Paper Chromatography Lab Report

The chromatography paper is a very long molecule (a polymer) in which thousands of rings of six atoms are covalently bonded together into long chains which form tiny tubes within the paper. The water molecule has been spilt leaving oxygen between each ring The long chains are held together by side-to-side hydrogen bonding(O-H) between the chains, so there is also weak dipole-dipole moment. These side by side long chains are called sheets of cellulose. The sheets are held one on top of the other by Van der Waals forces. The geometry of the short, carbon-hydrogen bonds minimizes the distance between layers and, therefore, Van der Waals forces are maximized. The paper has polar and nonpolar parts; it has a strong affinity to itself and materials

containing hydroxyls, in this case water and isopropanol have hydroxyls. Paper loses its strength when wet because the hydroxyls get between the cellulose chains and weaken the attraction between them.

As the isopropanol travels up the paper, due to the capillary action which is caused by the greater attraction of the solvent molecules to the surface of the chromatography paper then the intermolecular bonding between the solvent atoms. The solvent will hydrogen bond with the paper, the slightly negative oxygen of the paper will bond with the slightly positive Hydrogen of the solvent, and the slightly negative oxygen of the solvent will bond with the positive Hydrogen of the paper. As the solvent starts to move past the pigment, some of its molecules are sucked toward the surface of the paper and stick there temporarily before being pulled back again into the liquid they came from. This exchange of molecules between the surface of the paper and the solvent is adhesive effect called adsorption. The liquid's molecules will stick fast (adhesive) to the paper surface and also to each other (cohesive), so that each molecule pulls the next one along. The capillary action only occurs when the adhesive forces; attraction between the solvent and paper molecules are stronger than the cohesive forces, which is the intermolecular attraction between the solvent

molecules. As the solvent is moving higher up the paper, the pink pigment molecules move with the isopropanol, because the pink pigment is more attracted to the isopropanol molecules due to probably weak dipole-dipole, and london dispersion forces than to the chromatography paper molecules. The solvent and the pink pigment would have a weak dipole-dipole moment because both of them are weakly polar and polar bonds have dipole-dipole moment and everything has london forces. As the concentration of the solvent is decreased, hence the polarity increases, the orange pigment moves up the paper with the diluted solvent because the orange pigment is attracted to the higher polarity solvent. But the pink pigment molecules are less attracted to the higher polarity solvent, hence less soluble. Therefore during the lab as the concentration was increased the pink pigment affinity for the paper increased and its affinity for the solvent decreased, so it was absorbed more into the fibers of the paper. Which made the pigment move slower up the paper then the solvent. It was the opposite for the orange pigment, it moved higher up and faster as more water was added to the solvent. These pigments travel up the paper until the bonds between the solvent and pigment became so weak that the pigment had to break the attraction and leave itself imprinted at a certain height up the paper. Since two different pigments were observed during the lab, it is assumed that the water soluble marker was a mixture, made of different molecules. Since the pink and orange pigments moved at different rates as the solvent moves up the paper, it because different molecules formed different bonds with the chromatography paper and the solvent.

Error:
During the lab, the dotted pencil touched the 50% solvent.

This should not have happened because it changed the Rf value. Instead of moving up the paper, if the dotted line touches the solvent, the pigment would dissolve and move down the paper to the solvent. Since all of our trials for the 50% concentration was done on the same paper, it affected all the trails. For next time, do each trail on a different piece of chromatography paper.

Regular water was used instead of distilled.

During the experiment of the 50% concentration of isopropanol, the chromatography paper touched the walls of the graph. The experiment was repeated a 2nd with the 50% concentration but again the paper touch the glass walls. This is not good, because the solvent has the potential to attract to the glass. This results to inaccurate data because the pigment would be travelling towards the side of the paper and the Rf value would change.

Conclusion: The pink pigment solubility decreases as the concentration of isopropanol solvent decreases. The orange pigment solubility increases as the concentration of isopropanol decreases.

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