Qualitative Analysis Essay
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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...
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