In chemistry, chromatography is used as a physical separation technique for mixtures. Our most recent experiment utilized this technique for dye separation. Dyes are put together physically, not chemically, so they can separate using this method.
In the experiment, we followed the scientific method in multiple ways. To begin with, FD&C dyes were used as a control; in addition, different solvents were used. This meant that the experiment followed the scientific method strictly.
For the experiment, nine sheets of rolled paper, each with a set of dye dots, were dipped in three different mobile solvents. There were three different types of dye: FDC, Cra-Z-Art, and Vis-a-Vis. The three solvents were chromatography solvent, alcohol, and sodium chloride. The papers used were also a solvent, but they were stationary solvents, not mobile. The dots of dye on the paper landed just above the mobile solvent line, never touching the liquid. When the papers sat in the solvents for extended periods of time, the dye began to migrate up the paper along with the solvent. The rate at which the dye moved was dependent on the level of attraction between the dye and the paper. The more attracted the dye was to the paper, the less vertical movement occurred. From our experiment, we deduced that the nature of solvents and dyes follows a “like dissolves like” pattern because of shared chemical structure. Similarly, the different attraction levels observed throughout the experiment were determined to be the result of different chemical composition, or molecular structure.
Our conclusions can be supported by both qualitative and quantitative data. In terms of quantitative data, the proof is simply in the papers themselves. It is apparent that the color of the dyes changed as a result of the experiment. Regarding quantitative data, it is possible to calculate the extent of dye movement in relation to the mobile solvent by using a retardation factor formula. A retardation factor formula is the distance from the pencil line on the bottom of the paper to the dot, divided by the distance from the pencil line to the solvent front near the top of the paper.
Retardation Factor Formula
I calculated the Rf factor for the FDC dye and alcohol solution, and the Cra-Z-Art dye and NaCl solution.For the FDC, I measured 4 centimeters for the migration distance of substance, and five centimeters for the migration distance of solvent front, resulting in a retardation factor of .8. The Cra-Z-Art dye migration distance of substance measured 2.33 centimeters, and the migration distance of solvent front measured 3.5 centimeters, resulting in a Rf of .66571.
Chromatography Dye Movement Diagram
Another way to explain why some dyes travel further is particle diagrams. The more attracted the dye is to the paper, the less vertical movement. The image above exhibits the vertical movement of dye along with solvent and the level of dye separation, while the image below shows how our dye movement appeared on one of our papers.
This lab was quite useful for the reason that it reinforced my previous understanding about chemical/physical properties and changes. It reminded me that solubility is always a physical change, because bonds are not broken, only dissolving occurs during the process. Similarly, it reinforced my knowledge that while color change is an indicator of a chemical change, occasionally this is inaccurate and the change is only physical. My previous understanding about matter classification and separation techniques was also reinforced, because I was reminded that matter is classified by atoms, diatomic atoms, compounds, and mixtures. In this lab, we were working specifically with homogenous mixtures. In terms of separation, it reminded me that separation techniques have to match the type of mixture trying to be separated.