Experiment 22
QUALITATIVE ANALYSIS
Qualitative analysis deals with the separation and identification of the components of matter what is present, not how much. The commonest scheme for the separation and identification of
groups of ions in solution is based on separation by precipitation. The ions are separated into
five groups as classified as follows.
Group No.
Group Name
Precipitated Form
Example Ion
I
Chloride
Insoluble chlorides
Ag+
II
Acid Sulfide
Cu2+
III
Basic Sulfide
Sulfides insoluble in
acidic solution
Sulfides insoluble in
base or insoluble
hydroxides
IV
Carbonate
Insoluble carbonates
Ca2+
V
Soluble
All soluble salts
Na+
Ni2+
Because ions in any given group would precipitate along with ions in later groups (eg. silver
carbonate, sulfide and hydroxide are all insoluble as well as the chloride), the groups must be
separated in sequence. In the analysis, you will work with each group separately. An unknown
with ions from different groups can be successfully analyzed as long as the ions are separated
and identified in the proper sequence.
Definitions
precipitate: a solid which settles to the bottom of the test tube as it forms by chemical reaction.
centrifuge: to place a test tube in a device which spins it rapidly forcing the precipitate to the
bottom of the tube.
supernatant: the liquid above a precipitate.
decant: to pour off the supernatant liquid without disturbing the precipitate.
21-
1
washing: to add small portions of a wash liquid, mixing, centrifuging and decanting after each
addition.
Qualitative Analysis General Scheme
1. All ions in solution: add HCl and centrifuge any precipitate which forms.
Precipitate: GROUP I - Chloride group: PbCl2, AgCl, Hg2Cl2
Decantate: Other groups
2. Make decantate slightly acidic, add thioacetamide and heat.
Precipitate: GROUP II - Acid sulfide group: HgS, CuS, CdS, PbS
Decantate: Remaining groups
3. Make decantate slightly basic, add thioacetamide and heat.
Precipitate: GROUP III - Basic sulfide group: NiS, MnS, CoS, Fe2S3,
Cr(OH)3
Decantate: Remaining groups
4. Remove sulfide and add carbonate ion to decantate.
Precipitate: GROUP IV - Carbonate group: CaCO3, SrCO3, BaCO3
Decantate: GROUP V - Soluble group: Na+, K+, NH4+
GROUP I - THE CHLORIDE GROUP
This is the first and the simplest of the cation groups containing just Ag+, Hg22+, Pb2+.
The mercury(I) is not the simple ion Hg+ but consists of two atoms held together by a covalent
bond with a double positive charge. Mercury(I) must be distinguished clearly from mercury(II)
which is a single atom with a double charge Hg2+ and is not a member of group I.
Silver(I), mercury(I) and lead(II) occur together in this first group because all form an insoluble
precipitate with chloride ion. Since none of the other ions we will study form insoluble chlorides,
group I is easily separated from all of the others by addition of chloride ion in the form of
hydrochloric acid.
After separation of group I by precipitation with chloride, the cations in the group are separated
from each other and identified with some characteristic reaction which confirms their presence
or proves their absence in the solution.
The following flow diagram outlines the separations and confirmatory tests for group I.
Now make up a test solution containing all three ions of group I by mixing 2 drops of AgNO3
solution, 2 drops of Hg2(NO3)2 and 2 drops of Pb(NO3)2. Carry out the scheme in the
21-
2
following procedure on this mixture and simultaneously on your unknown and note all
observations. From a comparison of your observations of the known and unknown solutions,
you should be able to identify the ion(s) present in your unknown.
21-
3
FLOW DIAGRAM FOR GROUP I
Ag+, Hg22+, Pb2+


HCl

↓
AgCl, Hg2Cl2, PbCl2
(white solid)


hot water
__________↓_______________
↓
↓
2+
Pb
AgCl, Hg2Cl2

(white solid)




K2CrO 4
NH3

_________↓________
↓
↓
↓
PbCrO4
Ag(NH3)2+
Hg & HgNH2Cl
(yellow solid)

(grey-black solid)

HNO3

↓
AgCl
(white solid)
21-
4
Procedure
Set up the well plate in the following manner.
To this well
B1
B2
B3
B4
A.
Add this reagent
8-10 drops of 6M HCl
3-4 drops of K2CrO 4
5-7 drops of 6M NH3
8-10 drops of 6M HNO3
Precipitation of the Group
To a 0.5 mL microcentrifuge tube add 5 drops of sample solution and add 12-16 microdrops of
6M HCl (B1) with your disposable pipet and mix. Be sure to rinse your pipet with deionized
water after each reagent. If a precipitate forms, continue adding HCl dropwise until no more
precipitate forms when a drop of the acid is added to the supernatant liquid. Centrifuge and
decant.
If at any point you are unsure if a precipitate is present, centrifuge.
Decantate:
Contains ions of Groups II, III and IV. If these are known to be
absent, discard.
Precipitate:
AgCl, PbCl2, Hg2Cl2
B.
Separation of Pb+2:
Add 12-16 microdrops of HOT water to the precipitate from A in the microcentrifuge tube.
Heat nearly to boiling in the hot water bath. Centrifuge and decant while still hot. Put 3-4 drops
of the decantate into an empty well of your well plate or into a microcentrifuge tube.
Decantate:
Pb+2 in solution
Precipitate:
AgCl and Hg2Cl2
C.
Confirmation of Pb+2
Add a few drops of K2CrO 4 (B2) solution to the decantate from B. Formation of a yellow
precipitate of PbCrO 4 confirms lead(II).
Precipitate:
PbCrO 4, yellow precipitate. Confirms Pb+2 present.
21-
5
D.
Separation of Ag + and Hg2+2 and confirmation of Hg2+2:
Add 12-16 microdrops of 6M ammonia solution (B3) to the precipitate from B in the
microcentrifuge tube. Stir, centrifuge and decant a small portion into another empty tube. If a
portion of the precipitate remains undissolved and turns dark gray or black, the presence of
mercury(I) is confirmed.
Decantate:
Ag(NH3)2+ in solution
Precipitate: Hg0 and HgNH2Cl, black to gray precipitate. Confirms Hg2+2
present.
E.
Confirmation of Ag +:
Cautiously add 6M HNO3 (B4) dropwise to the solution from D until slightly acidic when
tested with litmus paper. The appearance of a white precipitate confirms the presence of silver.
Precipitate:
AgCl, white. Confirms Ag+ present.
EQUATIONS
A.
Ag+ + Cl- → AgCl
Pb+2 + 2 Cl- → PbCl2
Hg2+2 + 2 Cl- → Hg2Cl2
B.
PbCl2 → Pb+2 + 2 Cl-
C.
Pb+2 + CrO 4-2 → PbCrO 4
D.
AgCl + 2 NH3 → Ag(NH3)2+ + ClHg2Cl2 + NH3 + OH- → Hg(NH2)Cl + Hg0 + Cl- + H2O
E.
Ag(NH3)2+ + Cl- + H+ → NH4+ + AgCl
21-
6