Unformatted text preview: action? Dakota State University page 191 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual Experiment 19: qualitative analysis
Purpose: To determine the cations present in an unknown solution
Pernoid: Not a real word Background:
See “Basic Laboratory Procedures”; Litmus paper, Bunsen burner
One of the oldest questions asked by chemists is “what is it?” As I understand it,
the Native Americans would chew a particular type of leaf to relieve headaches; chemists
wanted to know what was in it that did so. The result is aspirin.
The concept of extracting organic materials and analyzing them to determine their
chemical composition is beyond the scope of this course (in fact, these topics are covered
in Organic Chemistry). However, in this experiment, we will be performing the General
Chemistry equivalent; qualitative analysis.
In qualitative analysis, we ask simply “what is in it.” We are not worried about
how much there is (this is “quantitative analysis”), just, yeah or nay, is this metal present?
It will take you several weeks to complete this procedure; this experiment replaces a “lab
practical,” because in it, you will have to be particularly wary of your technique,
observation skills, note taking skills, and labeling skills.
Your technique will be tested because cross contamination can cause “false
positives” (that is, you will think ions are present that really are not). What’s more, if
you fail to clean your equipment carefully, you can cause contamination that will result in
the same problem. Also, be careful of the equipment you choose; we are looking for
metal ions, so choosing things such as metal scoopulas can cause problems. At the same
time, you will have to keep in mind what ions are (or might be present) in each container
at all times. You will not be able to keep all solutions you make, but you must be very
careful not to discard a solution you will need later on. Remember that we are beginning
with a mixture of all possible ions; do not throw away solutions if you are not sure that it
is no longer needed.
You observation skills will be critical. Often it is challenging to tell if a positive
is sufficient to call it positive, or determine if it is simply a contaminant. We will have
ways to help you differentiate, but you will have to always be wary of what you add, and
Note taking is of critical importance. The smallest observation, which does not
seem to be significant when you first write them down, could be the determining factor
when deciding what is present and what is not. All too often, a student will ask me a
question, such as, “what does this mean?” I usually cannot help, not because I am not
willing, but because I was not present to witness the entire procedure, so I often do not
know where the student is or what might have happened to get them there. The only
witness is the analyst; any lost notes, then, are simply lost forever. Dakota State University page 192 of 232 Experiment 19: Qualitative Analysis General Chem...
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