R/H Chemistry
Background
Chromatography is a laboratory technique which is used to separate the components of certain types of mixtures.
The physical property polarity is one of the most common properties exploited by this technique.
Chromatography has been used by scientists for over 100 years, originally for the separation of plant pigments, hence the “chroma” part of the name.
One of the original media was paper. In paper chromatography the components of the mixture are dissolved in a solvent (mobile phase). The solvent rises through the paper (stationary phase) using capillary action. The distance a particular component moves along the paper is dependent upon its attraction to the paper compared with its attraction to the solvent. If it is more attracted to the paper, it will not move very far along the paper. If it is more attracted to the solvent, it will move further along the paper. In this way the components of the mixture become separated.
The distance a component travels along the paper can be compared to the distance the solvent moves, giving what is known as the R f
value. A lower R f
value usually means that the component is more polar, as it is more attracted to the paper than the solvent, while a higher R f value usually means that the component is relatively less polar and is more attracted to the solvent.
Purpose
In this lab you will separate the components of a sample of food coloring and determine which food dyes are present in your sample by comparing the R f
values with standard values.
Materials food dye mixtures (food coloring) watchglass sodium chloride solution, 0.1% NaCl, about 5 mL
18 x 100 mm test tube chromatography paper pencil ruler toothpick
Safety: apron and goggles. Food dyes are slightly hazardous by ingestion, inhalation, eye and skin contact. They can stain your skin and your clothes.
Procedures
1.
Put a small amount of food coloring onto a watchglass.
2.
Using a pencil, place a dot on the chromatography paper in the middle, above the narrowed portion of the paper. Write your initials and period and the name of the food dye in pencil on the top.
3.
Using a clean toothpick, spot the chromatography paper by putting the toothpick into the food coloring sample and then touching the tip of the toothpick gently onto the chromatography paper on the pencil dot. Repeat the procedure a couple of times to increase the concentration of the sample, trying not to increase the size of the dot.
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4.
Wait a couple of minutes for the sample to dry on the chromatography paper.
5.
While the sample is drying, obtain a large test tube.
Solvent front
Dye component 1
Dye component 2
Etc.
6.
Prior to putting the solvent into the test tube, gauge the amount of solvent you will need by holding your chromatography paper next to the test tube. You want only the tip of the paper to touch the solvent.
Pour enough 0.1% salt solution (solvent) into the test tube to do this.
7.
Using the toothpick as hanger, place the chromatography paper with sample into the test tube in such a way that the tip of the paper is in the solvent, below the sample dot.
8.
Allow the solvent to move up the paper by capillary action until the solvent front is within 1-2 cm of the top of the chromatogram.
9.
Remove the chromatogram from the test tube.
10.
With a pencil, mark the solvent front, i.e. how far the solvent traveled to the top of the chromatography paper. Measure the distance, in cm, and record in the Data and Analysis table.
11.
In pencil, trace the shape of each dye band or spot to mark the location of each separated band.
12.
Measure the distance traveled by each dye component, in cm. Measure from the dot at the bottom to the center of each band. Record in the Data and Analysis Table.
13.
Calculate R f
for each component by dividing the distance traveled by each component with the distance traveled by the solvent. Leave as a decimal. Record in the Data and Analysis Table.
14.
Tape your chromatogram into your lab notebook.
Data and Analysis Table
Distance (cm) traveled R f
Observed color
Food Dye (from
Standards Table)
Identify the food dyes found in your sample by comparing your R f values with those in the Food Dye Standards table below. Record in your Data and Analysis table.
Food Dye Standards
Dye
Yellow No. 5
Yellow No. 6
Blue No. 1
Yellow
Color R f
0.71
Dark orange 0.52
Dark turquoise blue 0.88
Blue No. 2
Green No. 3
Red No. 3
Red No. 40
Dark navy blue
Dark blue-green
Red
Dark red
Adapted from Flinn Publication 11173
0.35
0.90
0.10
0.31
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