Lab 4: Food Dye Chromatography
- Page ID
- 514166
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\dsum}{\displaystyle\sum\limits} \)
\( \newcommand{\dint}{\displaystyle\int\limits} \)
\( \newcommand{\dlim}{\displaystyle\lim\limits} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)PURPOSE
- To use paper chromatography to characterize standard food dyes.
- To use paper chromatography to identify food dyes used to coat candies.
INTRODUCTION
Most substances we encounter daily are mixtures, not pure compounds. Similarly, chemicals synthesized in laboratories often contain impurities such as reaction byproducts or leftover reactants. Consequently, a significant focus in chemistry is developing methods to separate and identify the components of mixtures.
Many separation techniques exploit physical differences among mixture components. For example, filtration separates substances in different physical states (e.g., solids from liquids), centrifugation uses differences in density, and distillation takes advantage of variations in boiling points. In this experiment, we will use paper chromatography to separate and analyze a mixture of food dyes.
Chromatography techniques involve three key components:
- Analyte: The mixture to be separated.
- Mobile phase: A liquid or gas that carries the analyte through the system.
- Stationary phase: A porous material that interacts with the analyte, slowing its movement.
Separation occurs because components of the mixture interact differently with the stationary phase. Components with stronger interactions move more slowly, while those with weaker interactions move faster, allowing the mixture to separate. These interactions are influenced by specific physical properties and experimental conditions, which can help isolate individual components.
Paper chromatography is a simple type of liquid chromatography that uses paper as the stationary phase. The analyte is applied near the bottom of the paper, which is then placed upright in a tank containing the mobile phase, with the paper’s edge submerged. Capillary action draws the mobile phase up the paper, carrying the analyte. Due to varying interactions with the stationary phase, different components of the mixture travel at different speeds. The separation is complete when the solvent front (the furthest point reached by the mobile phase) nears the top of the paper.
The separation is analyzed by measuring how far each component travels relative to the solvent front. This ratio, the Rf value, is unique for each substance under specific conditions and can be used to identify the components.
In this experiment, you will use paper chromatography to calculate Rf values for common food dyes and analyze the dyes in candy coatings. By comparing Rf values and colors, you will identify the dyes present in the mixtures.
SAFETY
- The food dyes may stain the skin. Avoid contact.
- Ethanol is flammable. Avoid open flames.
EQUIPMENT AND CHEMICALS NEEDED
- 600-mL beaker
- Watch glass or plastic wrap
- Porcelain spot plates
- Chromatography paper
- Scissors
- Stapler
- Pencil
- Pointed (not flat) toothpicks or capillary tubes
- 0.10 % NaCl (aq)
- 50/50 ethanol/water solution
- Standard dye solutions: F,D&C Blue No. 1, Blue No. 2, Green No. 3, Red No. 3, Red No. 40, Yellow No. 5, Yellow No. 6
- M&M® candies (or other similar color-coated candies)
EXPERIMENTAL PROCEDURE
A. Preparing the Developing Tank and Paper.
- Prepare a developing tank by pouring 25 mL of the mobile phase (0.10 % solution of NaCl in H2O) into a 600 mL beaker and covering the beaker with a watch glass or plastic wrap. Ensure the solution level is less than 1 cm from the bottom of the beaker. The air above the mobile phase must become saturated with solvent vapor so that the solvent does not evaporate from the stationary phase as the chromatogram develops. Therefore, be sure to keep the developing tank covered at all times.
- Cut a 16 cm × 8 cm piece of chromatography paper. With a pencil, draw a horizontal line 1.5 cm from the bottom (long) edge of the chromatography paper. Draw vertical tick marks along this line every 1.7-1.8 cm to fit 8 spots. Do not form it into a cylinder yet.
B. Preparing the Standard Food Dyes.
- Use a small porcelain spot plate to obtain a drop of each of the dye solutions. The dyes are as follows:
|
Label |
F,D&C Dye |
Common Name |
|---|---|---|
|
B1 |
F,D&C Blue No. 1 |
Brilliant blue FCF |
|
B2 |
F,D&C Blue No. 2 |
Indigo carmine |
|
G3 |
F,D&C Green No. 3 |
Fast green FCF |
|
R3 |
F,D&C Red No. 3 |
Erythrosine |
|
R40 |
F,D&C Red No. 40 |
Allura red |
|
Y5 |
F,D&C Yellow No. 5 |
Tartrazine |
|
Y6 |
F,D&C Yellow No. 6 |
Sunset yellow |
C. Preparing the Candy Food Dyes.
- To separate the dyes used to coat candies, you must first prepare a solution of the coating dyes with the following procedure. Place 3 M&M’s of the same color into a medium test tube. Add about 3 mL of a 50/50 mixture of water and ethanol and swirl the solvent until the candy coating has dissolved. Decant the solvent/dye solution from the M&M’s into an empty well in your spot plate. Be sure to decant before the chocolate center is exposed. You and your partner may coordinate with other experimenters so that each pair must prepare and analyze only one candy color solution that can be shared with the group.
D. Spotting the Paper
- Using a toothpick and one of the standard dye solutions, make a spot on the chromatography paper at one of the marks. Try to keep your spots less than 2 mm in diameter. Repeat the spotting using the other dyes. Use a different toothpick for each dye. Allow the dyes to dry then reapply to each spot at least 5 times or until a sufficiently dark spot has been reached. With a pencil, note the name of the dyes below the spots.
- The dye solutions prepared with the candies are not as concentrated as those of the standard dyes. You must spot these dye mixtures several (10 or more) times to obtain a sufficiently dark spot. Again, dry each spot between applications of the mixture to maintain as small and concentrated a spot as possible.
E. Developing the Chromatogram
- When the spots have dried, form the chromatography paper into a cylinder, and staple the edges of the paper together, making sure to leave a gap between the edges. If the edges come into contact, the solvent will not travel at a uniform speed up the chromatography paper and the components of the mixture will not move in a straight line.
- Place the chromatography paper into the developing tank, do not let it touch the sides of the tank, and quickly replace the cover. Make sure the level of the mobile phase is below the line of dyes on your paper. Allow the chromatogram to develop.
- When the solvent front is approximately 1 cm from the top of the chromatography paper, remove the chromatogram and lay it flat on a paper towel. Immediately mark the position of the solvent front with a pencil. The front will continue to move as the paper dries so you must mark this position now. Measure and note the distance the solvent front traveled (Dsolvent).
- Draw an ellipse around each spot on the developed chromatogram and draw a horizontal line through the center of each spot. If a spot shows significant “tailing” make your horizontal line through the darkest part of the spot. Use the distance from the starting line (not the bottom of the paper!) to these horizontal lines to determine Ddye for each dye. Record distances and calculate the Rf values using, Rf = Ddye / Dsolvent
PRE-LAB QUESTIONS
- Compare and contrast what the stationary and mobile phases mean in chromatography. You will have to do a little research to find this answer.
- Identify the stationary phase and the mobile phase in this lab.
- Given the following paper chromatogram for 4 dyes developed by a student in a previous semester. Calculate Rf value for each spot. Show your calculations.
DATA AND OBSERVATIONS
Characterization of Standard Dyes
Distance solvent front traveled, Dsolvent [mm] _____________________
|
Dye |
Color |
Distance Traveled, Ddye [mm] |
Rf |
|---|---|---|---|
|
F,D&C Blue No. 1 |
|||
|
F,D&C Blue No. 2 |
|||
|
F,D&C Green No. 3 |
|||
|
F,D&C Red No. 3 |
|||
|
F,D&C Red No. 40 |
|||
|
F,D&C Yellow No. 5 |
|||
|
F,D&C Yellow No. 6 |
Characterization of Dyes Used to Coat M&M Candies
Distance solvent front traveled, Dsolvent [mm] _____________________
|
M&M Color |
Dye Color |
Ddye [mm] |
Rf |
F,D&C Food Dye |
|---|---|---|---|---|
|
Red |
||||
|
Yellow |
||||
|
Green |
||||
|
Blue |
||||
|
Orange |
||||
|
Brown |
||||
POST-LAB QUESTIONS
- Why is a pencil used to draw the lines on your piece of paper rather than a pen?
- Why is it important for the initial spots to be small?
- What is the range of possible values for Rf? (lowest to highest)
- Discuss the advantages and disadvantages of waiting for ~120 minutes for the eluting solvent front to move.