A red precipitate not just a red color to the

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Unformatted text preview: ions (Ni+2 , Co+2 , Mn+2 , Al+3 , and Cr+3 ; many qualitative analysis schemes include Fe +3 and Zn+2 in group III) precipitate on the addtion of ammonium sulfide, leaving behind groups IV and V. Separation of group III ions from groups IV and V: Heat the decantate from group II in a hot water bath to reduce the volume to Ni+2,Co+2,Mn+2,Al+3,Cr+3,group IV and V NH4OH,Na2S NiS,CoS,MnS,Al2S3,Cr2S3 HCl Ni+2,Co+2 KNO2 C oNO2 Groups IV and V MnS,Al2S3,Cr2S3 Aqua Regia Ni+2 DMG NiDMG Mn+2,Al+3,Cr+3 NaOH Mn(OH)2 KClO3 MnO4- Al+3,Cr+3 HNO3,NH4OH Cr+3 Na2O AlAluminon Dakota State University AlOH HCl,Aluminon CrO4-2 page 197 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual approximately 1/4th the original volume. To this, add ½ mL of conc entrated ammonium hydroxide. Check the alkalinity with litmus paper; if it is not alkaline, continue to add concentrated ammonium hydroxide until it is. Once alkaline, add ½ mL of sodium sulfide. The group III ions should precipitate out as sulfides. Centrifuge and decant; the solid is to be tested for group III ions; hold onto the decantate to test for groups IV and V ions. Separation of Ni+2 and Co+2 from Mn+2 , Al+3 and Cr+3 : Add 1 mL of dilute HCl to the precipitate; this should dissolve the nickel and cobalt salts, while leaving the rest as solids. Centrifuge and decant. Verification of Co+2 : To the decantate from the above separation step, add potassium nitrite, KNO2 , dropwise. If cobalt is present, a yellow precipitate should form. Continue adding potassium nitrite until no further precipitate seems to form. Centrifuge and decant. Verification of Ni+2 : To the decantate from the above step, and ½ mL of dimethylglyoxime (DMG). A red precipitate (not just a red color to the solution; it should be cloudy) indicates the presence of nickel. Separation of Mn+2 from Al+3 and Cr+3 : Dissolve the solid from the above separation step using as little aqua regia as possible. Once the precipitate has dissolved, neutralize it with 10 M NaOH dropwise until the solution shows alkaline to litmus paper. If a precipitate forms, it is manganese hydroxide. Centrifuge and decant. Verification of Mn+2 : Add just a little bit (about 1/4th the size of a pea) of potassium chlorate, KClO 3 , to the decantate from the above step. A pinkish or purple color is the confirmation of manganese. Separation of Al+3 from Cr+3 : Neutralize the decantate from the above separation step with concentrated nitric acid. Add concentrated ammonium hydroxide dropwise until the solution is alkaline. If a precipitate forms, it is aluminum. Centrifuge and decant. Verification of Al+3 : Dakota State University page 198 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual Dissolve the precipitate from the above separation step using dilute HCl. Add several drops of aluminon reagent. If a precipitate forms, it is aluminum. Verification of Cr+3 : Add a LITTLE BIT of sodium bismuthate, Na2 BiO 3 , to the decantate from above. A yellow color is verification of chromate. Group IV: These are the last of the ions that can...
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