QualGroupB - Foothill Co llege- Chem istry 1C Name:...

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Foothill College-Chemistry 1C Name: Dr. L.J. Larson 1 Revised/Printed 6/27/08 QualGroupB.doc Separation and Identification of Group B Cations (Bi 3+ , Fe 3+ , Mn 2+ , Cr 3+ and Al 3+ ) Objectives To understand the chemical reactions involved in the separation and identification of the Group B cations. To complete a flow diagram summarizing the qualitative analysis scheme for the Group B ions. To successfully identify the Group B cation(s) in an unknown. Background Chemistry and Discussion To separate the Group B cations from Groups C and D, ammonia solution is added to the supernatant left after precipitation and removal of the Group A cations. Addition of ammonia to this supernatant results in the formation of a buffer solution. The pH of the buffer solution is adjusted to between 9 and 10 and, under these conditions, the Group B cations precipitate as hydroxides while Groups C and D remain in solution. The hydroxides of Group B have characteristic colors that may be useful in their identification. As part of your analysis, you will test known samples to determine the colors of the Group B hydroxide precipitates. Unfortunately, the color of the precipitate alone cannot always clearly identify the ions present in an unknown since light colored precipitates may be masked by darker ones. Further analysis is usually needed. Before analyzing the Group B precipitate, any lead(II) ion that did not precipitate during the Group A analysis is removed from the Group B precipitate by addition of sulfuric acid. The Group B hydroxides will dissolve and then stay in solution upon addition of H 2 SO 4 while lead(II) hydroxide dissolves and then reprecipitates as insoluble PbSO 4 . The Group B1 (Bi 3+ , Fe 3+ , Mn 2+ ) and B2 (Cr 3+ , Al 3+ ) cations are then separated from each other by adding NaOH to make a strongly alkaline solution followed by the addition of H 2 O 2 . The hydroxides of aluminum and chromium (III) are amphoteric and will dissolve in excess strong base, forming the complex ions Al(OH) 4 and Cr(OH) 4 . Hydrogen peroxide acts as an oxidizing agent in alkaline solutions, resulting in the conversion of the green colored Cr(OH) 4 ion to yellow CrO 4 2– . The Group B1 cations are insoluble in strongly alkaline solution containing H 2 O 2 : the Bi 3+ and Fe 3+ ions form insoluble hydroxides while Mn 2+ is oxidized to Mn 4+ by the H 2 O 2 yielding insoluble MnO 2 . Thus, after making the solution strongly basic with NaOH and adding H 2 O 2 the B1 cations are present as precipitates while the B2 cations are in solution. The two subgroups are then separated by centrifuging and decanting. The precipitate containing the Group B1 cations is heated to destroy excess H 2 O 2 that could interfere with later tests and then dissolved in hot hydrochloric acid. The ions present in solution are Bi 3+ , FeCl 4 and Mn 4+ . (Iron (III) forms a complex ion with chloride ions in solution.) The confirmation test for the Group B1 cations can be performed without further separation;
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This note was uploaded on 09/30/2009 for the course CHEM CHEM 100A taught by Professor Chem100a during the Spring '09 term at UCSD.

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QualGroupB - Foothill Co llege- Chem istry 1C Name:...

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