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Unformatted text preview: istry I and II Lab Manual Finally, labeling will be very important, and is keyed in with your note-taking.
This week, you will be storing solutions that you might not need for three more weeks. If
you do not label the solutions carefully, you will be faced with the uncomfortable
situation in which you are trying to remember “what this was” and “which one do we
Remember that you will be graded on accuracy. If you lose track of where you
are, or if something goes wrong, you will have to go back to the first step to separate out
ions from earlier groups. This will go faster than the first time since you will not have to
perform the verification tests for ions you already know are present, but it will be time
consuming none the less.
To keep track of what is happening, there are a few initial concepts that you
should keep in mind. These steps are, primarily, all we are doing.
The order of the tests are critical, because one ion can often lead to a false positive later
in the analysis for another ion. Separation steps are designed to separate these earlier
ions from later ones. Typically, separation is based on solubility; add a reagent, to which
some ions will precipitate but others will not, and separate the solid from the liquid. In a
separation, you will want to keep both the precipitate (typically containing the ions for
the immediate tests) and the decantate (the liquid, containing ions for later analyses).
Sometimes, tests later on can be sensitive, so we have to make sure the separation was
fairly efficient. In this case, we will wash the precipitate (typically) to make sure there
are no other ions of appreciable concentration to cause problems. Washing is usually
based on solubility. Once the solid and liquid are separated, you will add a wash solution
to the solid, agitate the solution, re-centrifuge and decant. Most of the time, you want to
keep the solid, but (unless otherwise denoted) discard the wash solution.
The first indication that the ion you are seeking is present is the precipitation in the
separation, but how do you know that the precipitate contains a specific ion (especially
since many separation techniques will separate out more than one ion at a time)? The
verification step is your way to ensure that the precipitate formed is indeed the ion you
are seeking. A negative on the verification step means the ion was not present, and the
precipitate was something other than what you thought it was.
Preparation of “Support Solutions”: Dakota State University page 193 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual There are two principle issues that you will have to resolve when running these analyses.
The first is “what does a positive test look like?” Some are obvious, some are more
subtle; it is always very helpful to be able to see a positive to compare the unknown with.
The second question is “how do I know...
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