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CHEM 121L General Chemistry Laboratory Revision 2.0 Paper Chromatography of a Metal Cation Mixture To learn about the separation of substances.To learn about the separation technique of chromatography.To learn how to use properties of known substances to identify unknown substances.In this laboratory exercise we will separate and identify the cations in an aqueous mixture that 3+ 2+ 2+ possibly contains Fe , Ni or Cu salts. The separation will be achieved using Paper Chromatography. Identification of the cations will be affected by simultaneously running separate solutions, each containing a single salt of these ions, through the chromatographic system and comparing the results against those of the unknown solution. Separation of substances is a key component in most chemical processes. Reaction products need to be separated from associated byproducts, complex natural systems need to be separated to obtain a desired component, etc. Thus, in some sense, the science of chemistry is really the science of separations. And, over the years, chemists have developed an enormous array of techniques for the separation of substances. In 1906 the Russian scientist Mikhail Semenovich Tswett reported separating the different colored pigments of plant leaves by passing a liquid extract of the leaves through a column of calcium carbonate (think powdered chalk). He coined the term chromatography, from the Greek wordsχρώµα(chroma or“color”) andγραφειν(graphein or “to write"), to describe this process. Presently, chromatography is the general name applied to a series of separation methods that employ a system with two phases of matter; a mobile phase and a stationary phase. The separation process occurs because the components of the mixture have different affinities for the two phases and thus move through the system at different rates. A component with a high affinity for the stationary phase moves more slowly, whereas one with a high affinity for the mobile phase moves more rapidly. Paper chromatography, probably the simplest chromatographic system, employs a strip of porous paper for the stationary phase. A drop of the mixture to be separated is placed on the paper, which is then dipped into a liquid, the mobile phase. The liquid travels up the paper as though it were a wick. The separation occurs as the liquid moves along the paper, carrying along with it, most rapidly, those components with a low affinity for the paper and leaving behind those with a high affinity for the paper. In this type of chromatography, the paper used is a highly purified cellulose with sufficient adsorbed Water that the stationary phase is actually aqueous.
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In a given chromatographic system, using the same type of paper, each substance can be characterized by a constant called the Retention Factor, R . By definition: f Distance from Origin Spot Travels R =(Eq. 1) f Distance from Origin Solvent Front Travels The R value is a characteristic property of each species in a mixture. The retention factor simply f measures the fraction of the distance each species travels, relative to the distance the solvent travels. In order for this technique to be effective, the paper and eluting solvent must be chosen such that each compound in the mixture has a different Rfvalue.
P a g e|3 Notice as well each compound’s Rfis not the only thing determining this method’s Resolution. As each compound migrates, their spot broadens as the material diffuses away from the center of the spot. The reasons for this broadening are complex and will not be discussed here. Note only that the longer the system is allowed to develop, the broader will be the spots. And, this breadth also affects the Resolution. Thus, the breadth of each spot is another important parameter of a paper chromatographic system. And, the usual quandary when establishing a chromatographic system is that the system should run long enough that the Rf’s of the various species will be different, but not so long that broadening causes the spots to overlap. In general, Paper Chromatography is considered to be a low resolution technique. Other chromatographic techniques have been developed to significantly improve resolution. We will use this chromatographic technique to separate a mixture of salts (Cupric Nitrate, Nickel Nitrate and Ferric Nitrate) dissolved in water. The presence of each salt can be detected independently, once the separation has been completed. Ferric Nitrate, Fe(NO3)3, which contains iron, produces a rust color on wet paper. Cupric Nitrate, Cu(NO3)2, reacts with ammonia to produce a deep blue color. The Nickel Nitrate, Ni(NO3)2, reacts with the organic reagent Dimethylglyoxime (DMG) to produce a pink color.
P a g e|4 PreLab Questions3+ 2+ 2+ 1. How are each of the cations, Fe , Ni and Cu , visualized on the paper? 2. In the procedure below, you are specifically directed to identify each original spot using a penciled mark. Why do you suppose an ink pen cannot be used for this purpose? 2+ 3. Suppose Zn where also a cation in the mixture. And further suppose the resulting chromatogram shows a spot corresponding to this cation at 2.7cm from its origin. If the 2+ solvent traveled 8.8cm from the same origin, what is the Rf?value for Zn
P a g e|5 Procedure1. Prepare about 25 mL of eluting solution (mobile phase). This is a mixture of 19 mL of acetone and 6 mL of 8 M HCl. Pour this into a 1000 mL beaker; which will act as the chromatography tank. Cover the beaker with a piece of aluminum foil. This allows the atmosphere within the beaker to become saturated with solvent vapor and helps to give a better chromatographic separation. 2. Obtain a piece of chromatography paper.Handle this from the side edges or with gloves on as the oils from hand can alter the properties of the paper surface sufficiently to affect the running of the chromatogram. Draw a pencil line about 2 cm from the long edge of the paper. This line will indicate the origin. 3. Draw out 3 capillary tubes, using the micro Bunsen burner, to use as a "spotter". (Your instructor will show you how to do this.) Using a different "spotter" tube for each solution, transfer a drop of each solution listed below to the penciled line to give a spot about 0.5 cm in diameter. Each spot should be about 3 cm apart. Allow the spot to dry momentarily, then transfer a second drop of each solution to the appropriate spot. With a pencil, identify each spot on the paper. The solutions are: 1) Ferric Nitrate, Fe(NO3)3 2) Nickel Nitrate, Ni(NO3)2 3) Cupric Nitrate, Cu(NO3)2 4) An unknown mixture of the above salts. 4. Form the paper into a cylinder without overlapping the edges. Fasten the paper at the top and bottom with staples. 5. Carefully place the paper cylinder into the chromatography tank and replace the cover. Wait as the solvent front moves up the paper. Do not move the beaker. If the movement of the solvent front appears to stall, burp the fumes in the chromatography tank by briefly opening the cover. 6. When the solvent has risen to within 1 cm of the top of the paper, remove the paper from the tank and quickly mark the solvent front with a pencil. Remove the staples and dry the paper with a hot air dryer. If no hot air dryer is available, dry the paper over aluke warmHot Plate. 7.Working in the fume hood, hold the paper just over the top of a large evaporating dish containing about 100 milliliters of concentrated ammonia solution. A deep blue color will indicate the presence of cupric nitrate. Any white "smoke" which appears is really Ammonium Chloride due to a reaction between Hydrochloric Acid in remaining eluting solution and the Ammonia: Hydrochloric Acid + Ammonia Ammonium Chloride or
8.
9.
HCl(aq) + NH3(aq)
NH4Cl(s)
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While the paper is still moist with ammonia, spray the paper with the DMG solution. A pink color will indicate the presence of nickel nitrate. If no pink color appears for the "Known" nickel salt, again hold the paper over the ammonia dish. This should help “bring out” the nickel spots.
For each spot, measure the distance the spot traveled and the breadth of each spot.
Data Analysis1. Calculate the Rfvalue for each known salt. 2. Calculate the Rfvalue for each spot produced by the unknown. 3. Identify the cation salts in the unknown mixture.
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P a g e|8 Post Lab Questions1. If you had not been told there were only three possible compounds in your unknown mixture, would you be able to determine the number of compounds in your mixture based on your chromatographic results? Explain. (Hint: Consider what would happen if two compounds have the same Retention Factor.) This is a problem with any separation technique. 2. Two extreme values for RfExplain what each value means in terms of theare 1 and 0. compound's affinity for the paper versus the eluting solution. 3. Why do you mark the solvent front immediately upon removal of the filter paper? 4. Amino acids, the building blocks of proteins, can easily be separated via paper chromatography. In fact, Insulin, the first protein whose amino acid composition was determined, had its composition determined by paper chromatography. Consider a mixture containing three amino acids that has been separated via paper chromatography. It is found the retention factor for each component is as follows: Component R f Glycine 0.37 Proline 0.65 Leucine 0.76 Sketch a diagram of a chromatogram. Assume the piece of paper is 20cm high and the solvent front runs 17cm from the original spot, dotted 1cm above the bottom of the paper. Be as quantitative as possible.
