography experiments. Included are videos and audio descriptions for
preparing and running the experiment, self check pictures for equipment set-up,
questions, Rf calculation instructions, quiz, and exercises. The site is at the
NT Curriculum Project of the University of Wisconsin-Madison.
http://genchem.chem.wisc.edu/labdocs/modules/paprchrom/paprchrom.desc.htm

2. Chromatography. A detailed description and tutorial of paper chromatography used
to separate a mixture of metallic ions. This qualitative technique can be used in
conjunction with traditional qualitative labs to show another way to identify these
ions. The method also lends itself to quantitative analysis. The site includes slides
of the procedure, results, and questions for students.
http://chemistry.rutgers.edu/genchem/chrom.html

3. Analysis of Mr. Sketch Ink by Paper Chromatography. This site describes the
the absorption and reflection of light and how Mr. Sketch pens work. There is an
excellent discussion of stationary and mobile phases, and the effect of polarity on
the movement of particles in chromatography. A procedure for an experiment and
pictures of actual chromatograms are included.

http://wwwchem.csustan.edu/chem1002/mrsketch.htm

4. Paper Chromatography. This is an update of the previous site and contains more
information and an expanded procedure for chromatography.
http://wwwchem.csustan.edu/chem2000/Exp1/Bkg1.htm

5. Freshman Chemistry. This site contains excellent pictures and cartoons that illustrate
chromatography equipment and show how the stationary and mobile phases
work.
http://antoine.frostburg.edu/chem/senese/101/matter/slides/sld007.htm

6. Orange Sherbert. A more detailed discussion of retention factors and their use in
analyzing paper chromatograms is found here. There are also some practical
examples that explain some unexpected results in using this technique.
http://orangesherbert.eng.clemson.edu/webta/chromatography.html

7. Student Sites of Chromatography. A list of student sites describing various uses
of paper chromatography.
http://shsu.edu/~chemistry/CHM440/chromsites/chromsites.html

8. Pens, again. Here is another site describing the use of paper chromatography to
separate pen inks.
http://purchon.co.uk/science/chromato.html

9. Paper Chromatography. Descriptions of uses of paper chromatography with an
example and students questions for an unknown sample.
http://chemistry.rutgers.edu/genchem/chrom.html

10. Forensics use of Paper Chromatography. This is an example of the use of paper
chromatography to identify various inks for a forensics investigation. A simple
procedure is described.
http://wwwl.islandnet.com/~yesmag/projects/paper_chroma.html

11. Gas Chromatography. Descriptions and diagrams of the uses of gas chromatography.
http://www.yahoo.co.uk/Science/Chemistry/chromatography

12. Chromatography and Mass Spectroscopy. This is a more advanced discussion of
various gas chromatography techniques paired with mass spectrometry. This site
includes many links and videos to illustrate the discussions.
http://www.shsu.edu/~chemistry/primers/gcms.html