Qualitative Analysis: Identification of The Anion
Qualitative Analysis: Identification of The Anion
Qualitative Analysis: Identification of The Anion
PREPARATION OF THE
ANION TEST SOLUTION
FROM THE SOLUBILITY TEST DONE IN PART
A…
ANION ANALYSIS
TEST WITH SULFURIC ACID
Watch
Add out of
1 drop forconcentrated
formation
of prodout
sulfuric
Watch uct (a
togas
acidfor or a bi of
a small
formation
Add 1 drop of concentrated
solid sample
substance (1/4 sizeingrain
different
of prod
sulfuric uct
acid(a
togas or a bi of
a small
of rice)from the sample)
color
solid sample
substance (1/4 sizeingrain
different
of rice)from the sample)
color
If
no
ev
id
en
t
re
ac
ti
on
,
w
ar
m
th
e
te
st
tu
be
ge
nt
ly
.
TEST WITH SULFURIC ACID
Anions that react with cold sulfuric acid:
ANIONS VISIBLE RESULT REACTION EQUATION
Carbonate Effervescence CO23- + H20 → CO2 + H20
Chloride Pungent odor; Blue litmus to red Cl- + H2SO4 → HCl + HSO4-
Iodide Violet powder – starch powder blue 3I- + 2H2SO4 → I3- + SO42- + 2H20 + SO2
I- + H2SO4 → HI + HSO4-
9I- + 4H2SO4 → 3I3- + S + 3SO42- + 4H2O
12I- + 5H2SO4 → 4I3- + H2S + 4SO42- + 4H2O
Anion Precipitate
CNS- White
CO32- White
Br- Cream colored
I- Light yellow
PO43- Yellow
CrO42- Red
S2 - Black
THE CHLORIDE GROUP TEST
Centrifuge
Addrofesidue and
Wash r esidue with 5
5distilled
drops
drops
Centrifuge
Wash and5
with
Add
discard
of 3M
water.
drops 5distilled
drops
HNO
ofDiscard wash
discard
solution.
liquid.
water. Discard wash3
of 3M
solution.
liquid. HNO3
**If
**If
all
all
dis
dis
sol
sol
ves
ves
CN
CN
SS--,,
Cl
Cl--,,
Br
Br--,,
II--,,
SS-2 -2
are
are
abs
abs
ent
ent
**If
**If
not
not
all
all
dis
dis
sol
sol
ves
ves
,,
on
on
e
e
or
or
mo
mo
re
re
of
of
the
the
se
se
cou
cou
ld
ld
be
be
pre
pre
sen
sen
tt
THE CHLORIDE GROUP TEST
Centrifuge
Add
Add6twoM ammonia
dropsand
solution
Centrifuge
Add dropwiseand
Add6M
0.5
discard
until
M AgNO
two ammonia
drops
litmus3
reddropwise
solution
discard
0.5 M
solution
solution.
turns
until
AgNO
blue.
red litmus3
solution
solution.
turns blue.
Ye
llo
w
pr
ec
ipi
ta
te
:
Ph
os
ph
at
e
Re
d
pr
ec
ipi
ta
te
:
Ch
ro
m
at
e
Ad
Add
Ad
d 3M
Centrifuge
drops
d 3M
BaCl
Add
discard
the
drops
BaCl
of
to
the
2
THE SULFATE GROUP TEST
2 NH
2 and
Centrifuge
solution
2 and
0.3M
litmus
discard
solution
oflitmus
to 0.3M CaCl
3 dropwise
drops
ofNH
test
0.3Mto 5
3 dropwise
drops two
solid.
of testtest
CaCl
solid.
and
solution
0.3M
and
until
to 5
drops
alkaline
solution
two until
2 drops
test alkaline
2
**No
**No
pre
pre
cipi
cipi
tate
tate
::
STO
STO
P!
P!
No
No
sulf
sulf
ate
ate
gro
gro
up
up
pres
pres
ent
ent
(SO
(SO44
2-,
2-
,
CrO
CrO
2-,
2-
4 ,
4
PO
PO4433
--,
,
C
C22O O44
2-)
2-
)
**Ye
**Ye
llow
llow
pre
pre
cipi
cipi
tate
tate
::
CrO
CrO
2-
42-
4
**No
**No
n-
n-
yell
yell
ow
ow
pre
pre
cipi
cipi
tate
tate
::
pro
pro
cee
cee
d
d to
to
nex
nex
tt
step
step
Add
Add
more
THE SULFATE GROUP TEST
solution
litmus
more
litmus
five
Add 6 M HCl
Add 6 Mfive
solution HCl
until blue
drops
turns
until
red
drops
turns
blue
red
If
pr
e
ci
pi
ta
te
di
ss
ol
v
es
,
S
O
2-
2-
4
4
is
a
b
se
nt
.
Confirmatory Tests
ANION ANALYSIS
Summary/Notes on Confirmatory Tests
ANION CONFIRMATORY TEST
CNS-
I-
Br-
Cl-
S2-
CO32- Omit if no effervescense observed in test with Sulfuric
Acid
SO42-
CrO42- Omit if unknown solution is colorless
PO43-
C2O42-
CH3COO-
NO3-
S2O32-
NO2-
MnO4- Omit if unknown solution is colorless
CNS-
Add 1 drop of 0.1 M Fe(NO3)3 to 5 drops of
test solution
Discussion:
The chemical equation for this confirmatory step is:
CNS-(aq) + Fe3+ (aq) → FeSCN2+ (aq)
The addition of Fe(NO3)3 causes the CNS- ion to react with the Fe3+ ion
to form a dark red complex ion in the form of FeSCN2+.
I-
Acidify 6 drops of the test solution with 6M HNO 3
Submerge test tube in boiling water for 30 seconds and cool in cold water
If the CCl4 layer becomes yellowish or reddish brown, Br - is present in the sample.
Discard the CCl4 layer and then extract the remaining Br- by adding fresh CCl4 and
shaking.Use aqueous layer for Bromide test
Br-
Discussion:
The chemical equation for the
confirmatory test is:
10 Br-(aq) + 2MnO4-(aq) + 16H+(aq) → 5Br2(aq) + Mn2+(aq) + 8H2O
Moisten a small piece of filter paper with PbOAc sol’n and place it on the convex side of the
watch glass
Place the watch glass over the beaker with the moist paper inside the beaker
If the paper turns black or silvery black in color, S - is present in the sample.
S2-
Discussion:
H2S(g) + Pb2+(aq) → PbS(s) + 2H+
Some sulfides dissolve in non-oxidizing
acids like HCl forming solutions which
contain H2S. Hydrogen sulfide occurs as a gas
at room temperature and is only slightly
soluble in water. However, it is not a very
stable substance. It could be easily oxidized
by air or oxidizing agents like HNO3, Fe3+,
Cr2O72-, NO2-, Pb2+.
CO32-
Put a small bit of the sample into a test tube and add 3 drops 3M
HCl
Quickly drop a medicine dropper with short glass tube into the Ba(OH)2 and insert the drop
of Ba(OH)2 into the test tube such that the rubber bulb covers the top of the test tube
Discussion:
The chemical reaction is: SO42-(aq) + Ba2+(aq) → BaSO4(s)
Sulfate ion is very unreactive compared to the other anions. It is not readily
reduced or oxidized. However, sulfate ion undergoes ppt reaction with Ba 2+ or Pb2+
ion.
In this confirmatory test, the SO 42- ion reacts with the Ba2+ ion forming a white
ppt which is insoluble n sol’ns of strong acids or any other common reagent.
CrO42-
Add 1M HNO3 drop by drop to 3 drops of test sol’n in a test tube,
until it turns acid in litmus test.
Discussion:
The equation below demonstrates the chemical reaction for ths confirmatory test:
H3PO4- (aq) + 12 MoO42- (aq) + 3 NH4+ (aq) + 22 H+ → (NH4)3PO4 · 12 MoO3 (s) +
(aq)
12 H2O
Phosphates of the alkali metals are soluble. Most other phosphates, such as FePO 4,
CrPO4, BiPO4, Ca3(PO4)2 and AgPO4 are only sparingly soluble. Phosphate ion also forms a
bright yellow precipitate with ammonium molybdate—ammonium phosphomolydate.
C2O42-
Add a drop of 6 M CH3COOH and 2 drops of 0.2 M CaCl2 solution to 2 drops of test solution
If the smell of the mixture is like vinegar, then acetate is present. If not proceed to the succeeding steps.
If the same sweetish odor of ethyl acetate can be detected coming from the sample as from the control, then the sample
contains acetate.
CH3COO-
Discussion:
One of the natures of acetate is that it
has the odor of vinegar whenever H2SO4 is
added or any solid containing acetate. Also,
warming the solution could also enhance the
odor.
NO3-
Place of 2 drops of test solution in a 5 mL test tube. Add 10 drops of conc H 2SO4 + a grain
of FeSO4.
Discussion:
Nitrate ions only respond to a specific test reagent, consisting of
acidified iron (II) sulfate. Nitrate ion will oxidize iron (II) to iron (III), and
brown nitrogen (IV) oxide gas will evolved. The chemical reactions are a
follows:
3Fe2+ (aq) + NO3- (aq)
+ 4 H+ (aq) → 3Fe3+ (aq) + NO (aq) + 2 H2O (l)
Discussion:
Thiosulfates are unstable in acidic solutions. They will decompose to
sufite and sulfur as shown below:
S2O32- (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O
NO2-
5 drops of test solution + 6 M HOAc
Add drop by drop until it is acidic to litmus.
Discussion:
The following are the chemical reactions involved in the confirmatory test:
HNO2 (aq) + Fe2+ (aq) + H+ (aq) → Fe3+ (aq) + NO (aq) + H2O
NO (aq) + Fe2+ (aq) ↔ Fe(NO)2+ (aq)
Nitrites when subjected to acidic solutions are converted to nitrous acid,
HNO2.
MnO4-
2 drops test solution + 2 drops 0.02 M Na2C2O4 + 2 drops 6 M
H2SO4
Discussion:
Oxalates, though unresponsive in neutral and basic solutions, tend to slowly
oxidize in acidic mediums. This oxidation process is due to strong oxidizing agents
like MnO4- ion. The reaction converts the oxalate ion to carbon dioxide and
bleaches the purple permanganate ion.
The reaction is:
5 C2O42- (aq) + 2 MnO4- (aq) + 8 H+ (aq)→ 10 CO2 (g) + 2 Mn2+ (aq) + 4 H2O
Qualitative Analysis
CONCLUSION AND
RECOMMENDATIONS
Qualitative analysis is an effective culminating activity for
chemistry laboratory subjects. It includes and combines
both laboratory techniques and chemical reactions which
will test one’s knowledge of the subject involved.