QUALITATIVE ANALYSIS
ACIDIC SULFIDES
EXPERIMENT 12
OBJECTIVE
The objective of this experiment is the separation and identification of As, Bi, Cu and Pb ions in a
solution.
EQUIPMENT AND CHEMICALS
1M Bi(NO3)3
1M As(NO3)3
1M Pb(NO3)2
1M Cu(NO3)2
6M HCl
1M K2CrO4
6M HNO3
15M NH4OH
6M NH4OH
1M thioacetamide
2M NH4Cl
4M KOH
12M HCl
3% H2O2
15M H2SO4
1M SnCl2
6M NaOH
1M ammonium acetate
magnesia mixture
centrifuge
dropper
6 small centrifuge tubes
unknowns
stirring rod
water bath (hot plate)
pH paper
DISCUSSION
The sulfide salts of Cu+2, Pb+2, Bi+3 and As+1 are insoluble. The precipitation of these ions is
accomplished by the addition of H2S to form the sulfide salts.
Cu +2
+ S −2 → CuS ↓
Pb + 2
+ S − 2 → PbS ↓
Bi + 3
As + 3
+ S−2
→ Bi 2 S 3 ↓
+ S − 2 → As 2 S 3 ↓
Separation of Arsenic from Lead, Copper and Bismuth
The addition of a alkaline solution of thioacetamide produces a soluble arsenic sulfide complex
while the bismuth, lead, and copper sulfides remain as solids..
As 2 S 3 ↓
+ 3 S −2 →
117
2 AsS 3 − 3
Precipitation of Antimony
The arsenic complex is reacidifed to precipitate back out the arsenic.
2 AsS 3 −3
+ 6 HCl →
As 2 S 3 ↓
Test for Arsenic
The arsenic sulfide is dissolved in a basic 3% hydrogen peroxide solution which oxidizes the As(III)
to As(IV) ions. The formation of a white precipitate upon the addition of magnesia mixture confirms
the presence of arsenic.
Mg +2
+ NH 4 +1 + AsO 4 − 3 →
MgNH 4 AsO 4 ↓
Separation of Copper and Bismuth from Lead
The precipitated sulfides of copper, bismuth and lead are dissolved by the addition of hot nitric acid.
CuS ↓ + HNO 3 → Cu +2
PbS ↓ + HNO 3 → Pb + 2
Bi 2 S 3 ↓ + HNO 3 → Bi + 3
Separation of Lead from Copper and Bismuth
The dissolved lead ions are then reacted with concentrated sulfuric acid to form an insoluble lead
sulfate.
Pb +2
+ SO 4 − 2 → PbSO 4 ↓
To confirm the presence of lead, the lead (II) sulfate is dissolved in ammonium acetate and reacted
with potassium chromate to form a yellow precipitate.
Pb +2
+ CrO 4 − 2 → PbCrO 4 ↓
Test for Copper and Bismuth
Aqueous ammonium hydroxide is added to the copper and bismuth ion solution that precipitates out
bismuth as a barium hydroxide. At the same time, the formation of a blue copper complex confirms
the presence of copper.
Cu +2 + NH 4 OH → Cu(NH3 ) 4 + 2
Bi + 3
+ NH 4 OH → Bi(OH) 2
118
The presence of bismuth is confirmed by the addition of sodium stannite that produces a black
precipitate.
Bi(OH) 2
+ 3 Sn(OH)3
→ Bi ↓
PROCEDURES
PREPARATION OF GROUP II CATIONS
1.
Add 2 ml of the sample solution containing As, Sb, Pb, Bi and Cu.
2.
Add 2 drops 6 MHCl.
3.
Heat for several minutes in a hot water bath.
4.
Centrifuge and discard any precipitated (Group I cations).
5.
Add concentrated NH4OH drop-wise until the solution is basic to pH paper (turns to blue).
6.
Add 10 – 15 drops of M thioacetamide and heat for several minutes in a hot water bath.
7.
Cool and centrifuge.
8.
Add an addition 2 drops of thioacetamide and heat, to test for complete precipitation.
9.
Cool, centrifuge and discard liquid.
10.
Wash the precipitate with 5 drops and 2M NH4Cl
11.
Centrifuge and discard the liquid.
12.
Add an additional 5 drops of 2M NH4Cl, centrifuge and discard liquid.
13.
Add 3 ml of 4M KOH and 3 drops of 1M thioacetamide to the precipitate.
14.
Heat in a hot water bath for at least 5 minutes.
15.
Centrifuge while warm.
16.
Decant off the supernatant and save the precipitate for step 30.
119
TEST FOR ARSENIC
17.
Add drop wise concentrated HCl to the precipitate from step 16 until acidic, then add an
additional 12 drops.
18.
Heat in a boiling water bath for 5 minutes.
19.
Cool and centrifuge
20.
Decant and discard the liquid and use the precipitate for the next step.
21.
Add 10 drops 6M NH4OH and 6 drops 3% H2O2 to the precipitate from step 20.
22.
Heat in a hot water bath for 5 minutes and cool.
23.
Add 5 drops of 6M NH4OH and 5 drops of magnesia mixture.
24.
The formation of a white precipitate indicates the presence of As.
TEST FOR LEAD
30.
Wash the precipitate from step 16 with 5 drops of water and discard the liquid.
31.
Add 15 drops 6M HNO3 and heat in a hot water bath until the precipitate dissolves.
32.
Cool and centrifuge.
33.
Discard any precipitate.
34.
Transfer the solution to a crucible and carefully add 5 drops concentrated H2SO4.
35.
Carefully heat the crucible until the appearance of SO3 fumes, (there must be fumes).
36.
After the dense fumes appear, cool to room temperature.
37.
Add 10 drops of water to the residue.
38.
Decant the liquid and save for step 43.
39
Extract the residue with 3 drops of 1M ammonium acetate.
40.
Stir thoroughly
120
41.
Add 1 drop of 1MK2CrO4.
42.
The formation of a yellow precipitate indicates Pb. (a yellow solution is not a positive test,
there must be a yellow precipitate).
TEST FOR COPPER
43.
Add 15 M NH4OH to the supernatant from step 38 until it is basic.
44.
The formation of a deep blue colour confirms the presence of Cu
45.
Centrifuge and save the white Bi(OH)2 precipitate for step 47.
TEST FOR BISMUTH
46.
In a separate test tube, add 3 drops 1M SnCl2 then add 6M NaOH drop-wise until any
precipitate which forms dissolves (this is now a solution called sodium stannite).
47.
Add 2 drops of the sodium stannite solution prepared in step 46.
48.
The immediately formation of a black precipitate confirms the presence of Bi.
DETERMINATION OF AN UNKNOWN SOLUTION
24.
Obtain an unknown solution from the instructor that may contain any combination of Cu, As,
Sb, Pb or Bi.
25.
Record the unknown number.
26.
Using the procedure starting at step 1, substitute 2 ml of unknown in place of the knowns
and determine which ions are in the unknown.
121
122
NAME __________________
DATE ___________________
SECTION ________________
IDENTIFICATION OF Bi, As, Cu, and Pb
REPORT SHEET
EXPERIMENT 12
PRECIPITATION OF INSOLUBLE SULFIDES
1.
Did you get a precipitate after adding the thioacetamide and heating?
2.
What are the possible known ion(s) in this precipitate?
TEST FOR ARSENIC
3.
After the addition of ammonium hydroxide and magnesia mixture, did you get a precipitate?
Yes or no
If no, what did this indicate?
If yes, what did this indicate and what confirmed this?
TEST FOR LEAD
4.
After the addition of nitric acid and heat in a crucible with sulfuric acid and the addition of
ammonium nitrate and potassium chromate, was there a precipitate?
Yes or no
If no, what did this indicate?
If yes, what did this indicate and what confirmed this?
123
TEST FOR COPPER
5.
After adding 6M NH4OH was added did a blue color form?
Yes or no
If no, what did this indicate?
If yes, what did this indicate and what confirmed this?
TEST FOR BISMUTH
6.
After the addition of sodium stannite, did a precipitate form?
Yes or no
If no, what did this indicate?
If yes, what did this indicate and what confirmed this?
INDENTIFICATION OF A UNKNOWN
UNKNOWN NUMBER _________
Based on your analysis of the unknown solution, circle all the metal ions you found present:
Bi
Pb
As
124
Cu