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Separation Of A Dye Mixture Using Chromatography

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  • Pages: 4
  • Word count: 844
  • Category: Chemistry

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The Rf values of seven known color dyes and three unknown samples were observed in order to compare results and discover the hidden identities of the three samples. Three solvents, H2O, Isopropyl alcohol, and a 2% NaCl solution, were introduced to the dotted chromatography strips to show three different ratios for each dye on account of polarities. We found that all of the Unknown samples contained more than one pigment, sample one yielded Red #3 and Blue #1. Unknown sample two also revealed the same Blue #1, along with Green #3. Lastly, unknown sample three was composed of yellow #5 and Red #3 (The red #3 may have been accompanied with a blue dye, as the hue appeared to be slightly blue).


The purpose of this experiment is to identify substances within a sample (in this case, the color dyes) through the use of chromatography. Utilizing the Rf value, we can observe the ratio of movement certain substances make. The “movement” is determined by polarity, solubility, and the choice of solvent to be used. Each combination will result in a unique ratio, thus allowing us to compare unknown samples and discover their identity. The Rf value is essentially the rate the dotted sample will move in accordance to the solvent rising up the chromatography strip via capillary action. As the solvent gets sucked up to the solvent front, the sample dissolved into the paper will “grab” onto the moving solvent.

Polarity plays a huge role on the ratio, as it affects the substance’s ability to migrate with the solvent. A higher polarity will yield a much higher ratio. The less attraction (polarity) there is between the solvent and the sample, the lower the dot will travel. The reason we had tested three differing solvents for each dye is so we would have three unique ratios to compare to, raising the chances of a viable match with an unknown dye. Solubility also follows the same rule. A soluble drop will “follow” the solvent up to the solvent front whereas a non- soluble substance will not budge. Another notable occurrence is that the dotted substance never will travel downwards below the base line. In separation of the components, they have little to no opposition of gravity as the capillary action will carry it straight upwards along with the solvent, resulting in never having a negative Rf value.


To begin our experiment, we prepared chromatography paper to be used in beakers or Erlenmeyer flasks. After all of the paper had been marked with pencil (rather than pen, as the ink would alter data results), we “dotted” them using toothpicks with the corresponding color dye. Whilst one partner performed the dotting, the other gathered 20 mL of each solvent- H2O, Isopropyl alcohol, and the 2% NaCl solution. With all of our materials gathered, we began the process of chromatography by inserting our paper strips into three flasks containing one of each solvent. The method used was to test each dye one by one with all three solvents, recording our findings. We then moved through all of the dyes with this method. After the dyes had been completely finished, we moved onto the unknown color dyes. Here, we used the same method and compared the findings for the unknowns with our previous data. The identities of all the mystery dyes were found.


In this experiment, the method of paper chromatography is practiced to determine unknown identities of three mystery dyes. Using the results received for known color dyes (controlled variable), we can compare with the results obtained from the mixed dyes. The unknown dyes yielded multiple pigments indicating that they were mixtures. Separation of components allowed us to accurately determine that unknown dye one had Red #3 and Blue #1; unknown dye two was composed of Blue #1 and Green #3; and unknown dye three was comprised of Yellow #5 and Red #3. We were unsure of if there had been a blue dye present within unknown sample 3, as Red #3 appeared to be almost purple rather than red.

Obviously, errors have occurred- my results on Rf values for the controlled color dyes turned out to be differing from the results of the unknown dyes. Many contributing factors could have taken part, perhaps the dots of color dyes were not consistent, or the flasks were not cleaned thoroughly. One error performed was overlooking “Handle the [chromatography] paper by the edges so the analysis area is not… contaminated”. Disregarding this importance resulted in our test for Blue #2’s Rf value to reveal a yellow streak to the side. Since both Yellow #5 and #6 have higher Rf values, the unintentional presence of the dye may have pulled some of the Blue dye higher. All and all, Blue #2 was not one of the unknown mixtures as I still had the valid results for the two other solvents; both the process of identifying the unknowns and the overall outcome of the experiment was not affected.

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