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
KNO2 C oNO2 Groups IV and V MnS,Al2S3,Cr2S3
Aqua Regia Ni+2
DMG NiDMG Mn+2,Al+3,Cr+3
KClO3 MnO4- Al+3,Cr+3
Dakota State University AlOH
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
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
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.
These are the last of the ions that can...
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