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Unformatted text preview: it is not a contaminant?” To answer these
questions, you will want two solutions in addition to the unknown; the standard, and the
blank. Run all three samples (the standard, the blank and the unknown) simultaneously.
Anything you do to one solution, do them to all solutions, and always remember to keep
them separate. Many qualitative analysis schemes recommend running the standard, then
repeating the procedure for the unknown. It is more efficient, and therefore faster, if you
can keep the solutions separate so you can run all three at the same time.
The standard contains all possible ions for the entire procedure. To make it, add 5 drops
of each ion’s nitrate into the same clean dry test tube (AgNO3 , Pb(NO3 )2 , Bi(NO3 )3 ,
Cu(NO3 )2 , Sn(NO3 )4 , Ni(NO3 )2 , Co(NO3 )2 , Mn(NO3 )2 , Al(NO3 )3 , Cr(NO3 )3 , Ba(NO3 )2 ,
Ca(NO3 )2 , and KNO3 .
The blank is the opposite of the standard; it contains NO ions. Use the appropriate
volume of distilled
Ag+, Pb+2, Groups II - IV
water. If a positive ever
appears in the blank, this
is an indication that
solutions picked up a
Groups II - IV contaminant, and you
will want to re-run the
tests to ensure that you
have no false positives. AgCl
Add NH3 Pb+2
Add K2CrO4 Ag(NH3)4+
Add Nitric Acid PbCrO4 A gCl Dakota State University Group I: Ag+ and Pb+2
Group I ions
include Hg+2 as well) are
put together because
they all have insoluble
(or relatively insoluble)
salts of chloride. This
fact allows for easy
separation of these ions
(aside from lead, as its
chloride is still partially page 194 of 232 Experiment 19: Qualitative Analysis General Chemistry I and II Lab Manual soluble) from other ions in the solution.
Separation of Group I from Groups II through V
To one milliliter of your solution, add dilute HCl dropwise until no more precipitate
seems to form. Do not add the HCl too quickly, since excess chloride ions can cause the
silver to re-dissolve. Centrifuge and decant; keep both the solid (which contains group I
ions) and the decantate (which contains groups II through IV ions). Add one drop of HCl
to the decantate to verify that no more precipitate forms; if precipitate does form, repeat
this separation step.
Separation of Ag+ from Pb+2
The solubility of lead (II) chloride increases with temperature, while silver chloride
remains insoluble. Add one milliliter of distilled water; heat the solution in a hot water
bath, but do not allow the solutio n to come to a boil. Decant the solution hot (you will
not be able to centrifuge). Repeat this step one more time.
Verification of Ag+
To the solid silver chloride, add concentrated ammonium hydroxide dropwise until the
solid dissolves (if it is not silver, it will not dissolve). Once all of the silver has
dissolved, acidify the solution with dilute nitric acid. If the solid was silver chloride, the
precipitate should re-form.
Verification of Pb+2
To the warm liquid containing lead ions, add potassium chromate, K2 CrO4 . If the
decantate contained lead ions, yellow lead (II) chromate precipitate will form.
Group II ions are characterized as having insoluble sulfide salts. For our analysis,
we will be looking for Pb+2 (remaining from group I), Bi+...
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