Qualitative Analysis
Qualitative analysis is used in the determination of the identity of
a
substance. It is different from quantitative analysis, which deals with
the
determination of the amount of a substance. In this experiment,
qualitative
analysis techniques are used to determine whether or not a sample
contains a
certain ion. When using this method, an unknown and a reactant are
mixed. The
result of the reaction leads to a conclusion about the presence or
absence of
certain ions in the unknown. Many ions react in similar ways, and
although the
addition of one reagent to an unknown may not identify the ion,
it limits the
possibilities as to what the ion could be. A sequence of
reactions used to
analyze a sample is called a scheme, and it usually
requires a large number of
reagents and separation steps. For this
experiment, the unknown may contain
anywhere from 2 to all of the following
cations and anions: Cations Anions Ag+
Cl- Ba2+ SO42- Fe3+ PO43- Cu2+
Cr3+ The following reagents are used to identify
the ions: 1M H2SO4 2M HCl 2M
NH4OH (labeled as NH4+) 2M NaOH .1M Ba(NO3)2
(labeled as .1M Ba2+) .1M AgNO3
(labeled as .1M Ag+) The first four are used to
identify the cations, and the
last two, used in conjunction with the first four,
are used to identify the
anions. The identification of the ions is mainly based
on solubilities. This
means that something must be known about the solubility
characteristics of
the different ions in the presence of the available reagents.
The point
of the first part of the experiment is to learn which reagents cause
the ions
to form precipitates, and which reagents dissolve the precipitates
formed by
the ions. This information is used to make the flow charts for
the
identification on the unknown ions. For example, it is important to know
that a
certain reagent will dissolve the precipitate formed by one ion, while
it will
not dissolve the precipitate formed by another ion. This can be used
to
distinguish between two different precipitates present in a solution, or
to
confirm which ion formed the precipitate and therefore was present in
the
solution. When carrying out the reactions, avoid adding an excess of
reagent to
the solution. This is because some precipitates redissolve in an
excess of the
reagent. Therefore, in cases where one drop of reagent produces
a precipitate, 3
or more drops could completely dissolve the precipitate
without it ever being
visible to the eye. This would cause a large error in
the scheme developed to
identify the unknown ions. Experimental: The first
part of the experiment
consists of reacting the cations and anions with the
reagents in order to see
what the reaction will result in (precipitate or no
precipitate). The cations
were each reacted with the first four reagents
listed in the introduction
(H2SO4, HCl, NH4+, and NaOH). Then, the anions
were each reacted with Ba2+ and
Ag+. This was done by placing 2 drops of
the ion in the test tube and then
adding 2 drops of reagent. Each cation was
reacted with each of the 4 reagents
before moving on to the next cation to be
tested. Prior to performing the
reactions, a chart was made like the one in
the data and calculations section.
As each reaction was performed, the
chart was filled in with the observation of
what happened. If there was no
change, NR was written in the chart for "no
reaction." If a precipitate
formed, the color of the precipitate was written
in the chart. If there was
no precipitate but there was a color change in the
solution, that was also
recorded. As each reaction was carried out, it was
sometimes difficult to
determine whether a precipitate formed or not. If there
was uncertainty, the
test tubes had to be placed into the centrifuge in order to
separate the
precipitates from the solution. There are some very important
things to
remember when using the centrifuge. First, when tubes are placed in
the
centrifuge, a tube with an approximately equal volume of solution should
be
placed exactly opposite each sample tube to counterbalance it (use a test
tube
filled with an equivalent amount of water if necessary). Second, the
centrifuge
should come to a stop before it is opened and the test tubes
removed. This is to
avoid injury. Once the tubes were removed from the
centrifuge, it was obvious
whether there was a precipitate present or not. If
a solid has settled onto the
bottom or side of the test tube, there was a
precipitate present. If the tube
appears to contain the same solution as
before the test tube was placed in the
centrifuge, no reaction occurred. The
next part of the experiment consists of
determining which reagents dissolve
certain precipitates. This information can
be especially helpful when
determining the ions present in the unknown. The
precipitates tested were
AgCl, BaSO4, and Ag3(PO4). They were reacted with HCl,
H2SO4, NH4OH, and
NaOH. This was done by making the precipitate using the
information from the
first chart, and then adding 2 drops of reagent. For
example, the precipitate
AgCl was made by reacting Ag+ with HCl. Four samples of
this were prepared,
and each of the reagents was added to the samples to see if
the precipitate
dissolved. A chart was filled in with the results of the
reactions. In the
final part of the experiment, the unknown was tested to
determine which ions
were present in it. This was done using flow charts created
with the
information from part 1 of the experiment (see data and
calculations
section). To test for the ions in unknown #2, it was first made
into a solution
by adding 25 mL of distilled water to the sample in a 100 mL
beaker. It was
mixed until all of the solid dissolved. To speed up the
dissolving process, the
beaker was held in the palm of the hand in order to
slightly heat the solution.
Once the solution was ready, it was tested
for the ions by following the flow
charts. For each step, 2 drops of reagent
was added to 2 drops of unknown
solution. To test for the cations, the cation
flow chart was followed. First,
HCl was added to the solution. There was
no reaction, so H2SO4 was added to
another sample of the unknown solution.
This also resulted in no reaction, so
NaOH was added to another sample of
the solution. A rusty-brown precipitate
appeared, which meant either PO43- or
Cu2+ was present. To determine which one
of these ions was present, NH4+ was
added to the solution, and a rust-colored
precipitate formed. This confirmed
the presence of Fe3+. Next, the unknown had
to be tested for anions. The
anion flow chart was followed. 2 drops of unknown
were reacted with Ba2+, and
there was no reaction. 2 more drops of unknown were
reacted with Ag+, and
white and tan precipitates formed. H2SO4 was added to the
test tube
containing the precipitates, and a white precipitate was left. This
confirmed
that PO43- was present but dissolved when the H2SO4 was added (as was
found
in part 1 of the experiment), leaving the Cl-. Therefore, the anions
present
were Cl- and PO43-. Data and Calculations: The data charts and the
flow
charts are on the following pages. Unknown #2 contains Fe3+, Cl-, PO43-.
Net
ionic equations for precipitates and reactions on flow charts: Ag+ + Cl-
à AgCl
Ba2+ + SO42- à Ba SO4 Fe3+ + 3OH- à Fe (OH)3 Cu2+ + 2OH- à Cu
(OH)2 Cr3+ +
3OH- à Cr (OH)3 3Ag+ + PO43- à Ag3(PO4) Ag3(PO4) + H2SO4 à
Ag2(SO4) + H3PO4
Thought process for flow charts: 2 separate flow charts
had to be made, one for
the cations and one for the anions. Starting with the
cations, the flow chart
must begin by listing all cations possibly present
because the unknown can
contain any number of them. HCl was the first reagent
added on the flow chart
because it only produced a precipitate with one of
the cations, Ag+. This was
determined using the data chart from part 1 of the
experiment, where the
precipitates formed with each reagent were clearly
delineated. By beginning the
flow chart with reagents that produce fewer
precipitates and ending it with the
ones that produce more, the chart was
easier to follow during the testing for
ions. Therefore, the next reagent
used on the chart was H2SO4. Since the Ag+
precipitated out as AgCl, the ions
left to react with H2SO4 were Ba2+, Fe3+,
Cu2+, and Cr3+ (these were the
ions that were left unreacted by the HCl). By
referring to the data chart
from part 1 of the lab, it was found that H2SO4 only
formed a precipitate
with Ba2+(BaSO4). This meant that the Fe3+, Cu2+, and Cr3+
ions were left
unreacted. NaOH was added to these ions and it formed a
rust-colored
precipitate with Fe3+, while it formed a blue precipitate with
Cu2+. To
confirm whether the precipitate was from Fe3+ or Cu2+, NH4+ was added
to the
unknown. If a rusty-brown precipitate appeared, the ion present
was
Fe3+(Fe(OH)3). If no precipitate formed but the solution turned dark
blue, Cu2+
was present. The only ion left unreacted after the NaOH was added
was Cr3+. If a
bluish-white precipitate formed when NH4+ was added to the
Cr3+, it confirmed
the presence of Cr3+(Cr(NH4)3). The anion flow chart began
with all three of the
anions listed (SO42-, Cl-, PO43-). Ba2+ was the first
reagent added because it
formed a precipitate with only one of the ions,
SO42-(BaSO4). The ions left
unreacted were Cl- and PO43-. Ag+ was added to
these. At this point, a white
precipitate could form with Cl-(AgCl), or a
yellow precipitate could form with
PO43-(Ag3(PO4)). If the precipitate
was purely white, the ion was Cl-. This was
confirmed by adding H2SO4 to the
precipitate, which would result in no reaction.
This is because H2SO4
does not dissolve AgCl (from part 1 of the experiment).
Then, the
addition of NH4+ would dissolve the precipitate and prove that only Cl-
was
present. However, if the unknown contained both Cl- and PO43-,
both
precipitates would form, but the two colors of the precipitates would
be
indistinguishable from each other. A yellow precipitate would be seen, but
it
would be impossible to tell if there was also a white precipitate
present.
Therefore, H2SO4 would again be added to the precipitates. If a
white
precipitate appeared, it would mean that PO43- had been present, but it
was
dissolved by the H2SO4. It would leave only the AgCl precipitate visible
(H2SO4
dissolves Ag3(PO4), but not AgCl), but because the precipitate
originally had a
yellow color to it, it would be known that both the Cl- and
the PO43- ions were
present in the unknown solution. If the solution had no
precipitate left (turned
clear) when the H2SO4 was added to the yellowish
precipitate, it would indicate
that only PO43- was present in the solution.
This is because H2SO4 dissolves
Ag3(PO4), and there were no other
precipitates left in the solution to be seen.
Results and Discussion: In
conclusion, unknown #2 contained Fe3+, Cl-, PO43-.
This was determined
using qualitative analysis, and the purpose of the
experiment was therefore
fulfilled. One possible source of error for this lab
could occur in part 1,
where the reactions of different reagents with different
ions are recorded in
data charts. If there is incorrect information about
whether or not a
precipitate formed, it will most likely result in an incorrect
flow chart and
an incorrect identification of the ions in the unknown. That is
why it is
important to use the centrifuge if there is uncertainty about a
precipitate,
or the reaction should be performed again. Another source of error
would be
to add too many drops of reagent to the ion or sample of unknown. This
is
because a precipitate may form with the reagent, but dissolve in an excess
of
the reagent. Therefore, the precipitate could form but then be dissolved
without
ever being seen. This would also result in an incorrect flow chart
and an
incorrect identification of the unknown ions. Another source of error
could
occur in making or following the flow chart. Incorrect reasoning when
designing
the flow chart will make it difficult to correctly identify the
ions in the
unknown, and not following the flow chart correctly would
obviously cause error
in the final results.