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

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Foothill College-Chemistry 1C Name: Dr. L.J. Larson 1 Revised/Printed 6/28/08 QualGroupD.doc Separation and Identification of Group D Cations (Cu 2+ , Ni 2+ , Mg 2+ and Zn 2+ ) Objectives To understand the chemical reactions involved in the separation and identification of the Group D cations. To complete a flow diagram summarizing the qualitative analysis scheme for the Group D ions. To successfully identify the Group D cation(s) in an unknown. Background Chemistry and Discussion The Group D cations are characterized as having hydroxides, oxides and oxalates that are soluble in an ammonia/ammonium ion buffer solution. Three of the Group D cations, Cu 2+ , Ni 2+ and Zn 2+ , form complex ions with ammonia. These complexes ions are very stable and prevent the cations from precipitating when oxalate is added even though their oxalates are relatively insoluble. Many complex ions are highly colored and the color of your solution at this stage may help in identification of the cations in your unknown. Testing known solutions to determine the color of the complex ions formed is recommended. Magnesium hydroxide will precipitate in aqueous ammonia, however the precipitate can be prevented from forming if the solution contains a significant concentration of NH 4 + . This can be understood by considering the common ion effect and the following equilibrium: NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) In an NH 3 /NH 4 + buffer solution, this equilibrium is shifted left, reducing the hydroxide ion concentration so that Mg 2+ does not precipitate. Magnesium oxalate has a K sp value that is relatively high and under the conditions of the experiment should not have precipitated if a limited amount of oxalate was added in a previous separation step. The presence of ammonia and possibly oxalate ions in the supernatant liquid containing Group D can interfere with characterization tests for the cations in this group. Therefore, prior to testing the ammonia and oxalate, if present, must be removed from the solution. This is accomplished by heating the solution in a crucible over a low flame to dryness, adding concentrated HNO 3 and then reheating to dryness. THIS HEATING MUST BE DONE IN THE HOOD! The following reactions occur: NH 4 + (aq) + Cl (aq) NH 4 Cl(s) NH 3 (g) + HCl(g) NH 4 + (aq) + NO 3 (aq) NH 4 NO 3 (s) N 2 O(g) + 2H 2 O(g) And if oxalate is present: 2NO 3 (aq) + C 2 O 4 2– + 4H + (aq) 2NO 2 (g) + 2CO 2 (g) + 2H 2 O(g) Note that if oxalate is not present in your sample, then the HNO 3 need not be added prior to heating; NH 3 will still be removed as shown in the reaction that does not involve the nitrate ion given above. After heating to dryness, the residue contains the oxides of the Group D cations: Copper (II) oxide is black, nickel oxide is
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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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QualGroupD - Foothill Co llege- Chem istry 1C Name:...

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