Separation and Identification of Group A Cations

Foothill College-Chemistry 1C Name:

Separation and Identification of Group A Cations

(The Chloride Gang: Ag+ and Pb2+)

Objectives

• To understand the chemical reactions involved in the separation and identification of the Group A cations, silver and
lead (II).

• To complete a flow diagram summarizing the qualitative analysis scheme for the Group A ions.
• To successfully identify the Group A cation(s) in an unknown solution.

Background Chemistry and Discussion

Of the cations encountered in our qualitative analysis scheme, only two form insoluble chlorides, Ag+ and Pb2+. These can be
separated from a mixture of the other cations by adding a slight excess of hydrochloric acid. A slight excess helps to insure
more complete precipitation of both ions since their solubility is reduced in excess Cl– through the common-ion effect. On the
other hand, a large excess of hydrochloric acid should be avoided, since both silver ion and lead (II) ion form chloro complex
ions, PbCl42–(aq) and AgCl2–, to some extent in excess HCl. After precipitating the Group A cations as chlorides, they are
physically separated from the remaining mixture by centrifuging and decanting the supernatant liquid into another test tube
for further analysis. Refering to the Ksp values of AgCl and PbCl2, note that PbCl2 is significantly more soluble than AgCl. In
fact, PbCl2 tends to not precipitate completely in the Group A separation. Any remaining lead (II) ion, if present, will be
removed from the sample solution by precipitation early in the Group B analysis.

Both silver chloride and lead (II) chloride are white solids. Therefore, the color of the precipitate formed when HCl is added
is not indicative of its identity and further analysis is needed for identification. The solubility of PbCl2 increases
approximately threefold as the temperature of the solution increases from 20°C to 100°C. The solubility of AgCl increases
very little over this temperature range. Thus, in hot water appreciable amounts of PbCl2 will dissolve while AgCl remains
insoluble. Addition of KI(aq) to a solution that contains Pb2+ will yield PbI2(s), a golden yellow precipitate. Of the two
Group A ions, only the silver ion is soluble in aqueous ammonia due to the formation a complex ion. From this information a
separation and identification scheme for the Group A cations can be developed.

Reagents Available 1-M KI
6-M HNO3
6-M HCl
6-M NH3
Dilute, Known Solutions of Ag+ and Pb2+

Safety and Waste Disposal

All of the Group A cations are toxic and 6M HCl, NH3 and HNO3 are irritants. Avoid contact and wash immediately if
any is spilled or splashed on you. Wear eye protection at all times.

As you perform the experiment, collect all waste solutions in a waste beaker. This mixture should then be discarded in
the appropriate waste container. DO NOT POUR ANY OF THE SOLUTIONS DOWN THE DRAIN.

Unknowns and Knowns

Separate known samples containing Ag+ and Pb2+ are provided for your use. Testing known samples is helpful in this
analysis since doing so will allow you to observe what a positive test looks like. It is usually convenient to test a known
sample simultaneously with your unknown. To test a known sample, you can either prepare a known mixture of the Group A
cations or the known Group A cation solutions can be tested separately. In the case of a known mixture of the cations, steps
in the procedure that are required for separation of the cations must be followed before the confirmation tests can be
conducted. If a known is prepared that contains only one of the cations, then the confirmation test can be conducted directly
on the known. To prepare a known sample for testing, add 2 to 3 drops of the solution(s) containing the cation(s) you wish to
test to about 0.5 mL of water. Note that the experimental conditions, such as pH, for the known test must be the same
as that for the unknown.

Dr. L.J. Larson 1 Revised/Printed 5/19/08
QualGroupA.doc

Foothill College-Chemistry 1C Qualitative Analysis Group A

Outline of Procedure: Use a flow diagram in your notebook to record all
observations for each step in the procedure.

Chemistry and Relevant Background Information: Experimental Procedure:
1. Precipitation of Group A cations: 1. Use test tubes that will fit into the centrifuge.
The Group A cations (Ag+ and Pb2+) are the only cations in (a) Measure 10 drops of a solution to be tested for the Group
our qualitative analysis scheme that form insoluble chlorides. A cations into a test tube. Add 6 M HCl, dropwise, with
Addition of 6M HCl separates the Group A cations from shaking just until no more precipitate appears to form. Stir
Groups B–D. thoroughly; then centrifuge. Do not decant yet.
In the space below, write the balanced net ionic chemical
equations, including phase labels, for the precipitation (b) Test for complete precipitation by adding 1 drop of 6 M
reaction that occurs when HCl(aq) is added to a solution HCl to the clear supernatant. If cloudiness is observed or a
containing silver ions. precipitate forms, add another drop, stir and centrifuge.
Repeat this process until no new precipitate forms upon
In the space below, write the balanced net ionic chemical addition of the 1 drop of hydrochloric acid.
equations, including phase labels, for the precipitation
reaction that occurs when HCl(aq) is added to a solution (c) If an unknown that also contains Groups B through D is
containing lead (II) ions. being tested, decant the supernatant liquid into a clean test
tube and save it for further analysis. DO NOT FORGET TO
LABEL THE TEST TUBE!

2. Separation and Identification of Pb2+ ion: (d) Wash the precipitate twice by adding 10 drops of cold,
deionized water then stir, centrifuge and decant. The washes
The solubility of PbCl2 increases considerable in hot water can be discarded.
compared to cold water while the solubility of AgCl remains 2.
(a) Prepare a boiling water bath. Add about 20 drops of
relatively constant. Hot water can thus be used to separate deionized water to the precipitate from step 1 and heat the
mixture in the boiling water bath for at least 3 minutes,
PbCl2(s) from AgCl(s). The precipitate from step 1 is treated stirring constantly.
with hot water. After centrifuging and decanting, the
resulting supernatant is tested for the presence of Pb2+ by (b) Centrifuge quickly and decant the hot supernatant into a
clean test tube. Set aside any remaining precipitate to further
adding KI(aq). The formation a golden yellow solid analyze. DO NOT FORGET TO LABEL IT!
confirms the presence of Pb2+.
(c) Add three drops of 1 M KI solution to the test tube
In the space below, write the balanced net ionic chemical containing the supernatant liquid. Carefully observe and
record your observations.
equation, including phase labels, for the reaction that
occurs when KI(aq) is added to a solution containing Pb2+

ions.

3. Identification of Ag+ ion: 3.
Any precipitate remaining from step 2b is tested for silver (a) If Pb2+ was confirmed in step 2(c), wash any remaining
ion by first adding 6 M NH3. Silver chloride, if present, will
dissolve as it forms the Ag(NH3)2+ complex ion. precipitate from step 2(b) by adding 10 drops of deionized
In the space below, write the balanced net ionic chemical
equation, including phase labels, for the reaction that water and heating the mixture in the boiling water bath for 2

occurs when NH3(aq) is added to AgCl(s). minutes, stirring continuously. Centrifuge quickly and

discard the supernatant. Repeat this wash step one more
time. If Pb2+ was not confirmed, this washing step can be

skipped; go directly to step 3(b).

To verify the presence of Ag+, the resulting solution is (b) Add 10 drops of 6 M NH3 to any remaining solid from
treated with nitric acid. If Ag(NH3)2+ is present, it is either step 2(b) or 3(a), stir and centrifuge. Decant the
decomposed by the acid and AgCl(s) is again formed. supernatant into a clean test tube.

Reappearance of a white precipitate confirms the presence of (c) Add 6 M HNO3 drop wise with stirring to the supernatant
Ag+ in the original solution. liquid until the solution just tests acidic to litmus paper.
Carefully observe and record your observations as the
In the space below, write the balanced net ionic chemical solution is acidified.

equation, including phase labels, for the reaction that

occurs when HNO3(aq) is added to a solution containing
Ag(NH3)2+.

Dr. L.J. Larson 2 May 19, 2008
QualGroupA.doc

Foothill College-Chemistry 1C Qualitative Analysis Group A

Prelab Exercise: Name:

1. Explain why adding a slight excess of hydrochloric acid insures more complete precipitation of the Group A cations, but
a large excess should not be used.

2. What is the purpose of washing the precipitate with hot water in step 3(a) of the procedure? Be as specific as possible in
your answer.

3. For an unknown solution that contains at least one of the Group A cations, answer the following questions.
a) Upon adding 6 M HCl to the unknown solution, a white precipitate forms. What cation(s) may be present in the
unknown?

b) When the white precipitate from (a) is treated with hot water, a white precipitate and colorless supernatant result.
Adding KI(aq) to the supernatant results in no reaction. What conclusion can be made about the presence or absence
of the cations present in the unknown?

c) When the precipitate from (b) is treated with 6 M NH3(aq), it dissolves. If HNO3 were then added to the resulting
solution, what would you expect to observe? Give the balanced net ionic equation, including phase symbols, for the
reaction that occurs upon addition of the HNO3.

Dr. L.J. Larson 3 May 19, 2008
QualGroupA.doc

Foothill College-Chemistry 1C Qualitative Analysis Group A

4. Complete the flow diagram below for Group A analysis. For each step, indicate the chemical form of each Group A
cation that is present at the given step (for example Ag+, AgCl, Ag(NH3)2+,, etc.). Also indicate colors of solutions and

precipitates where known.

snt ≡ supernatant
ppt ≡ precipitate
∆ ≡ heat

Ag+, Pb2+, Groups B, C, D snt
6 M HCl

ppt

water, ∆ ppt
snt 6 M NH3

1 M KI

6 M HNO3

Dr. L.J. Larson 4 May 19, 2008
QualGroupA.doc

Foothill College-Chemistry 1C Qualitative Analysis Group A

Follow-up Questions: Name:

For Numerical Problems, you must show all work for credit!

1. Using the Ksp values given in Table 2 of the Introduction to Qualitative Analysis, calculate the molar solubility of
a) AgCl

b) PbCl2

2. A 0.50 gram sample of AgCl(s) is shaken with 5.0 mL of 6.0 M NH3 until there is no more net reaction. (Kf for
Ag(NH3)2+ = 1.7 x 107)
a) Write the net ionic equation, including phase symbols, for the chemical reaction that occurs.

b) Does any solid AgCl remain? If so, what mass remains?

3. Suppose 6 M NH3 is accidentally added instead of hot water in the step used to separate Pb2+ ions from Ag+ ions? Is it
still possible to identify the ions present using this sample? If identification is still possible, use a flow diagram to show

the steps needed to complete the analysis.

Dr. L.J. Larson 5 May 19, 2008
QualGroupA.doc