Group I, Precipitation of Insoluble Chlorides
Your unknown may contain Ag+, Hg22+ and/or Pb2+ ions.
PROCEDURE: Be sure to record all reagents added, observations, conclusions, and equations.
A. Precipitation of Group I Chlorides.
1.
If the unknown is for Group I only, prepare 1 mL of cold 12 M HCl to which has been
added I drop of 6 M nitric acid. (The nitric acid minimizes reduction of silver by the mercurous
chloride.) Using a spatula, place about 50 mg of unknown powder into this solution. Stir
vigorously for about 30 seconds with a glass stirring rod (NOT the metal spatula!). Dilute with
at least an equal volume of cold water and again stir vigorously for about 30 seconds. Cool
thoroughly, centrifuge and decant (or filter). The precipitate is a mixture of Group I chlorides.
Go to step B with the precipitate. (The solution at this point may still contain lead ions. It may
be neutralized with ammonia solution and tested for lead ions, following Procedure B.2 below.)
2.
If the unknown is a general unknown containing cations from any or all of the qualitative
analysis cation groups. Treat 100 mg of the unknown powder with 1 mL of 12 M HCl. Immerse
in boiling water, stir continuously until all has dissolved except Group I. Add 1 mL of water,
and stir. Cool thoroughly and centrifuge (or filter). The precipitate is a mixture of Group I
chlorides. Use part or all of the solution for Group II precipitation.
Reactions and Comments:
Hg22+ + 2Cl-  Hg2Cl2 Least Soluble
Ag+ + Cl-
 AgCl
Pb2+ + 2Cl-  PbC12
Most Soluble
The lead chloride is soluble enough that some lead ions will remain in solution. The
mercury(I) and silver ions are effectively removed from solution, almost totally
precipitated. In 12 M HCl, lead and silver ions form complex ions, [PbCl4]2- and [AgCl2]-,
with the excess chloride ion. Dilution with water lowers the chloride concentration
sufficiently so that the precipitates can form.
If the lead ion concentration is too low, it may fail to precipitate. If the unknown is Group I
only, a test for lead should be made on the centrifugate (or filtrate) from the original
Group I precipitation.
1
B. Separation and Identification of Lead Ion.
1. Add 10 drops (0.5 mL) of hot water to the precipitate to dissolve lead chloride, Centrifuge
or filter, and repeat with an additional 10 drops of hot water. Save the solution from both
washings for step 3.2. Save the precipitate for Procedure C.
2. To the solution from B.1 (or A.1) add 2 drops of 1 M K2CrO4. A yellow precipitate of
PbCrO4 confirms the presence of lead.
Pb2+ + CrO42-  PbCrO4
C. Separation of Silver from Mercury, and Identification of Mercury (I) Ion
To the residue (precipitate) from Procedure B.1, add 5 drops of 4 M aqueous ammonia.
Centrifuge (or filter), and then repeat with 5 additional drops of ammonia. A black or gray
residue confirms the presence of mercury(I). Do not be fooled by a few black specks that
may be metallic silver. Save the solution (centrifugate or filtrate) for Procedure D.
The mercury undergoes auto-oxidation-reduction, resulting in the formation of finely
divided metallic mercury (black), and mercury(II) amidochloride, HgNH2Cl (white). The
mixture looks black or gray.
Hg2Cl2 + 2NH3  Hg + HgNH2Cl + NH4+ + ClThe ammonia dissolves AgCl by forming the diammine silver complex ion.
AgCl + 2NH3  [Ag(NH3)2]+ + Cl
D. Identification of Silver Ion.
Acidify the solution from Procedure C with 6 M nitric acid. Stir well and test with litmus to
verify that all the ammonia has been neutralized. A white precipitate or cloudiness (AgCl)
confirms the presence of silver.
The H+ from the nitric acid takes the ammonia away from the silver, freeing the silver ion to
recombine with chloride.
[Ag(NH3)2]+ + Cl- + 2H+  AgCl + 2 NH4+
2
Group I Flow Chart
Ag+, Pb2+, Hg22+
[ Cold HCl ]
AgCl, PbCl2, Hg2CI2
Pb2+ and Groups 2 - 5
[ Hot Water ]
Hg2CI2, AgCl
Pb2+
[ NH3 ]
[K2Cr04 ]
Hg + HgNH2CI
Black
[Ag(NH3)2 ]+ + Cl-
White
PbCr04
Yellow
[ HN03 ]
AgCl
White
3