Ion Exchange Separation of Chromium Complexes
Safety Note: Perchloric acid, HClO4, is a strong oxidant and can react explosively with some organic compounds such as acetone. In the present experiment, which is conducted entirely in aqueous solutions, there should be no hazard as long as the solutions are kept away from organic solvents. At the end of the experiment place all the solutions that contain chromium and perchloric acid into a labeled waste receptacle for them. The solutions should never be poured down the sink.
Preparation of Ion Exchange Column
A buret (about 10-mm inner diameter) may be conveniently used in this experiment. Fill the buret three-quarters full with distilled water. Push a small plug of cotton or glass wool to the bottom of the buret with a rod. With the stopcock on the buret open and a large beaker positioned under the tip of the buret, pour into the buret a slurry of Dowex 50W-X8 (50-100 mesh, H+ form) cation exchange resin in deionized water until a final resin height of approximately 15cm is achieved. Allow water to pass through the resin until the effluent is colorless (this should take less than 100ml). Then lower the water level so it coincides with the top of the resin, and close the stopcock. If the water level drops below the top of the resin, channels will develop. Because channeling reduces the separation efficiency of a resin, never allow the resin to dry.
Prepare 200ml each of 0.1, 1.0 and 4.0M solutions of HClO4. These solutions will be used to elute the desired complexes.
trans-Dichlorotetraaqupchromium (III) Ion, trans-CrCl2(OH2)4+
Prepare a 0.35M Cr(III) solution by dissolving 2.33 g (8.7mmol) of commercial CrCl3 6 H2O in a mixture of 25ml of H2O and one drop concentrated HClO4. Do no heat the solution to dissolve the solid. Important: Record the time of day when the dissolution of the CrCl3 6 H2O is performed. Portions of the Cr (III) solution will be used throughout the experiment.
Immediately after this 0.35M Cr(III) solution is prepared, add 5 ml of it to the cation exchange column previously prepared and drain until the solution and resin levels are the same. Elute with 0.1M HClO4. When the most intense portion of the broad green band begins to drip out of the buret, collect a 5-ml fraction and immediately record the spectrum of the fraction on a visible spectrophotometer in the wavelength range from 350 to 800 nm using glass cells of 1 cm path length.
Chloropentaaquochromium (III) Ion , CrCl(OH2)52+
When CrCl2(OH2)4+ is warmed briefly in aqueous solution, much of it is converted to the cation CrCl(OH2)52+. Place an Erlenmeyer flask containing 5 ml of the 0.35M Cr(III) solution in a boiling hot water bath for 3 mins. (Do not allow the solution inside the flask itself to come to a boil). After 3 min are up, remove the Erlenmeyer flask from the bath and immediately add 5ml of distilled water. Pour the entire solution into the same buret that was used above. Drain the solution to the resin level and flush the column with 0.1M HClO4 until the unreacted CrCl2(OH2)4 has been eluted. The desired complex, CrCl(OH2)52+ is then eluted with 1.0M HClO4. Collect 5 ml of the most intensely colored fraction that elutes. Measure the visible spectrum of the solution.
Hexaaquochromium (III) Ion, Cr(OH2)63+
Combine 5 ml of the 0.35 M Cr (III) solution with 5 ml of distilled water and boil for 5 min. Add all of this solution to the same buret and drain until the solution level reaches that of the resin. First rinse the column with 1.0M HClO4 to remove any unreacted CrCl2(OH2)4+ or CrCl(OH2)52+. Then elute the complex with 4.0 M HClO4. Collect 5 ml of the intensely colored portion of the eluted Cr(OH2)63+ solution and then record its visible spectrum.
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