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. NH4Cl 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 NH4Cl and then NH4OH 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 FeF6-3 ion, which will allow one to see
the blue Co(SCN)4-2.
5. 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 MnO4-1 under these conditions. This test may also be performed
on the original solution. To 1 mL of the original solution, add HNO3 until
distinctly acid then add NaBiO3 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.
9. In the confirmatory test for chromium, the precipitate may be yellow BaCrO4
mixed with some white BaSO4 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:
CH3CSNH2 + 3OH- -> C2H3O2- + NH3 + H2O + 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 NH4OH 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 NH4Cl 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 5.
4. Precipitate: NiS and CoS. Add 3 drops of concentrated HCl and 1 drop of
concentrated HNO3. 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 NH4OH 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 NH4SCN 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
when NH4SCN 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 HNO3 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 (H2O2) 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 MnO2. 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 NaBiO3 and 2
mL of 6 M HNO3 to one of the test tubes. Stir and centrifuge
immediately. A deep purple color in the supernatant liquid, due to the
formation of the MnO4-1 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-, CrO4-2, 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
NH4OH. 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 NH4C2H3O2 and 3 drops of aluminon.
Add NH4OH till basic to litmus. The formation of a dyed red precipitate of
Al(OH)3, called “lake”, confirms the presence of Al+3.
9. Decantate: CrO4-2 and Zn(NH3)4+2. Heat 5 minutes. Add BaCl2 until
precipitation is complete. A light yellow precipitate confirms Cr+3. Centrifuge
and decant.
10. Decantate: Zn(NH3)4+2. Acidify the solution with 6 M HCl dropwise until just
acidic to litmus. Add 3-4 drops of K4Fe(CN)6. The formation of a blue-green
precipitate of K2Zn2[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 H2S, 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
Al+3 + 3OH- -> Al(OH)3
2Fe+3 + H2S -> 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 + H2S
Al(OH)3 + 3H+ -> Al+3 + 3H2O
The sulfides of cobalt and nickel dissolve in a mixture of nitric and hydrochloric acids:
3CoS + 8H+ + 2NO3- -> 3Co+2 + 2NO↑ + 3S + 4H2O
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+2 + 6NH3 -> Co(NH3)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
|
|
CH3-C=NOH
CH3-C=N N=C-CH3
2
|
+ Ni(NH3)6+2 ->
|
Ni
|
+ 2NH4+ + 4NH3
CH3-C=NOH
CH3-C=N N=C-CH3
|
|
O-H…O
The cobalt hexaamine complex ion is destroyed by the addition of hydrogen ions:
Co(NH3)6+2 + 6H+ -> Co+2 + 6NH4+
The positive test for cobalt is the formation of the blue Co(SCN)4-2:
Co+2 + 4SCN- -> 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+3 + 6SCN- -> Fe(SCN)6-3
Fe(SCN)6-3 + 6F- -> FeF6-3 + 6SCNIn the evaporation of the solution in step 5, Fe+2 is converted back to Fe+3:
3Fe+2 + 4H+ + NO3- -> 3Fe+3 + NO↑ + 2H2O
The solution is made basic with NaOH to precipitate all the ions as hydroxides:
Mn+2 + 2OH- -> 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 (H2O2) is added to the mixture to oxidize the manganese hydroxide to
manganese dioxide and the chromite ion Cr(OH)4-, to the chromate ion CrO4-2:
Mn(OH)2 + H2O2 -> MnO2 + 2H2O
2Cr(OH)4- + 3H2O2 + 2OH- -> 2CrO4-2 + 8H20
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:
MnO2 + 2H+ + NO2- -> Mn+2 + NO3- + H2O
2Mn+2 + 5BiO3- + 14H+ -> 2MnO4- + 5Bi+3 + 7H2O
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)4- + 4H+ -> Al+3 + 4H2O
In this step the chromate ion is converted to the dichromate ion:
2CrO4-2 + 2H+ -> Cr2O7-2 + H2O
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:
Cr2O7-2 + Ba+2 -> BaCrO4
Zn+2 + 2OH- -> Zn(OH)2
Zn(OH)2 + 4NH3 -> Zn(NH3)4+2 + 2OHThe chromium is confirmed by precipitation as barium chromate:
CrO4-2 + Ba+2 -> BaCrO4
Acidifying the solution breaks down the zinc tetraamine complex:
Zn(NH3)4+2 + 4H+ -> Zn+2 + 4NH4+
The presence of zinc is confirmed by the formation of the mixed salt potassium zinc
ferricyanide:
3Zn+2 + 2K4Fe(CN)6 -> K2Zn3[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+3 + 3H2O
Al+3 + 3OH- -> Al(OH)3