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Analytical Chemistry II Anions are divided into six groups:
1- Carbonates and Bicarbonates group
2- Sulphur-containing anions
3- Halides
4- Cyanogen anions
5- Arsinic and phosphorous containing anions
6- Nitrogen- containing anions
1. Carbonates and Bicarbonates group, CO32-, HCO3
-
CO3
2- HCO3
-
Parent acid Carbonic acid (H2CO3) is a very weak acid, Heating of solution of H2CO3, CO2 will evolve.
H2CO3 ⇋ CO2 + H2O
Solubility Insoluble in water except (Na+, K+, NH4+) Soluble in water
1- Dry Reactions
a) dilute HCl
b) sulphuric acid
Decomposition with effervescence due to the evolution of CO2 gas, for both CO3 2- and HCO3
-
CO3-- + 2H+ ⇋ CO2 + H2O
NaHCO3+ H+ ⇋ CO2 + H2O + Na+
As HCl, eff. In addition the formation insoluble sulphate (with gp v metals)
BaCO3 +H2SO4 ⇋ ↓BaSO4 + ↑SO2 + H2O 2- wet reaction
a- with AgNO3
A white precipitate of silver carbonate is immediately formed.
CO3 -- +2Ag+ Ag2CO3
The precipitate is soluble in mineral acids (nitric acid) and in ammonia.
Ag2CO3 + 2H+ 2 Ag+ + CO2 + H2O
Ag2CO3+4NH3 2[Ag (NH3)2]+ + CO32-
The precipitate becomes yellow or brown if the mixture is boiled.
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Ag2CO3 ⇋ Ag2O +CO2
boiling
b- with BaCl2,
CaCl2 and MgSO4:
White precipitates of BaCO3, CaCO3 and MgCO3 will be
obtained with carbonate solution.
BaCl2 + NaCO3 BaCO3 + 2 NaCl
Ca++ + CO3 -- CaCO3
Mg++ + CO3 -- MgCO3
The precipitate is soluble in mineral acids
No ppt. on cold since all HCO3- are
soluble in water.
Mixture of CO32- & HCO-3
Both anions haves similar reactions, but CO32- form precipitates immediately on cold upon the addition of
CaCl2, BaCl2 or MgSO4, while the bicarbonates of these metals are soluble. Separation: Add excess CaCl2 (BaCl2 or MgSO4) to a solution of the mixture CO3
2- /HCO3- a white ppt.
indicates CO3--
, centrifuge or filter
Ca (HCO3)2 + 2 NH3 ⇋ CaCO3+ (NH4)2 CO3
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2. Sulphur-containing anions
Sulphide (S2-) Sulphites (SO32-) Thiosulphate
(S2O32-
)
Sulphates
(SO42-)
Parent
acid
H2S H2SO3
H2S2O3Decompos
es to give, H2O,
SO2 and S.
H2SO4
1- Dry
Reactions
a- Action
of dilute
HCl
H2S gas; evolved has rotten egg odour,
1- blackening of filter paper moistened with
lead acetate sol
2- turns filter paper moistened with
cadmium acetate solution, yellow
It bleaches the brown color of l2 solution,
changes the pink color of acid KMnO4 into
colorless and changes the orange color of
acid K2Cr2O7 into green.
H2S + l2 ⇋ 2l- + 2H+ +So
5H2S + 6H+ +2 MnO4- ⇋ 2Mn++ + 8H2O + 5So
3H2S + 8H+ + Cr2O7-- ⇋ 2Cr3+ + 7H2O + 3So
SO2 gas evolved has bunt
sulphur odor and turbid lime
water due to the formation
of the insoluble CaSO3
Ca (OH)2 +SO2 ⇋ CaSO3 +
H2O
SO2 has reducing character,
bleaches the brown color of
iodine, reacts with acid
KMnO4 and acid K2Cr2O7.
l2 + SO2 + H2O ⇋ SO3 +
2H++ 2l-
2 MnO4- + 5 SO2 +
6H+⇋2Mn++ + 5SO3 + 3H2O
Cr2O72- +3 SO2 + 8H+
⇋ 2Cr3++ 3SO3 + 4H2O
The solution
becomes turbid
due to the
liberated yellow
colloidal sulphur
with evolution of
SO2 gas.
S2O3-- + 2H+
⇋
H2S2O3 ⇋ H2O
+ SO2 + So↓
No reaction
with dil. HCl
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2- Wet
Reactions
Reaction
with BaCl2:
No visible
reaction
White ppt. of BaSO3 is
formed which is soluble in dil.
HCl.
Ba+++SO32- =BaSO3
No ppt. in dilute
solution, but a ppt. is
formed from very
concentrated solution
A white ppt. of BaSO4 is formed
which is insoluble in dil. HCl, even
upon boiling.
Ba+++ SO4-- = BaSO4 ↓ White
Reaction
with
AgNO3
a black ppt. of
Ag2S soluble in
hot dil. HNO3,
insoluble in
ammonia an
2 Ag++ S-- ⇋
Ag2S black
A white crystalline ppt. of
Ag2SO3, which on boiling with
water undergoes self redox
with the production of grey
ppt. of metallic silver.
2 Ag++ SO32-⇋ Ag2SO3
2 Ag2SO3 boil→2 Ago + Ag2SO4
↓ + SO2 ↑
White ppt. which changes
its color on standing to
yellow, brown and finally
black, due to the formation
of Ag2S.
2 Ag+ + S2O3--⇋ Ag2 S2O3
Ag2S2O3+ H2O ⇋ Ag2S +
H2SO4
No ppt. in dil solution, but a
ppt. may be formed in a very
concentrated solution.
Reaction
with FeCl3
a black ppt. of Fe2S3 is
formed which is soluble
in dil. HNO3
2Fe3++ 3S-- → Fe2S3
A drak red color of ferric
sulphite is produced on
cold.
2Fe3++ SO3-- → Fe2(SO3)3
A purple color of complex
disappears on boiling
Fe3++ 2S2O32- → (Fe(S2O3)2)-
2 S2O3--+ 2Fe3+ ⇋ 2Fe+++ S4O6
-
-
do not react with FeCl3
Reaction
with lead
acetate:
A black ppt. of
PbS is produced
Pb+++ S-- → PbS
A with ppt. soluble in
cold HNO3. On boiling
oxidation to PbSO4.
SO3--+ Pb++ →PbSO3
A white ppt. is soluble in
cold HNO3, on boiling a
black ppt. of PbS is formed.
Pb+++S2O3-- →PbS2O3
A white ppt. lead suphate, which is
insoluble in cold dil. mineral acids,
but soluble in ammonium acetate and
hydroxide solutions
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Special
Tests
Cadmium
carbonate test
S--+ CdCO3
→ CdS + CO32-
Zinc nitroprusside
test :
Add ZnSO4 + K4[Fe
(CN)6] + 1% sodium
nitroprusside
solution. salmon-
colored ppt. of zinc
nitroprusside
is formed Zn
(Fe(CN)5 NO).
Formation of thiocyanate :
By boiling with KCN +
NaOH, Cool, acidify and add
FeCI3, a blood red color of
ferric thiocyanate complex
is produced.
S2O3--+ CN- OH- →SCN-+
SO3-
Fe3++ SCN- Cool →Fe(SCN)2+
Hepar’s test
Sulpate is reduced by carbon to
sulphide
Transfer the fusion product to a
silver coin and moisten with a little
water, a brownish black stain of
Ag2S results.
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3. Halides
Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-)
Therefore the order of stronger halogen acid is from HI HBr HCl HF. Parent Acids: HF:coloress fuming highly corrosive
and itching liquid
HCl Colorless gas
with irritating
odor, fumes in
moist air,
HBr Colorless gas
with irritating odor,
fumes in moist air
HI Colorless gas
with irritating
odor, fumes
strongly in moist
air,
Dry Reactions
a- dilute HCl
Hydrochloric acid shows no reaction .This reaction can differentiate
carbonate and sulphur group from halides.
b- conc. H2SO4 2X-+ H2SO4 = 2 HX + SO42- X = may be CI-, I-, Br- and F-
HF colorless fumes, the glass
rod acquire oily appearance
due to the formation of silicic
acid and hydrofluorosilicic
acid.
2 F-+ H2SO4 → 2H F +
SO4—
4HF+SiO2→SiF4+ 2H2O
3 SiF4+ 3H2O → H2 SiO3+ 2
H2 SiF6
HCI gas is evolved
2CI-+H2SO4 → 2 HCI +
SO4--
identified by :
Formation of white
fumes of NH4CI when a
glass rod moistened
with ammonium
hydroxide
NH4OH + HCI
→ NH4CI + H2O
A mixture of HBr
and Br2 have
characteristic
brown color
2 Br-+ H2SO4 → 2
HBr + SO4--
2 HBr + H2SO4
→ Br2 + SO2+ 2
H2O
I2 which appears as
violet fumes. I2 can
be detected by
exposing the evolved
gas to paper
moistened with starch
solution, it changes
into blue.
2I-+ H2SO4 →2 HI +
SO42-
2HI + H2SO4 → I2 +
SO2 + 2H2O
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Conc.H2SO4 &
MnO2:
2X- + 4H++ MnO2 ⇋ Mn+++ 2H2O +X2
X = may be CI-, Br- and I-
The free halogen, (X2) could be detected by: 1- Bleaching of a moistened colored litmus paper.
2- Suffocating, and irritating odor. 3- Characteristic color of Br2 (brown), I2 (violet) and CI2 gas
4- I2 changes starch paper into blue, Br2 turns it orange. 5- CI2 and Br2 change a starch – KI into
blue due to the oxidation of I- to I2 produce a blue adsorption complex.
2- Wet Reactions
a- Reaction with
AgNO3:
No precipitate A white curdy ppt. insoluble in
nitric acid, soluble in KCN and
dil. ammonia solution to give
the ammine complex.
AgCI + 2NH3 →
[Ag(NH3)2]CI
A curdy, pale yellow
precipitate , sparingly
soluble in dilute, but readily
soluble in conc. ammonia
solution
AgBr + 2 NH3 →
[Ag(NH3)2]++ Br-
A curdy yellow ppt.
insoluble in dil.
ammonia but very
slightly soluble in
conc. ammonia
solution.
b- Reaction with
BaCI2 solution:
The white gelatinous BaF2 ppt. is partially
soluble in dil. HCI or HNO3
No ppt. is formed
c- Reaction with
FeCI3:
a white crystalline ppt. of the complex
salt, which is sparingly soluble in water
Fe3++ 6 F- → [FeF6]3-
CI - and Br - : do not react
with FeCI3
Reacts with FeCI3, due to
its strong reducing action
with the liberation of I2.
Reaction with lead
acetate
F-, Cl- and Br- form a white ppt with lead acetate, sparingly
soluble in cold more soluble in hot water, crystallize on
cooling
forms a bright yellow ppt of PbI2
which is soluble in hot water
and crystallizes on cooling as golden
spangles
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Chlorine water
test
Chloride and Fluoride do not
react with chlorine water
2Br-+ CI2 → Br2+ 2CI-
Br2+ CI2 →2 BrCI (yellow)
Br2+ CI2 (excess) + 2H2O
→2HOBr+2HCI Colorless
2I- + CI2 → I2+ 2CI-
I2+ 5CI2 (excess) + 6H2O
→ 2 HIO3+10HCI
Special Tests Boron fluoride test:
fluoride + borax and conc.
H2SO4.→ boronfluoride gas.
Na2B4O7 + H2SO4+ 5H2O →
4H3BO3 +Na2SO4
2NaF + H2SO4 → 2HF +
Na2SO4 H3BO3+3HF→
BF3+ 3H2O
Chromyl chloride test:
The solid chloride + powder potassium
dichromate in a tube, + conc. sulphuric acid.
The deep red vapors of chromyl chloride
CrO2CI2 are passed into sodium hydroxide
solution.→
Yellow color;
4CI-+ Cr2O7--+ 6H+ cond.→ 2CrO2 Cl2 + 3H2O
CrO2CI2 + 4OH- → CrO4-- + 2CI- + 2H2O
I- reacts with Cu++
forming a whit ppt.
of Cu2I2,
2Cu+++4I- ⇋ Cu2I2
+I2
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