1) Why is a pencil used to mark the chromatogram and not a ballpoint or ink pen? 2) Explain why...

Question:

1) Why is a pencil used to mark the chromatogram and not a ballpoint or ink pen?

2) Explain why the cation samples are repeatedly spotted and dried on the chromatogram.

3) The eluent is to be below the 1.5-cm line on the chromatographic paper. Describe the eluent were above the 1.5-cm line.

4) Explain why the center of the band is used to calculate the R{eq}_f{/eq} value for a cation of the band.

5) Suppose two cations have the same R{eq}_f{/eq} value. How might you resolve their presence chromatography?

Paper Chromatography:

Chromatography is a general name for a group of analytical techniques that serve to separate and identify components in a mixture. It is based on the use of a mobile phase and stationary phase where a sample mixture carried along by the mobile phase has its components interacting in a varying manner that serves to slow the progression of the components through the stationary phase but to different degrees. A popular chromatographic method encountered early in academic chemical laboratories is paper chromatography, where typically filter paper serves as the stationary phase and a solvent (or mixture of solvents) serves as the mobile phase. With the paper positioned vertically and one end in contact with the mobile phase, the mobile phase moves up the paper by capillary action carrying the sample and along with it and separating into spots on the paper.

Answer and Explanation: 1


> 1) Why is a pencil used to mark the chromatogram and not a ballpoint or ink pen? - Drawing the starting line and other demarcations in pencil is important as a pencil is primarily carbon (graphite) with maybe some binding components. A ball-point pen's ink, on the other hand, is made of a mixture of liquid components that may interact with the mobile phase and separate into its constituent components, which will interfere with the analysis of the sample mixture(s). The ink may also interact chemically with the samples changing their nature and give inaccurate results.

> 2) Explain why the cation samples are repeatedly spotted and dried on the chromatographic paper. - As can be seen from a completed chromatogram, the spots, as they move up the paper, spread as they move. If you were to apply the needed volume of cation sample at one time, it would bleed into a large spot on the paper. If this were allowed to move in the mobile phase, it would create a very diffused moving sample possibly contacting and interfering with adjacent samples. Also, depending on where the separated spots are visualized, the concentration of cation may be too dilute to give a good reading. By spotting and drying the sample multiple times, you are able to confine the spot to a small area and increase its concentration. This makes for better transport up the paper without contacting adjacent sample or reference mixtures as they too move up the chromatographic paper.

> 3) The eluent is to be below the 1.5-cm line on the chromatographic paper. Describe what would happen if the eluent were above the 1.5-cm line. - If you added too much eluent to the chromatographic container, the samples will simply diffuse and bleed into the eluent without progressing up the paper. We are depending on the capillary action of the eluent up the paper and the interaction of the samples with the paper in order to separate them. Submerging them at the start in the eluent will not permit this separation and identification method to do its job.

> 4) Explain why the center of the band is used to calculate the R{eq}_f{/eq} value for a cation of the band. - You will see that the bands created are diffuse and often appear like a tear-drop shape with a head and a tail. Setting a standard analysis procedure using the center of the bands helps to minimize perceptive errors that might occur if choosing the head or tail regions as the measurement points. This minimizes and standardizes the contribution of reading errors in the R{eq}_f{/eq} value determination.

> 5) Suppose two cations have the same R{eq}_f{/eq} value. How might you resolve their presence chromatography? - If initially two cations have the same R{eq}_f{/eq} values under given chromatographic conditions, the first thing to try is a change of eluent solvent or solvent parameters such as pH. Changing the pH could alter how the cations react with the anions present in the mobile phase. Changing the solvent or preparing a mixed solvent eluent could change again the interaction of the cations with the eluent and thus the stationary phase also. This could be accomplished by several methods, including cutting out the spot containing for the two cations, extracting them from the paper by soaking it in a solvent, concentrating the cations by reducing the solvent volume by evaporation or other means, or spotting them onto a fresh paper chromatogram. Once you try one of these methods, try your new or changed eluent.


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Retention Factor in Chromatography: Definition & Formula

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Chapter 1 / Lesson 9
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Understand what the retention factor in chromatography is by learning its definition. Learn how to calculate retention factor using the retention factor equation.


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