Ebube Agwaramgbo
Group 3 Cations Week 2
03/10/2022
The presence of Co2+ and Mn2+
6M NaOH is added to reddish solution containing an unknown mixture of ions.
This formed a large amount of precipitate. According to the hydroxide solubility rules, all
possible ions in solution would react with NaOH to form a solid hydroxide salt. However,
because Aluminum (III) and Zn (II) are both amphoteric and the solution was made
basic by the addition of NaOH meaning that these two ions are present as aqueous
complex ions. The mixture was then centrifuged and decanted. The supernate is saved
for later use. The precipitate was dissolved in 6M HNO3 in a hot water bath. The
addition of nitric acid causes an acid-base reaction to occur allowing the hydroxides to
form its conjugate base, water. The following solution is split into four aliquots.
A small piece of solid NaF is added to one of the aliquots and allowed to dissolve
until the solution gets cloudy. The lab introduction states that this is done to create a
complex ion FeF6, but due to no iron being present this reaction did not occur. KSCN*
is then added to the solution resulting in a bright blue solution. The lab introduction
sates that KSCN* can react with Co2+ to form a bright blue complex ion. The formation
of this bright blue ion confirms the presence of Co2+.
The presence of Mn2+
6M HNO3 and deionized water is added to a different aliquot. Then, solid,
NaBiO3 is added to the solution which causes a purple supernate to form. The NaBiO3
oxidizes manganese (II) in an acidic solution to form the purple permanganate ion. The
formation of a purple supernate confirms Mn2+.
Absence of Fe3+, Ni2+, Al3+, and Zn2+
.1 M KSCN is added to a different aliquot. The lab introduction states that KSCN
can react with Fe3+ to form a dark red solution. The fact that the color of the solution did
not change to blood red indicates that Fe3+ is not present in solution.
Absence of Ni2+
6M NH3 is added dropwise until the solution is slightly basic. The solution is
centrifuged and decanted. The precipitate is discarded as waste. Then an excess of five
drops NH3 are added to the solution, and it once again centrifuged and decanted with
the precipitate being thrown away. The lab introduction states that ammonia is added to
cause Fe3+ and Mn2+ to precipitate out of solution. The solution is a amber color due
to the presence of Co2+. The lab introduction states that adding dimethylglyoxime will
cause a precipitating reaction with Ni2+ to create the solid, strawberry red, precipitate
Ni(HDMG)2. No precipitate formed when H2DMG was added which infers that there is
no Ni2+ present.
Absence of Al3+
Add 6M HNO3, until the solution Is slightly acidic, to the supernate that was
saved for later use. 6M NH3 is then added to the solution until it is slightly basic and
then an additional two drops are added. The lab introduction explains that the addition
of ammonia causes Al3+ to precipitate as aluminum hydroxide. This would appear as a
translucent gelatinous precipitate. The fact that no precipitate formed indicates that no
Al3+ is present. Further testing for Al3+ stops and the solution is now used to test for
the presence of zinc ions.
Absence of Zn2+
Above, when the 6M NH3 was added to the supernate, two reactions happened
simultaneously. The lab introduction states that the addition of ammonia will cause zinc
to form the soluble complex ion [Zn(NH3)4]2+. Addition of 6M HCl breaks down the
complex ion and allows Zn2+ ions to be present within solution. Addition of zinc reagent,
[Hg(SCN)4]2-, should cause a white precipitate to form. While a precipitate did form
after the zinc reagent was added, the precipitate was red instead of white. Therefore,
Zn2+ is inferred to not be present in solution.