Group III: Notes, Procedures and Reactions

Transcription

1 Group III: Notes, Procedures and Reactions Cations in Group III: Ni +2 Co +2 Mn +2 Fe +3 Al +3 Cr +3 Zn +2 Group III Notes: 1. The sulfides in Group III precipitates oxidize readily in air, forming soluble sulfates, which would dissolve in washings. Thus the separation, washing, and treatment of these precipitates need to be carried out as quickly as possible and with a minimum exposure to air. NH 4 Cl is added to the wash water to decrease the tendency of Group III sulfides to become colloidal (a colloid is a very fine solid which will not easily centrifuge). Should a colloid develop, reheating can sometimes increase the crystal size. 2. The separation of cobalt and nickel is based upon the fact that these sulfides dissolve the least rapidly. This separation is not clean. After one minute of shaking, only a small amount of CoS and NiS will have dissolved but most of the other cations will have gone into decantate. Iron and aluminum compounds may require more time. Take careful note of how easily each compound dissolves at this point when you do them alone and adjust your unknown trials accordingly. 3. Dimethylglyoxime is an organic compound that reacts with Ni +2 to form the organometallic cage compound in alkaline, neutral, or very slightly acidic solutions. It is possible to test the original solution for nickel. To 2 ml of the original solution add 1 ml NH 4 Cl and then NH 4 OH till strongly basic. Centrifuge and decant and test the decantate with several drops of dimethylglyoxime. The typical pink-red precipitate shows the presence of nickel. 4. Fe +3, being present in many chemical may contaminate and mask the test for cobalt by formation of red Fe(SCN) 6-3. Addition of NaF will convert the Fe(SCN) 6-3 ion to the stable and colorless FeF 6-3 ion, which will allow one to see the blue Co(SCN) In step 5, an excess of hydroxide is essential to form the aluminum and zinc hydroxyl complexes and an excess of hydrogen peroxide is essential to convert Cr +3 to the chromate ion. 6. Sodium bismuthate is a powerful oxidizing agent in a nitric acid solution. It oxidizes Mn +2 to MnO 4-1 under these conditions. This test may also be performed on the original solution. To 1 ml of the original solution, add HNO 3 until distinctly acid then add NaBiO 3 and stir or shake well. Purple color in the solution indicates the presence of manganese. 7. Iron is easily picked up as an impurity during analysis because so many chemicals contain small amounts of iron and many laboratory items contain iron. Do not report Fe +3 unless the test is blood red, and light red color is due to contamination and should not be considered a positive test. 8. A lake is a colored precipitate produced by the absorption of a dye on the surface of an insoluble metallic oxide or hydroxide. Aluminum contaminates many reagents and trace tests for this element are common. Carefully note the quantity and appearance of the precipitate in the known Al test. Also note that it is relatively easy to lose the aluminum steps 3 and 5.

2 9. In the confirmatory test for chromium, the precipitate may be yellow BaCrO 4 mixed with some white BaSO 4 causing the test to be a light cream color. Group III Procedures: Cadmium sulfide is the most soluble of the Group II sulfides while zinc sulfide is the least soluble of the Group III sulfides. Therefore, the ph used must be such that CdS precipitates but ZnS does not. On the General Unknown, the ph adjustment to Group II should be made BEFORE the addition of thioacetamide as the hydroxides will easily redissolve while the Group III sulfides precipitate, they would not redissolve. For Group III thioacetamide is once again used as the Group reagent but the precipitation takes place in an alkaline solution. Under these conditions the thioacetamide rapidly releases sulfide ions as shown in the following equation: CH 3 CSNH 2 + 3OH - -> C 2 H 3 O NH 3 + H 2 O + S -2 When the Group III solution is made alkaline with ammonium hydroxide, the hydroxides of all the Group III ions precipitate, but the sulfide ions convert the hydroxides of nickel, cobalt, manganese, iron, and zinc to the less soluble sulfides. Aluminum and chromium remain as the hydroxides. Analysis: 1. To make the test solution use 1 ml of the known or unknown, 1 drop of 6 M HCl and 5 drops of thioacetamide (please follow the thio rules-use fume hood). If this is your General Unknown, boil down your decantate from Group II to no more than 2 ml and use the entire sample. Heat in a hot water bath for 5 minutes. Add 5 drops of concentration NH 4 OH and 5 more drops of thioacetamide and stir up any precipitate that forms. 2. Heat the test tube containing the mixture in a boiling water bath for 5 minutes, centrifuge and test for completeness by adding 2 drops of thioacetamide to the decantate and reheating. Wash the precipitate(s) with one ml portions of hot water in which a small amount of solid NH 4 Cl has been dissolved. 3. Precipitate: NiS, CoS, MnS, FeS, Al(OH) 3, ZnS, and Cr(OH) 3. Add 5 ml of 1 M HCl, stir quickly, cork the test tube, and shake vigorously for one minute. Centrifuge, and decante immediately. Save the decantate for step Precipitate: NiS and CoS. Add 3 drops of concentrated HCl and 1 drop of concentrated HNO 3. Heat in a hot water bath until the precipitate is dissolved. Separate and discard any free sulfur. Pour the solution into a 50 ml flask, place it on the hot plate briefly, just till bubbles are visible. Add 6 M NH 4 OH until the solution is just basic to litmus. Divide the solution into 2 test tubes. a. Ni +2 test. Add 5 drops of dimethylglyoxime solution to one test tube. A pink-red precipitate confirms the presence of Ni +2. b. Co +2 test. Acidify the solution in the second test tube with 1 M HCl. Add several crystal of NH 4 SCN and agitate the mixture. Pour 1 ml acetone down the side of the test tube so that layer forms on top of the decantate. A blue color in the acetone lay confirms Co +2. If the solution becomes red

3 when NH 4 SCN is added, Fe +3 is indicated. Destroy the red color by the addition of 1 M NaF to allow the blue Co(SCN) 4-2 ion to be observed. 5. Decantate: Mn +2, Fe +2, Al +3, Cr +3, Zn +2. Transfer the solution to a 50 ml flask, add 1 ml of 4 M HNO 3 and evaporate almost to dryness (the center of the flask will almost be dry). Add NaOH until basic to litmus and then add at least 10 drops excess. Mix thoroughly by swirling the flask. Cool. Add 2 ml hydrogen peroxide (H 2 O 2 ) and when the foaming ceases, boil gently for two minutes. Should the liquid/solid mixture begin to explode out of the flask, remove it from the heat briefly as needed. Transfer the mixture to a test tube, cool and centrifuge. Save the decantate and the first washing for step 7. During the second washing, divide the mixture of wash water and precipitate into two test tubes before centrifuging. Centrifuge both test tubes and discard the second was solution. 6. Precipitate: Fe(OH) 3 and MnO 2. This precipitate is now in two test tubes (from step 5). You will use one to test for manganese, the second to test for iron. a. Test for manganese: Add a small (1/2 small scoop) of solid NaBiO 3 and 2 ml of 6 M HNO 3 to one of the test tubes. Stir and centrifuge immediately. A deep purple color in the supernatant liquid, due to the formation of the MnO 4-1 ion confirms the presence of manganese. b. Test for iron: Dissolve the precipitate in the second test tube in 6 M HCl. Add several drops of KSCN. A deep red color due to Fe(SCN) 6-3 confirms the presence of iron. 7. Decantate: Al(OH) 4 -, CrO 4-2, Zn(OH) 2-2. Note that the presence of the chromate ion here will impart a yellow color to the solution (the other two ions are colorless). Acidify with 6 M HCl and then make distinctly basic with 6 M NH 4 OH. Centrifuge and decant. 8. Precipitate: Al(OH) 3. Wash the precipitate thoroughly and then dissolve in several drops of 1 M HCl. Add 2 drops of NH 4 C 2 H 3 O 2 and 3 drops of aluminon. Add NH 4 OH till basic to litmus. The formation of a dyed red precipitate of Al(OH) 3, called lake, confirms the presence of Al Decantate: CrO 4-2 and Zn(NH 3 ) Heat 5 minutes. Add BaCl 2 until precipitation is complete. A light yellow precipitate confirms Cr +3. Centrifuge and decant. 10. Decantate: Zn(NH 3 ) Acidify the solution with 6 M HCl dropwise until just acidic to litmus. Add 3-4 drops of K 4 Fe(CN) 6. The formation of a blue-green precipitate of K 2 Zn 2 [Fe(CN) 6 ] 2 confirms the presence of zinc. Centrifuge the precipitate for easier identification. Group III Reactions: In this group thioacetamide is again used as the group reagent to generate H 2 S, which precipitates Group III ions as the insoluble sulfides. As these are more soluble sulfides, a basic solution is used to provide a higher concentration of sulfide ions. Chromium and aluminum precipitate as their hydroxides. Hydrogen sulfide is a strong reducing agent and the Fe +3 ion is reduced to Fe +2 during the initial heating and precipitates as iron(ii) sulfide: Co +2 + S -2 -> CoS

4 Al OH - -> Al(OH) 3 2Fe +3 + H 2 S -> 2Fe +2 + S + 2H + then: Fe +2 + S -2 -> FeS The sulfides of manganese, iron, and zinc and the hydroxides of aluminum and chromium redissolve in an acid solution: MnS + 2H + -> Mn +2 + H 2 S Al(OH) 3 + 3H + -> Al H 2 O The sulfides of cobalt and nickel dissolve in a mixture of nitric and hydrochloric acids: 3CoS + 8H + + 2NO 3 - -> 3Co NO + 3S + 4H 2 O After boiling the solution, remove the NO (because it would destroy the confirmatory reagents) by adding excess ammonia, which forms the hexaamine metal complexes. Co NH 3 -> Co(NH 3 ) 6 +2 (the cobalt hexaamine complex is pink and the nickel hexaamine complex is blue) The presence of nickel is confirmed by the formation of a pink dimethylglyoxime nickel neutral complex: O H-O CH 3 -C=NOH CH 3 -C=N N=C-CH Ni(NH 3 ) 6 -> Ni + 2NH NH 3 CH 3 -C=NOH CH 3 -C=N N=C-CH 3 O-H O The cobalt hexaamine complex ion is destroyed by the addition of hydrogen ions: Co(NH 3 ) H + -> Co NH 4 The positive test for cobalt is the formation of the blue Co(SCN) 4-2 : Co SCN - -> Co(SCN) 4-2 During the test for cobalt, if iron is present the red iron thiocyanate complex may form. In order to see the cobalt test the iron must be removed in the form of the fluoride complex, which is colorless: Fe SCN - -> Fe(SCN) 6-3 Fe(SCN) F - -> FeF SCN - In the evaporation of the solution in step 5, Fe +2 is converted back to Fe +3 : 3Fe H + + NO 3 - -> 3Fe +3 + NO + 2H 2 O

5 The solution is made basic with NaOH to precipitate all the ions as hydroxides: Mn OH - -> Mn(OH) 2 The addition of excess NaOH dissolves the hydroxides of aluminum, chromium, and zinc by the formation of the soluble hydroxyl complex ions: Al(OH) 2 + OH - -> Al(OH) 4 - (colorless) Cr(OH) 3 + OH - -> Cr(OH) 4 - (green) Zn(OH) 2 + 2OH - -> Zn(OH) 4-2 (colorless) Hydrogen peroxide (H 2 O 2 ) is added to the mixture to oxidize the manganese hydroxide to manganese dioxide and the chromite ion Cr(OH) 4 -, to the chromate ion CrO 4-2 : Mn(OH) 2 + H 2 O 2 -> MnO 2 + 2H 2 O 2Cr(OH) H 2 O 2 + 2OH - -> 2CrO H 2 0 The manganese is identified in the presence of iron by oxidizing it to the purple permanganate ion MnO4-, by reaction with sodium bismuthate in a nitric acid solution: MnO 2 + 2H + + NO 2 - -> Mn +2 + NO H 2 O 2Mn BiO H + -> 2MnO Bi H 2 O The iron hydroxide is dissolved in a hydrochloric acid solution and then confirmed by the formation of the iron thiocyanate complex ion, which is blood red. The decantate from step 7 is acidified to break up the hydroxyl complexes and convert the aluminum and zinc to their cations: Al(OH) H + -> Al H 2 O In this step the chromate ion is converted to the dichromate ion: 2CrO H + -> Cr 2 O H 2 O Excess ammonium hydroxide precipitates aluminum and zinc hydroxides, converts the dichromate ion back to chromate and redissolves the zinc hydroxide by forming the soluble zinc tetraamine complex ion: Cr 2 O Ba +2 -> BaCrO 4 Zn OH - -> Zn(OH) 2 Zn(OH) 2 + 4NH 3 -> Zn(NH 3 ) OH - The chromium is confirmed by precipitation as barium chromate: CrO Ba +2 -> BaCrO 4

6 Acidifying the solution breaks down the zinc tetraamine complex: Zn(NH 3 ) H + -> Zn NH 4 + The presence of zinc is confirmed by the formation of the mixed salt potassium zinc ferricyanide: 3Zn K 4 Fe(CN) 6 -> K 2 Zn 3 [Fe(CN) 6 ] 2 + 6K + Aluminum hydroxide is dissolved in hydrochloric acid and then reprecipitated in the presence of the organic dye aluminon which colors the precipitate red: Al(OH) 3 + 3H + -> Al H 2 O Al OH - -> Al(OH) 3