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It was determined that
Ag+, Ni2+ and Mn2+ were present in the first
unknown solution while the actual cations present were Ag+, Ni2+
and Zn2+. For the second unknown solution, it was determined that Fe3+,
Bi3+, Al3+, Ni2+ and Zn2+ were
present in the unknown while the actual cations present were Fe3+,
Bi3+, Cu2+, Ni2+ and Zn2+.

 

For the first unknown
solution, it was provided that the solution contains 5 possible cations: Ag+,
Pb2+, Mn2+, Ni2+ and Zn2+. Hence,
based on this information, a separation scheme was developed. H2SO4
should be used first to precipitate the Pb2+ as only Pb2+ will
react with H2SO4 while the other ions show no visible
reaction. After the PbSO4 is separated from the solution, HCl can be
used to precipitate the Ag+ as only Ag+ will form a white
precipitate with HCl and the other ions show no visible observations. After the
AgCl is separated from the solution, there are only Mn2+, Ni2+
and Zn2+ remaining in the solution. A basic sulfide test can be
carried out to precipitate the black NiS, pink MnS and the white ZnS. After the
solids are separated from the mixture, HCl can be added to the solids to form
an aqueous solution with Mn2+ and Zn2+ while NiS will
remain insoluble. The insoluble salts can be separated from the mixture and Mn2+,
Zn2+ will be the only cations left in the solution. 19M NaOH can be
used to precipitate the Mn(OH)2 which will be further oxidized to
brown MnO(OH)2, while Zn2+ will form a colorless complex
ion HZnO2- in excess NaOH, thus Mn2+ and Zn2+
can be separated.

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Mn2+ was
concluded to be present in the first solution instead of the actual Zn2+.

This is because at the end of the experiment, when 19M NaOH was added to the
acidic solution containing either Mn2+ or Zn2+, a brown
colored yet transparent solution formed with some brown specks which looks like
precipitates. Since Mn2+ reacts with NaOH to form the white solid
Mn(OH)2 which is further oxidized to a brown solid MnO(OH)2,
the observed brown specks matches with this prediction. However, this is wrong.

Zn2+ was presented in the solution instead of Mn2+. The
brown specks detected can be the black NiS solid which was accidently drawn out
of the mixture during decanting. If the solution mixture was settled for a
longer time and was centrifuged, the wrong observation of the formation of a
brown solid can be fixed and the conclusion would be correct. Also, a
confirmatory test of Mn2+ could be carried out to clear the
confusion.

 

For the second unknown
solution, all ten cations were in the range of the unknown. So a more
complicated separation scheme was designed. Firstly, HCl can be added to
precipitate out the PbCl2 and AgCl as only Ag+ and Pb2+
show visible reaction with HCl. The 2 white solids can be separated by adding
hot water to them – PbCl2 is soluble in hot water so it forms an
aqueous solution of Pb2+ while the AgCl will remain insoluble. If
the white solid disappears, the presence of Pb2+ can be confirmed;
if the solid remained insoluble, the presence of Ag+ can be
confirmed. And the solution should be separated from the solids via
centrifuging, solid KI can be added to the solution to check if there are any
Pb2+ in the solution. For the rest of the solution containing all
possible 8 cations Ba2+, Bi3+, Fe3+, Mn2+,
Cu2+, Ni2+, Zn2+ and Al3+, an
acidic sulfide test can be used to separate the brown solid Bi2S3
and the black solid CuS from the solution. Then, nitric acid can be added
to the solid mixture to form aqueous solution of Bi3+ and Cu2+
again, and adding 15M NH4OH can separate them – a white solid of
Bi(OH)3 will form while a royal blue Cu(NH3)43+
complex ion will form. 3M NH4OH can be added to the rest of the
solution to separate the Ba2+, Fe3+, Mn2+, Ni2+,
Zn2+, Al3+. Brown solids of Fe(OH)3 and white
solids of Al(OH)3 will form. After the solids and the solution are
separated, 19M NaOH can be added to the solids to separate the Fe(OH)3
and Al(OH)3, the Fe(OH)3 will remain as a brown solid
while the Al(OH)3 forms a colorless complex ion Al(OH)4-
in excess NaOH. For the solution with the remaining ions Ba2+, Mn2+,
Ni2+ and Zn2+, basic sulfide can be added to separate
them. Only Ba2+ shows invisible solution with OH-/H2S
while the rest of the ions form black NiS, pink MnS and white ZnS. The Ba2+
in the aqueous solution can be confirmed by adding K2CrO4 to
the solution after the solution has been separated from the solids. The NiS,
MnS and ZnS solids can be separated by adding HCl to them, only NiS will remain
insoluble while MnS and ZnS will form aqueous solution of Mn2+ and
Zn2+. The NiS solid should be separated from the solution via
centrifuging and there will only be Mn2+ and Zn2+ remaining
in the solution. 19M NaOH can be added to the solution and the Mn2+
will form a white solid Mn(OH)2 which is further oxidized to brown
MnO(OH)2, whereas the Zn2+ forms a colorless complex ion
HZnO2- in excess NaOH, thus Mn2+ and Zn2+
can be separated.

 

Al3+ was
concluded to be in the second solution instead of the Cu2+ which was
actually presented. Cu2+ was concluded absence in the solution
because while H+/H2S was added to the solution, no
precipitation occurred as predicted (Bi2S3 and CuS should
precipitate if Bi3+ and Cu2+ were present). The dark Bi2S3
solids were not detected but Bi3+ was in the solution. This is
due to the fact that salts of Bi3+ are very soluble in acidic
solution. However, the precipitation of CuS did not occur as well. This can be
caused by the insufficient addition of the H+/H2S and so
no visible precipitation of the black CuS occurred. Since Cu2+ was
not detected through the acidic sulfide test, a confirmatory test for Cu2+
was carried out – 15M NH4OH were added to a 3 mL sample of the
original solution. A mixture of precipitates occurred and after centrifuging
the color of the solution was blue. However, Cu2+ was concluded to
be absence because the Ni2+ also react with excess NH3 to
form a complex ion Ni(NH3)62+ which is also
blue. And since the precipitation of black CuS did not occur as expected, it
was determined that Cu2+ was not in the solution. Al3+
was concluded to be in the solution because when the 3M NH4OH was
added to the solution, pale green precipitates of Ni(OH)2, brown
precipitates of Fe(OH)3 and white precipitates formed. The white
precipitates were thought to be Al(OH)3. In fact, the white solid
could be Zn(OH)2 which formed when the OH- is not in an
excess amount in the solution. A confirmatory test was carried out for Al3+
as well. After centrifuging the mixture, some red precipitates were in the test
tube which led to the conclusion that Al3+ was in the solution.

However, aluminon is not a specific reagent for Al3+, color
formation will also occur when Fe3+ or Be2+ are present.

It is possible that during that stage there were some Fe3+ remained
in the solution (insufficient amount of NaOH was added so not all Fe3+
reacted and formed Fe(OH)3 that can be precipitated out). Therefore,
the red solid was probably a Fe3+ complex with aluminon instead of a
Al3+ complex. Overall, the lab was successful as only one cation was
not identified in each separation. If a Mn2+ confirmatory test was
carried out for the first separation the Mn2+ could be successfully
eliminated and the prediction would be correct. If the acidic sulfide test was
done more carefully a black CuS precipitate could be detected and the existence
of Cu2+ can be confirmed and the prediction would be correct. 

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