QUALITATIVE ANALYSIS
SOLUBLE CHLORIDES
EXPERIMENT 11
OBJECTIVE
The objective of this experiment is the separation and identification of Ag, Pb, and Hg ions in a
solution.
EQUIPMENT AND CHEMICALS
1M AgNO3
1M Pb(NO3)2
1M Hg2(NO3)2
6M HCl
1M K2CrO4
4M HNO3
6M NH4OH
centrifuge
dropper
6 small centrifuge tubes
unknown
stirring rod
water bath (hot plate)
pH paper
DISCUSSION
The chloride salts of Ag+1 , Pb+2, and Hg+2 are insoluble. The precipitation of these ions is
accomplished by the addition of HCl to form the insoluble chloride salts.
Ag +
Cl − → AgCl ↓
+
Pb + 2
+ 2 Cl − → PbCl 2 ↓
Hg 2 + 2 + 2 Cl − → Hg 2 Cl 2 ↓
Separation of Lead
PbCl2 is about three times more soluble in hot water than cold water. Therefore, the PbCl2 can be
easily separated from AgCl and Hg2Cl2 by dissolving it in hot waste. The dissolved Pb+2 ion can be
combined with a CrO4-2 ion to form a yellow insoluble precipitate.
Pb +2
+ CrO 4 − 2 → PbCrO 4 ↓
Separation of Mercury
AgCl can be separated from Hg2Cl2 by the addition of NH3 water. The Ag forms a complex with the
NH3 to which is soluble.
AgCl + 2 NH3 ↔
Ag(NH3 ) 2 + + Cl −
Hg2Cl2 undergoes a reaction with the NH3 to form a black precipitate.
Hg 2 Cl 2 + 2 NH3
↔
Hg ↓ + HgNH2 Cl ↓ + NH 4 Cl
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Separation of Silver
To confirm the presence of Ag+, the silver complex is acidified with HCl. A white precipitate
(AgCl) confirms, Ag+.
Ag(NH3 ) 2 + ↔
Ag + + 2 NH3
2 NH3 + 2 H + ↔
Ag + +
Cl − →
2 NH 4 +
AgCl ↓
PROCEDURES
PREPARATION OF GROUP I SOLUBLE CHLORIDES
1.
Place 3 ml of the sample solution containing all three ions in a small test tube and add 15
drops of 6M HCl.
2.
Heat the solution in a hot water bath for several minutes to coagulate the precipitate (solid).
3.
Cool the solution and centrifuge.
4.
Decant (pour off) the supernatant (liquid) into a second small test tube and label as “other”.
5.
Wash the precipitant (solid) with 2 ml of water, centrifuge and combine the liquid with the
second test tube labeled “other”.
TEST FOR LEAD
6.
To the precipitate (solid) add 2 ml of water, stir and heat in a boiling water bath.
7.
Stir and immediately centrifuge.
8.
While still warm, decant the supernatant (liquid) into a small test tube.
9.
Wash the remaining precipitate with 1 ml of water, stir and heat in a boiling water bath, and
while still hot centrifuge and combine the liquid with the supernatant in step 7.
10.
Save the precipitant for step 13.
11.
To the supernatant add 6 drops of 1M K2CrO4.
12.
Centrifuge.
13.
The formation of a yellow precipitate confirms the presence of Pb+2.
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TEST FOR MERCURY
14.
To the precipitate from step 9, add 2 ml of 6M NH4OH and stir and centrifuge.
15.
Pour off the supernatant into a small test tube and save for step 18.
16.
To the precipitate from step 14, add 2 ml of 6M NH4OH and stir and centrifuge.
17.
Pour off the supernatant into the test tube in step 14.
18.
A black precipitate indicates the presence of Hg2+2.
TEST FOR SILVER
19.
Acidify the supernatant with 4M HNO3 until it is acidic (Dip a stirring rod into the solution
and touch it to the pH paper. If the pH paper turns red, the supernatant is acidic).
20.
Centrifuge.
21.
A white cloudiness or precipitate confirms the presence of Ag+.
DETERMINATION OF AN UNKNOWN SOLUTION
22.
Obtain an unknown solution from the instructor that may contain any combination of Ag, Pb
or Hg.
23.
Record the unknown number.
24.
Using the procedure starting at step 1, substitute 3 ml of unknown in place of the knowns
and determine which ions are in the unknown.
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112
NAME __________________
DATE ___________________
SECTION ________________
IDENTIFICATION OF Ag, Pb, AND Ag
REPORT SHEET
EXPERIMENT 11
PRECIPITATION OF SOLUBLE CHLORIDES
a)
Did you get a precipitate after adding the 6 M HCl and heating?
b)
What are the possible known ion(s) in this precipitate?
TEST FOR LEAD
1.
2.
After adding hot water to the above precipitate and centrifuging:
a)
If there was a precipitate remaining, what known ion(s) could be in it?
b)
In the supernatant, what known ion(s) could be in it?
When 1M K2CrO4 was added to the supernatant in step 10, was there a precipitate?
a)
Yes or no
b)
If no, what did this indicate?
c)
If yes, what did this indicate and what confirmed this?
TEST FOR MERCURY
1.
After adding 6M NH4OH to the precipitate in step 13 and centrifuging:
a)
if there was a precipitate remaining, what known ion(s) could be in it?
b)
In the supernatant, what known ion(s) could be in it?
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2.
When 6M NH4OH was added to the precipitate in step 15, did a precipitate still remain?
a)
Yes or no
b)
If no, what did this indicate?
c)
yes, what did this indicate and what confirmed this?
TEST FOR SILVER
1.
After adding 6M NH4OH was added to the precipitate in step 13, and acidifying the
supernatant with 4M HNO3, did a precipitate form?
a)
Yes or no
b)
If no, what did this indicate?
c)
If yes, what did this indicate and what confirmed this?
INDENTIFICATION OF A UNKNOWN
1.
What was the unknown number?
2.
Did you get a precipitate after adding the 6 M HCl and heating?
3.
What are the possible unknown ion(s) in this precipitate?
TEST FOR LEAD
1.
After adding hot water to the above precipitate and centrifuging:
a)
if there was a precipitate remaining, what unknown ion(s) could be in it?
b)
In the supernatant, what unknown ion(s) could be in it?
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2.
When 1M K2CrO4 was added to the supernatant in step 13, was there a precipitate?
a)
Yes or no
b)
If no, what did this indicate?
c)
If yes, what did this indicate and what confirmed this?
TEST FOR MERCURY
1.
2.
After adding 6M NH4OH to the precipitate in step 15 and centrifuging:
a)
if there was a precipitate remaining, what unknown ion(s) could be in it?
b)
In the supernatant, what unknown ion(s) could be in it?
When 6M NH4OH was added to the precipitate in 3a, did a precipitate still remain?
a)
Yes or no
b)
If no, what did this indicate?
c)
If yes, what did this indicate and what confirmed this?
TEST FOR SILVER
1.
After adding 6M NH4OH was added to the precipitate in step 13, and acidifying the
supernatant with 4M HNO3, did a precipitate form?
a)
Yes or no
b)
If no, what did this indicate?
c)
If yes, what did this indicate and what confirmed this?
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METAL(S) PRESENT IN UNKNOWN SOLUTION
UNKNOWN NUMBER _________
Based on your analysis of the unknown solution, circle all the metal ions you found present:
Ag
Pb
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Hg