Question

2. If two different compounds have the same Rr value, how might they be identified using paper chromatography? 3. Why is it important to keep the spots on the paper as small as possible? 4. If the solvent pool at the bottom of the beaker touched the spots on the 1.5cm line, what would happen to the spots? How would you address this error? 5. If you were teaching a student how to do paper chromatography, what three pieces of advice would you give to help your student avoid having to redo the experiment due to experimental error? Lab 9: Paper Chromatography Adapted from: "Chemical Principles in the Laboratory" by Slowinski and Wolsey Materials 600-mL beaker Whatman paper - 0.1 M each silver nitrate, cobalt (II) nitrate, copper (II) nitrate, iron III nitrate, and mercury (II) nitrate Running solution (500 mL 6.0M HC1, 400 mL ethanol, and 400 mL n-butanol) Developing solution in a spray bottle (8.0 g potassium ferrocyanide and 3.5 g potassium iodide in 1.0 L of solution) - Capillary tubes Objectives: To identify metal cations in an unknown solution Background: Chromatography is defined as the separation of mixtures into their constituents by preferential adsorption by a solid, as a column of silica or a strip of filter paper. This field of separation science is one of the most important in science today. This technique is the basis for identifying the substances present in a newly discovered plant for drug research, for identifying the components of an unknown mixture by a forensic laboratory and to understand the products of a chemical reaction when performing chemical research. Chromatography in its many forms is one of the tools of the analytical chemist In this experiment, you will perform paper chromatography of metal cations. You will use known solutions of cations to determine the composition of an unknown mixture. You will use several methods to develop and identify the cations on the paper substrate. Chromatography takes a number of forms in the modern laboratory high-performance liquid chromatography (HPLC), gas-liquid chromatography (GLC), column chromatography, thin-layer chromatography (TLC), and paper chromatography, to name the most common. All of these techniques have elements in common that give rise to the separation of the compounds in a mixture In 1906, Mikhail Semenovich Tswett, a Russian botanist, is credited with the first experimental work in this field. Tswett extracted the compounds responsible for the green color of leaves using ether and passed the extract through a column packed with CaCO (white chalk). The separated compounds appeared as colored bands on the column, thus the name chromatography For this technique to work the compounds do not need to be colored, most are in fact colorless compounds. There are a variety of ways to detect the position of the compound on the substrate and/or collect it when it flows off the substrate in the solvent. Two factors are common to chromatography: the movement and interaction of the sample through the substrate (solid phase) by a mobile phase (gas or liquid phase). The sample mixture of substances to be separated is placed in a very small spot on the substrate. The substrate is then treated with a mobile (gas/liquid) phase to begin the movement of the sample across or through the substrate. The liquid phase is composed of a mixture of solvents or a pure solvent in which the components of the sample to be separated are partially soluble. Components that are not soluble will be left in the original location of the sample on the substrate. The solid phase particles or material will interact with the sample and hold onto the molecules to varying degrees. The components of the sample mixture will have differing solubility in the liquid phase and varying attraction to the substrate. As the liquid phase moves through the substrate the molecules that are alike will tend to form a band and become separated from other molecules in the mixture. Some components of the mixture will quickly flow through the substrate. Others will move more slowly. In this way, the sample will be separated into its components. To quantify the separation and give a measure that can be used to identify a compound, a mathematical function called the retention factor, Rf, has been defined. distance traveled by spot distance traveled by solvent The Rf value will vary with different substrates and solvent mixtures. For a comparison to be made, conditions must be controlled, i.e., the substrate and solvent system must be the same as well as the temperature. To reduce variations, a known sample is generally analyzed at the same time as an unknown. When the chromatogram is analyzed afterward, a direct comparison can be made between the unknown and the known samples. Ifa component appears in the same area of the substrate in both samples, the scientist will know what particular compound is present in the unknown You will be using paper chromatography to separate a mixture of metal cations. The paper substrate will be spotted with known solutions of cations and several unknown mixtures of cations. The solvent will be a mixture of concentrated hydrochloric acid, ethanol, and butanol. A developing solution will be used to bring out hidden spots on the paper. The known spots will be used to determine the composition of the unknown. Procedure: Prepare a developing chamber by obtaining a CLEAN, DRY 600mL beaker, plastic wrap, a rubber band, and approximately 10 mL of the prepared running solution. Pour the solvent carefully into the bottom of the beaker and immediately cover with the plastic wrap secured with a rubber band. The solvent will saturate the atmosphere inside the beaker while you prepare the paper Obtain a piece of Whatman #1 filter paper 10 cm x 20 cm. Be careful to handle the paper by the edges ONLY! Fingerprints will be visible on the surface of the paper after developing has taken place. Using a PENCIL, mark a line about 1.5 cm from one of the 20 cm long sides of the paper. You must use a pencil for all marks because ink is a mixture of compounds that is soluble in the solvent that will be used. You would end up with a chromatogram of the ink. Make marks at 1 cm intervals along the line you just drew. Below the line, label the eight marks for the five cations being investigated (Ag', Co2, Cu Fe, Hg and the unknowns (UI, U2, and U3). See figure below Using the sample bottles provided and the associated capillary pipette, make a small spot of the solution at each of the labeled marks on the paper. The spot should be kept as small as possible to prevent the bleeding together of the substances on the paper as they move with the solvent. A separate capillary pipette must be used for each solution. If there is any doubt about which solution a pipette was used form, discard it and obtain a new one. Once the spots have been placed on the paper, carefully pick up the paper and wave it in the air to evaporate the water solvent used for the cation solutions. You must not start the chromatography process until ALL of the water has evaporated. Water will interfère with the interaction between the cations and the solvent. Once the water is gone, the paper and staple the edges together as shown by your instructor. The edges of the paper must make a cylinder with NOT touch each other Quickly remove the plastic wrap from the 600 mL beaker and lay the circular chromatogram on the bottom of the beaker with the spots at the bottom. The pool of solvent must not reach the level of the spotted chemicals on the paper. The paper must also not touch the walls of the beaker Watch the beaker as the solvent front moves up the paper toward the top of the beaker. When the solvent has reached within 1.5 cm of the top of the paper, open the beaker and remove the chromatogram. With your PENCIL, mark the paper where the solvent front reached along the top. While they are still visible, mark as many of the cation spots as possible by outlining them with your pencil and indicating the color. Allow the paper to completely dry in the air. If the odor bothers you, go to the fume hood and wave the paper (holding it by the edge) through the air to increase the evaporation of the solvent After the chromatogram is dry, spray with the developing solution and mark any spots that appear. Be sure to note the color of the spots. Allow the chromatogram to dry completely again and measure the distance the solvent front moved (from the 1.5 cm baseline at the bottom). Then, measure the distance that each cation spots has moved from the starting baseline. Calculate the Rr value for each of the known cations and for the spots found for each of the unknowns. By comparing colors and Rr values you should be able to determine the composition of each of the unknowns

Answer 1

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