Objectives:
1. To measure the Rf values of marker inks.
2. To analyze dyes based on their Rf values.
Materials:
Whatman No. 1 filter paper cut into 4x14cm rectangle
Pencil, tape, ruler
Al foil or seran wrap
600 mL beaker
100 mL 5% NaCl
Washable markers of various colors.
Petri dish or shallow bowl*
Pre-washed 100% white cotton fabric*
Rubber band*
Plastic pipet*
Circular filter paper*
70% rubbing alcohol
*optional
Introduction:
Chromatography is the technique of separating components of mixtures based on differences in polarity or size or both. There are many different types of Chromatography. In each case a mixture of solutes is dissolved in a moving solvent, called the “mobile phase.” The moving solvent carries the mobile phase through a structure holding another material called the “stationary phase.” The various constituents of the mixture travel at different speeds, causing them to separate. The mobile phase can be a liquid or a gas, and the stationary phase is a solid. The components of the mixture separate because as a result of their differences move through the system at different rates.
There are a number of chromatographic procedures that can be used. In thin-layer chromatography (TLC), the liquid mobile phase moves across a solid stationary phase that contains a thin layer of particles spread on a glass plate. Column chromatography makes use of a stationary phase of small particles packed into a tube. The mobile phase, which can be either a liquid or gas, passes through the solid phase. This experiment uses paper chromatography. We will separate the components of markers using paper as the stationary phase over which a mobile solvent, 5% NaCl (aq), moves.
Paper chromatography involves placing a small dot of sample solution (the washable marker) onto a strip of chromatography paper. The paper is placed in a jar containing a shallow layer of solvent and sealed. The solvent moves up the paper by capillary action. (Think of dipping a paper towel into water. The water flows up the paper towel by capillary action.)
As the solvent rises through the paper, it meets the sample and the sample dissolves in the solvent. The solvent will carry the sample up the paper as it rises.
If the sample is a pure compound as some dyes are, the compound will move up the paper as one spot. If the sample is a dye that is a mixture of compounds the sample will resolve into two or more spots.
There are opposing forces occurring to determine how easily components travel up the paper. Because of the different chemical structures of each kind of molecule, the chances are very high that each molecule will have at least a slightly different polarity, giving each molecule a different solubility in the solvent. The unequal solubilities cause the various color molecules to leave solution at different places as the solvent continues to move up the paper. The more soluble a molecule is, the higher it will migrate up the paper. Remember your “like dissolves like” for solubility. On the other hand, paper is made of cellulose, a polar substance. The more polar the dye molecules are, the more they will adhere or stick to the paper, making them travel up the paper more slowly. These various complex forces determine how far up the paper the different components travel.
If the sample separates into more than one “smear” or color, that means it must be composed of more than one kind of molecule.
The mixtures (washable markers) that are to be separated are spotted on the paper chromatogram and the chromatogram is then placed in the developing solvent. Once the chromatogram has been developed the retention factor (Rf) value is calculated. The Rf value is the distance traveled by the solute compound (dye) divided by the distance traveled by the solvent front (5% NaCl.)
Procedure:
Part I: Spotting the Filter Paper
1. Obtain the chromatography paper (or Whatman No. 1 filter paper.)
2. Cut filter
