Practical #3- Paper Chromatography

We did a practical regarding Paper Chromatography. Paper Chromatography is a separation technique which is used to separate a MIXTURE of solutes with different solubility and degree of adsorption. This method uses a solvent moving over a porous or adsorbent medium (e.g. paper or jel) to separate a mixture of solutes. In the practical we use rectangular strips of filter paper.







The aim of the experiment is to separate the different dyes in the green food dye.

Apparatus used:

-Capillary tube

-Green Food Colouring

-Chromatography paper (Filter paper_

-Boiling tube

-Pencil and Ruler

-Distilled Water

-Clothes Peg

-Test tube rack

Steps:
1) Draw a line 1.5cm away from the end of the paper with a pencil and ruler
2) Using the capillary tube, drop 3 drops of green food dye on the middle of the starting line.
3) Fill the boiling tube with water until the water fills about 1.5cm from the bottom of the tube
4) Make sure that there is no water at the sides of the boiling tube that can touch the paper. (This may result in the substances getting separated at other parts of the paper, resulting in inaccurate observations.
5) Clip the paper in place with a clothes peg
6) Watch the dye run up the paper
7) Leave the paper there for 10 minutes.
8) Take the paper out of the test tube and let it dry
9) Mark the solvent front and tips of the different colour of dyes.

This was the result I got after the experiment. Evidently, the blue dye has travelled further the yellow dye.

Conclusions made based on the observation:
-The blue dye is more soluble in the solvent (Distilled water) than yellow dye
-The yellow dye is possibly better adsorbed by the filter paper than the blue dye.








Calculating the Rf value

Rf is short for retention factor. Unknown substances separated by chromatography can be identified by Rf values.

Rf values = Distance moved by substance/Distance moved by the solvent

Identical dyes have the same Rf when separated under the same experimental condition (Temperature, Pressure)

In this case, the results appear as a spectrum instead of individual dots. We were then told to take a reference point for each substance. I took the top of each dye.

Component	Distance moved by sport/cm	Distance moved by solvent/cm	Rf
Blue dye	6.9	12.4	0.556
Yellow dye	5.3	12.4	0.427

Some questions to consider...
1) Is it possible to separate a mixture of colourless chemicals by chromatography? Explain your answer.
Yes. A locating agent can be used to convert a colourless chemical into coloured substances.

2) What factors could affect the Rf value of a chemical? How would these factors affect Rf values?
a) Temperature affects the solubility of substances, the higher the temperature , the higher the solubility of the substances.
b) Type of paper affects the distance travelled. If the dyes are more easily absorbed by the paper, the Rf value is lower.
c) Type of solvent affects because dyes may be more soluble in ethanol than water or vice versa.