Qualitative Inorganic Analysis

1
Qualitative Inorganic Analysis
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
2
Carbonates and Bicarbonates group
CO32-
HCO3-
I. General characters 1- Parent acid
Carbonic acid (H2CO3) is a very weak volatile
acid (stronger than HCN and boric acid) Heating
of solution of H2CO3, CO2 will evolve. H2CO3
CO2 H2O
Bicarbonates are considered to be the
first step of ionization of carbonic acid,
while in the second step carbonates are formed
H2 CO3
H CO32-
H HCO3-
3
2-Solubility
All carbonated with the exception of those of
the alkali metals (Na and K) and of
ammonium are insoluble in water. All
bicarbonates are soluble in water.
II. General Reactions
1- Dry Reactions a- Action of dilute HCl
Decomposition with effervescence due to the
evolution of CO2 gas, for both CO3 -- and HCO3-
CO3-- 2H CO2 ? H2O
NaHCO3 H CO2 ? H2O Na
This is a type of displacement reaction in which
stronger acid liberates the very weak carbonic
acid, which spontaneously decomposes to CO2
H2O.
4
Test for CO2 gas
The solid substance is placed in a test tube,
dilute HCl is added, which immediately displaced
the gas, which is evolved (upon warming) and
passed into lime water or baryta water contained
in another test tube.
The production of a turbidity indicates the
presence of carbonates or bicarbonates. CO2
Ca(OH)2 ? CaCO3 H2O CO2 Ba (OH)2 ? BaCO3
H2O With prolonged passage of CO2, the turbidity
formed due to the insoluble carbonates, slowly
disappears as a result of the formation of a
soluble bicarbonate. CaCO3 CO2 H2O
Ca (HCO3)2
Boiling
5
2- Wet Reactions
In order to carry out the wet reactions, a
solution of the substance in water must be done.
Bicarbonates are mostly decomposed on heating
with the liberation of CO2.
2HCO3- CO3-- H2O CO2 ? .
a- Reaction 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 Ag2CO34NH3 ? 2Ag (NH3)2
CO32- The precipitate becomes yellow or brown if
the mixture is boiled.
Ag2CO3 ? Ag2O CO2 ?
boiling
6
b- Reactions with BaCl2, CaCl2 and MgSO4 White
precipitates of BaCO3, CaCO3 and MgCO3 will be
obtained upon the addition of these reagents to
samples of carbonate solution. BaCl2 NaCO3
? BaCO3 2 NaCl Ca CO3 -- ? CaCO3 Mg
CO3 -- ? MgCO3 The precipitate is soluble in
mineral acids For HCO3- No ppt. on cold since
all bicarbonates are soluble in water
Ba 2HCO3- Ba(HCO3)2
Soluble
H2O CO2 ? BaCO3

Boiling
7
III. 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
CO32- /HCO3- a white ppt. indicates CO3-- ,
centrifuge or filter
Contrifugate
White ppt. May be HCO3-
CaCO32-




H

CO2 H2O
Confirmatory test 1) Boil
2) Add ammonia
solution
white ppt.
Ca (HCO3)2 2 NH3 CaCO3 (NH4)2 CO3
8
Sulphur-containing anions
This group of anions, are 1- Sulphide (S2-) 2-
Sulphites (SO32-) 3- Thiosulphate (S2O32-) 4-
Sulphates (SO42-) 5- Perasulphate (S2O82-).
I. General characters
1- Parent Acids
a- Hydrogren sulphide or Hydrosulphuric acid
(H2S) It is a gas with offensive rotten egg
odour and poisonous. In solution it gives a weak
acid, which ionizes in two steps H2S
H HS- (hydrosulphide ion)
HS- H S-- (sulphide ion) Both HS-
and S-- ions give the same reactions.
9
b- Sulphurous acid(H2SO3) This acid is only
known in solution (like H2CO3). It has moderate
strong acidity. Like H2CO3 in water present in
equilibrium as follows
heat
H2SO3
H HSO3-
H2O ?SO2
H SO3--
Acid sulphite
c- Thiosulphuric acid (H2S2O3) It is not known
in the free form, and decomposes to give, H2O,
SO2 and S. It's more stronger than sulphurous
acid in solutions. It consists of SO32- solution
and S, which upon boiling gives S2O32-.
d- Sulphuric acid (H2SO4) It's a colourless
oily liquiud (B.P. 3300C).
General properties of H2SO4 1- Acid properties
It is one of the strongest acids, ionize in
dilute solutions in two steps,
H2SO4 H HSO4- (hydrogen sulphate)
HSO4- H SO4-- (sulphate)
10
Metals can liberate hydrogen from H2SO4
solution.
H2SO4 Zno ZnSO4 H2 Being a
strong acid can replace weak acids like, boric
acids, hydrocyanic acid and volatile acids or
their decomposition products due to its high
B.P.
2NaCl H2SO4 Na2SO4 2HCl 2-
Dehydrating properties Conc. H2SO4 has a
great tendency to combine with water to from
stable hydrates H2SO4.x H2O. So it is used as a
dehydrating agent for certain substance, and
used mostly in the dissectors. It causes
charring for certain organic substances as sugars
due to the vigorous abstracting of water from
theses substances. 3- Oxidizing properties
It's considered to be as moderately strong
oxidizing agent when heated with most reducing
agents
H2SO4 H2O SO2 O It is
reduced to SO2, while with active reducing agents
it may be reduced to So or H2S.
heat
11
2-Solubility
All Na, K and NH4 salts of sulphur containing
anions are soluble in water. Sulphides Other
sulphides are in-soluble except those of Ca,
Ba, Sr2 dissolve due to hydrolysis.
Sulphites Other sulphites are all in-soluble.
Thiosulphates Most S2O32-are soluble, Ag,
Pb, Hg2 Ba salts
are slightly soluble. Sulphates All
sulphates are soluble except Pb, Ba and Sr.
Ca Mg salts are
slightly soluble.
12
3-Complexing agent Thiosulphate form complex
with Fe3
Fe3 2S2O3-- (Fe(S2O3)2)- purple
color
4-Reducing agent
Sulphides, sulphites and thiosulphates are
reducing agents. They reduce solutions of I2,
KMnO4 and K2Cr2O7 with varying activities in
acidified solutions.
H
I2S2- 2I-So lodine (brown)
Colourless
2KMnO4 5S2- 16H 2Mn 5SO4-- 8H2O
2K
13
I2SO32-H2O SO42-2I-2H
2 MnO4- 5 SO3-- 6H 2Mn 5SO4--
3H2O
Cr2O7-- 3SO32- 8H 2Cr3 3SO4--4H2O
I22S2O3-- H S4O62-2I-
Tetrathionate
H
Fe32S2O32- S4O62-Fe2
8MnO4- 5 S2O3-- 14H 8Mn10SO4--7H2O
4Cr2O72- 3S2O32- 26H 8
Cr36SO4-- 13 H2O
14
II. General Reactions
1- Dry Reactions a- Action of dilute HCl

1. Sulphide S2-

H2S gas evolved upon adding dil. HCl to a solid
sample. The gas evolved has its characteristic
rotten egg odour, and could be identified by 1-
blackening of filter paper moistened with lead
acetate sol.
S-- 2H H2S
H2SPb PbS black
2- alternatively, a filter paper moistened with
cadmium acetate solution, turns yellow
H2S Cd CdS
Yellow H2S has reducing character, It reacts
with l2 solution, acid KMnO4, acid K2Cr2O7
15
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
2- Sulphite SO32-
Upon treatment of SO3-- with dil. HCl, SO2 gas
will evolve, due to the decomposition of the
liberated unstable H2SO3
SO--3 2H H2SO3 SO2
H2O
The evolved SO2 gas has a characteristic bunt
sulphur odor and turbid lime water (like CO2)
due to the formation of the insoluble CaSO3 which
is soluble upon prolonged passage of SO2 due to
the formation of soluble calcium bisulphite.
Ca (OH)2 SO2 CaSO3 H2O
16
CaSO3 SO2 H2O Ca(HSO3)2.
SO2 like H2S 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
3- Thiosulphate S2O32-
No immediate change on cold, but on warming with
dil. HCl or standing, the solution become turbid
due to the liberated yellow colloidal sulphur
with evolution of SO2 gas. This is due to the
decomposition of the produced unstable
thiosulphuric acid.
S2O3-- 2H H2S2O3 H2O
SO2 So Thiosulphate has the same action of
sulphite with HCl in addition to formation of
yellow colloidal precipitate.
17
4- Sulphate SO42- No reaction with dil. HCl.
2- Wet Reactions
a- Reaction with BaCl2 Add BaCl2 reagent to
neutral sample solution
1- S2- No visible reaction
2- SO32- White ppt. of BaSO3 is formed which is
soluble in dil. HCl. Ba SO32-
BaSO3
3- S2O3-- No ppt. in dilute solution, but a
ppt. is formed from very
concentrated solution. 4- SO4-- A white ppt.
of BaSO4 is formed which is insoluble in dil.
HCl, even upon boiling.
Ba SO4-- BaSO4
White
18
b- Reaction with AgNO3 Add AgNO3 reagent to the
neutral sample solution
1- S2- a black ppt. of Ag2S is formed which is
soluble in hot dil. HNO3, insoluble
in ammonia and KCN solution
2 Ag S-- Ag2S
black
2- SO32- A white crystalline ppt. of Ag2SO3 is
formed, which on boiling with
water undergoes self oxidation reduction with the
production of grey ppt. of
metallic silver.
2 Ag SO32- Ag2SO3
White
2 Ag2SO3 boil 2 Ago Ag2SO4 SO2
19
Silver sulphite is soluble in nitric acid,
ammonia and in excess sulphite to give a complex
salt, which on boiling gives a grey ppt. of
metallic silver
Ag2 SO3 SO3-- 2(AgSO3)-
2(AgSO3)- boiling 2Ago SO4-- SO2

3- S2O3-- Forms white ppt. of silver
thiosulphate which changes its color
on standing to yellow, brown and finally
black, due to the formation of
Ag2S. Ag2S2O3 is soluble in
excess S2O3-- to give a complex ion.
2 Ag S2O3-- Ag2 S2O3
Ag2S2O3 H2O Ag2S H2SO4
Ag2S2O3 3S2O3-- 2(Ag(S2O3)2)3-
20
4- SO42- No ppt. in dil solution, but a ppt.
may be formed in a very
concentrated solution.
c- Reaction with FeCl3 Add FeCl3 reagent to the
neutral sample solution
1- S2- a black ppt. of Fe2S3 is formed which is
soluble in dil. HNO3
2Fe3 3S-- Fe2S3

black
2- SO3-- A drak red color of ferric sulphite is
produced on cold.
2Fe3 SO3-- Fe2(SO3)3
3- S2O32- A purple color of complex ferric
thiosulphate is produced which
disappears on boiling as tetrathionate and Fe2
are formed from the oxidation of
S2O32- with Fe3, even on cold
Fe3 2S2O32- (Fe(S2O3)2)-
2 S2O3-- 2Fe3 2Fe S4O6--
4-SO42- do not react with FeCl3.
21
d- Reaction with lead acetate Adding
lead acetate reagent to the neutral sample
solution.
1- S-- A black ppt. of PbS is produced
Pb S-- PbS
2- SO32- A with ppt. of lead sulphite which is
soluble in cold HNO3. On boiling
oxidation to PbSO4 which is a white ppt. occurs.
SO3-- Pb PbSO3
3- S2O3-- A white ppt. of lead thiosulphate is
formed which is soluble in cold
HNO3, on boiling a black ppt. of PbS is formed.
PbS2O3-- PbS2O3
4- SO4-- A white ppt. lead suphate, which is
insoluble in cold dil. mineral
acids, but soluble in ammonium acetate and
hydroxide solutions (Na and K)
22
Pb SO42- PbSO4
PbSO4 4 CH3 COO- (Pb (CH3COO)4)2-
SO42-
PbSO4 3OH- HPbO2- H2O SO42-
Plumbites
III. Special Tests

1. Sulphide S2-

Cadmium carbonate test
The sulphide solution is shaken with CdCO3
powder, a canary yellow ppt. of CdS is produced.
S-- CdCO3 CdS CO32- This test
could be used for the identification and
separation of S2- when present in a mixture with
other sulphur containing anions, or those anions
which do not react with CdCO3.
23
2- Sulphite SO32-
Zinc nitroprusside test Add to cold saturated
ZnSO4 solution, equal volume of K4Fe (CN)6
solution, add few drops of 1 sodium
nitroprusside solution. This solution is added
to the SO32-solution,a salmon-colored ppt. of
zinc nitroprusside is formed Zn (Fe(CN)5 NO).
The latter reacts with moist SO2 to give a red
ppt. of Na5Fe(CN)5 SO3
3- Thiosulphate S2O32-
Formation of thiocyanate By boiling with KCN
solution (poison), in the presence of NaOH, Cool,
acidify and add FeCI3, a blood red color of
ferric thiocyanate complex is produced.
S2O3-- CN- OH- SCN- SO3--
boil
Fe3 SCN- Cool Fe(SCN)2
24
4- Sulphate SO42-
Hepars test Sulpate is reduced by carbon to
sulphide by heating on a piece of charcoal in
the presence of Na2CO3 in the reducing zone of
the flame
MSO4 Na2CO3 Fusion Na2SO4 MCO3
Na2SO4 C Na2S 4 CO
Transfer the fusion product to a silver coin and
moisten with a little water, a brownish black
stain of Ag2S results.
S-- 2H2O 2 OH- H2S
H2S 2 Ag Ag2S H2
25
IV. Analysis of Mixtures
1- Mixture of S2-, SO32-, S2O32- and SO42-
Separation is carried first shaking the mixture
solution with CdCO3 powder. The centrifugate is
allowed to react with BaCl2 solution which will
precipitate BaSO4 and BaSO3 leaving S2O32-as
soluble centifugate. The precipitated BaSO4 and
BaSO3 can be separated by the solubility of
BaSO3 in excess dil. HCI.
S2-, SO32-, S2O32- , SO42- Solution CdCO3
Yellow ppt.
Centrifugate
S2-
BaCI2
Centrifugate
White ppt.
BaSO3BaSO4
S2O32-
HCl
Heat
HCl
White PPt SO42-
Centrifugate SO32- confirm by reducing character
SO2 So
26
2- Mixture of CO32- and SO32- or S2O32-
This type of mixtures are considered to be
difficult, due to the interference occur upon
the addition of dil. HCI which liberates CO2 and
SO2 gases which turbides lime water and
disappears on prolonged passage. SO2 can be
detected by its reducing characters as discussed
before, but CO2 has non reducing
characters. Therefore SO32- or S2O32- ions must
be firstly oxidized into SO42- by an oxidizing
agent such as H2O2,K2Cr2O7 or KMnO4 and dil.
H2SO4 and warm, CO2 will only evolve which can
be test with lime water.

3- Mixture of H2S and SO2 gases
In order to differentiate between these two gases
which evolve upon the addition of dil. HCI to
sulphides, sulphites and thiosulphates and having
similar reducing properties. A paper moistened
with lead acetate solution changes into black
when exposed to H2S gas, SO2 can cause turbidity
to lime water
27
Halides
This group of anions, are 1- Fluoride (F-) 2-
Chloride (Cl-) 3- Bromide (Br-) 4- Iodide (I-)
Fluorides, chlorides, bromides and iodides are
known as halogens. They are characterized by
their higher electronegativity
As the ionic size increases, the tendency to
loose electrons increases and therefore iodide
ion is firstly and easily oxidized into free I2
by loosing readily an electron followed by Br -
when present in a mixture. However it's difficult
to oxidize F- into F2, hence F- ions are highly
stable to held strongly a proton. Therefore the
order of stronger halogen acid is from HI ? HBr ?
HCl ? HF.
28
I. General characters
1- Parent Acids
a- Hydrofluoric acid HF It's coloress fuming
highly corrosive and itching liquid (B.P.
19.4oC). Soluble in water producing the weakest
acidic solution in the halogen acid series.
b- Hydrochloric acid HCl Colorless gas with
irritating odor, fumes in moist air, extremely
soluble in water to form acidic solution.
Concentrated HCI contains 37 of HCI gas.
c- Hydrobromic acids HBr Colorless gas with
irritating odor, fumes in moist air and is
extremely soluble in water forming very strongly
acidic solution. On standing the solution becomes
yellow due to the oxidation to bromine.
d- Hydroiodic acid HI Colorless gas with
irritating odor, fumes strongly in moist air,
soluble in water forming the strongest acidic
solution of the haloacid series. the solution is
colorless, becomes brown on standing due to the
liberated iodine.
29
2-Solubility
All the salts of CI-, Br- and I- are soluble
except Ag, Hg22, Cu salts, their lead salts
are slightly soluble in cold water, soluble in
hot water. The alkali metal salts of fluorides,
ammonium and silver salts are soluble, other
salts are insoluble or sparingly soluble.
3-Reducing agent Cl- has very weak reducing
character. Br- and I- have reducing character,
they can react with oxidizing agent like
chlorine water to give Br2 or I2. I- has strong
reducing power than Br- so it react with FeCl3,
H2O2 and nitrite solutions.
II. General Reactions
1- Dry Reactions a- Action of dilute HCl
Hydrochloric acid shows no reaction upon
treatment of the solid sample with it even on
heating. This reaction can differentiate
carbonate and sulphur group from halides.
30
b- Action of concentrated H2SO4
Decomposition of the halides occurs upon the
addition of the strong non-volatile concentrated
H2SO4 to the solid sample, this occurs in the
cold, completely on warming with the evolution of
HX which can be recognized by a) the fumes
evolved. b) Confirmatory chemical test 2X-
H2SO4 2 HX SO42- X may be CI-, I-, Br-
and F-
1- For Fluoride
Fluoride gives a characteristic reaction when
treated with conc. H2SO4. Hydrofluoric acid is
produced which is colorless and fumes with moist
air. due to the corrosive and itching action of
the gas on the glass in presence of H2O, the
test tube or the glass rod subjected to the
evolved HF gas acquire oily appearance due to
the formation of silicic acid and
hydrofluorosilicic acid. This test is
considered to be specific for fluoride anion,
even in the presence of other halides.
4HF SiO2 SiF4 2H2O
glass
2 F- H2SO4 2H F ? SO4--
3 SiF4 3H2O H2 SiO3 2 H2 SiF6
silicic acid
hydrofluoro silicic acid

31
2- For chloride
HCI gas is evolved upon treatment with conc.
H2SO4 which can be identified by
2CI- H2SO4 2 HCI SO4--
1- Formation of white fumes with moist air due
the formation of droplets of hydrochloric
acid. 2- Pungent irritating odor. 3- Changing a
blue moistened litmus paper into red. 4-
Formation of white fumes of NH4CI when a glass
rod moistened with ammonium hydroxide solution
is exposed to the evolved gas. NH4OH HCI
NH4CI H2O
3- For Bromide
A mixture of HBr and Br2 may be formed which have
characteristic brown color especially on
warming. At the same time sulphuric acid will be
reduced into SO2, H2S or S
2 Br- H2 SO4 2 HBr SO4--
2 HBr H2SO4 Br2 SO2 2 H2O
32
4- For iodide
Since HI is the most active reducing agent, so it
is readily oxidized to iodine 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 6HI H2SO4
3 I2 S 4H2O 8HI H2SO4 4 I2
H2S 4H2O
c- Action of concentrated H2SO4 and MnO2
If the solid halide is mixed with an equal
quantity of precipitated manganese dioxide,
concentrated H2SO4 added and the mixture gently
warmed. Chlorine, bromine and iodine are evolved
from CI-, Br- and I- but F- liberates HF since
it has no reducing properties.
2X- 4H MnO2 Mn 2H2O X2
X may be CI-, Br- and I-
33
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 (greenish tint). 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.
CI2 2KI 2KCI I2
Br2 2KI 2KBr I2
2- Wet Reactions
a- Reaction with AgNO3 To 1ml of the salt
solution add AgNO3 reagent.
1- Fluoride No precipitate, since AgF is soluble
in water.
2- Chloride A white curdy ppt. of AgCI which is
insoluble in nitric acid,
soluble in KCN and Na2S2O3 as other silver
halides. The precipitated AgCI is soluble in dil.
ammonia solution to give the ammine complex.
34
Ag CI- AgCI
AgCI 2NH3 Ag(NH3)2CI
Silver ammine chloride
Ag(NH3)2 CI 2H 2 NH4
AgCI AgCI is reprecipitated upon treatment of
the ammine complex with acid.
AgX 2CN- Ag (CN)2- X-
Soluble complex
AgX 2 S2O3--
Ag(S2O3)23-X-
3- Bromide A curdy, pale yellow precipitate of
AgBr, sparingly soluble in
dilute, but readily soluble in conc. ammonia
solution
Ag Br- AgBr
AgBr 2 NH3
Ag(NH3)2 Br-
4- Iodide A curdy yellow ppt. of AgI is formed
which is insoluble in dil. ammonia
but very slightly soluble in conc. ammonia
solution.
Ag I - AgI
35
There is a periodicity in character of three
silver halides. Since AgI is the most insoluble
one, followed by AgBr and AgCI. Therefore AgCI
will be dissolved in dil. ammonia, followed by
AgBr in conc. Ammonia solution but AgI does not
This is also attributed to that the conc. of
silver ions (Ag) produced form the dissociation
of silver ammine complex according to its
instability constant is insufficient to exceed
the high solubility product of AgCI, approach
that of AgBr (partially soluble) but exceeds that
of AgI.
Ag 2NH3
Ag(NH3)2
Instability constant (Ag) (NH3)2

_________________
Ag(NH3)2
Therefore when Br- or iodide solutions are added
to AgCI, yellow ppt. of AgBr or AgI are formed.
AgCI Br- (or I-) AgBr (or
AgI) CI-
AgBr I- AgI Br-
36
b- Reaction with BaCI2 solution
Only fluoride gives a white gelatinous ppt. when
BaCI2 reagent is added to sample solution.
Ba 2F- BaF2 The white gelatinous
BaF2 ppt. is partially soluble in dil. HCI or
HNO3 No ppt. is formed in case of other halides.
c- Reaction with FeCI3 Add few drops of FeCI3
reagent to concentrated sample solution. 1- F-
a white crystalline ppt. of the complex salt,
which is sparingly soluble in water
Fe3 6 F- FeF63-
2- CI- and Br- do not react with FeCI3
3- lodide reacts with FeCI3, due to its strong
reducing action with the liberation of I2.
d- Reaction with lead acetate
Precipitates of Pbx2 are formed in cold solution
when lead acetate reagent is added to sample
solutions.
37
F-, Cl- and Br- form a white ppt with lead
acetate, sparingly soluble in cold more soluble
in hot water, crystallize on cooling
Pb 2 F- PbF2
Pb 2 CI- PbCI2
Pb 2 Br- PbBr2
Iodide forms a bright yellow ppt of PbI2 ? which
is soluble in hot water and crystallizes on
cooling as golden spangles.
e- Chlorine water test
Chloride and Fluoride do not react with chlorine
water .
Chlorine water oxidizes I- and Br- into I2 and
Br2 which can be extracted with chloroform or
carbon tetrachloride as violet color or brown or
yellow color of I2 and Br2, respectively.
Iodide react first with chlorine water before
bromide as it has more reducing character.
38
Chlorine water reagent is added drop wise to a
solution of iodide or bromide as
excess chlorine water converts Br2 into yellow
bromine monochloride or into colorless
hypobromous acid or bromic acid and the organic
layer turns pale yellow or colorless. Also,
excess chlorine water oxidized I2 to colorless
iodic acid.
2Br- CI2 Br2 2CI-
bromine monochloride
Br2 CI2 2 BrCI (yellow)
Br2 CI2 (excess) 2H2O 2HOBr2HCI
hypobromous acid
Colorless
Br2 5CI2 (excess) 6H2O 2
HBrO310HCI
bromic acid
2I- CI2 I2 2CI-
I2 5CI2 (excess) 6H2O 2 HIO310HCI
iodic acid
39
III. Special Tests
1- For Fluorides Boron fluoride test
When fluoride is mixed with borax and moisten
with conc. H2SO4. The formed HF and boric acid
react to produce boronfluoride gas. If the
mixture introduced into the flame tinged green by
BF3 gas.
Na2B4O7 H2SO4 5H2O 4H3BO3Na2SO4
Borax boric
acid
2NaF H2SO4 2HF Na2SO4
H3BO3 3HF BF3 3H2O
2- For chlorides
Chromyl chloride test
This test is a specific test for chloride even in
the presence of other halides. It's classified
as dry reactions test because, it is carried out
on the solid sample
40
The solid chloride is mixed with three times its
weight of powdered potassium dichromate in a
tube, an equal bulk of concentrated sulphuric
acid is added, the tube is attached to another
tube by a pent tube, dipped into a NaOH
solution. The deep red vapors of chromyl chloride
CrO2CI2 which are evolved are passed into sodium
hydroxide solution. The resulting yellow
solution in the test tube contains sodium
chromate this confirmed by perchromic acid
test, which is carried out by acidifying with
dil. H2SO4, adding 1-2 ml alcohol or ether,
followed by a little H2O2 solution. The organic
layer is colored blue.
4CI- Cr2O7-- 6H cond. 2CrO2 Cl2 ?
3H2O
CrO2CI2 ? 4OH- CrO4-- 2CI- 2H2O
2 CrO4-- 2H Cr2O7-- H2O
Cr2O7-- 7H2O2 2 CrO83- 5H2O 4H
Blue in ether or
amyl alcohol
It is possible to test for CrO4--also by lead
acetate
CrO4-- Pb Pb CrO4
Yellow
41
N.B. 1- Some CI2 may also be liberated owing to
the reacting. 6CI- Cr2O7-- 14H
3CI2 2Cr3 7H2O and this decreases the
sensitivity of the test.
2- Fluorides give rise to the volatile CrO2F2
which is decomposed by water, and hence should
be absent or removed.
3- Nitrites and nitrates interfere, as nitrosyl
chloride may be formed.
4- Bromides and iodides give rise to the free
halogens, which yield colorless or pale
yellow solution with NaOH.
6 Br- Cr2O7-- 14H 2 Cr3 3Br2
7H2O
6 I- Cr2O7-- 14H 2Cr3 3I2
7H2O
Br2 2OH- OBr- Br- H2O
(hypobromide)
I2 2OH- OI- I- H2O
(hypoiodide)
42
3- For iodides
A) lodide is readily oxidized in acid solution
(dil. H2SO4) with nitrite solution or H2O2 into
free l2
2I- 2NO2- 4H I2 2NO 2H2O
2I- H2O2 2H I2 2H2O
B) I- reacts with Cu forming a whit ppt. of
Cu2I2, the I- being oxidized to free I2. Thus
a white ppt. in brown solution is formed on
treating I- with CuSO4 solution.
2Cu 4I- Cu2I2 ?I2
C) I- reacts with mercuric chloride solution
mercuric iodide HgI2 will be precipitated as
yellow-scarlet red ppt. which dissolves in excess
iodide forming soluble colorless complex.
HgCI2 2I- HgI2 2CI-
Scarlet red
HgI2 2I- (HgI4)2-
Soluble complex
Nessler's reagent
43
IV. Analysis of Mixtures
1- Mixture of F-, Cl-, Br- and I-

• The F- is separated by treating the mixture
  solution acidified with
• CH3COOH with Ba(NO3)2 or Ca (NO3)2

Centrifuge
White PPt.
Centrifugate BaF2

CI-, Br- and I- Confirmed by Conc.H2SO4
test
b) for the centrifugate ( Cl-, Br- and I-), carry
out chlorine water test for both I- and Br
( or get rid of I- by oxidation to I2 using H2O2
or nitrite and extract I2 by chloroform then
test for Br- in aqueous solution
c) For CI-, carry out chromyl test on a solid
sample.
44
2- Mixture of chlorine / chloride and Br2 / Br-
Chlorine is tested for by its smell, bleaching
effect, while Br2 is tested by shaking with
chloroform, it give brown color. CI- and Br-could
be tested after removal of chlorine and bromine
by shaking with metallic mercury (till the smell
of CI2 disappears and the liquid doesn't bleach
litmus paper). Insoluble Hg2CI2 and/or Hg2Br2
are formed. Test for CI- and or Br- in the clear
supernatant (centrifugate(.
CI2 2Hgo Hg2CI2 ?
Br2 2Hgo Hg2Br2 ?
3- Mixture of chloride and iodide
Add AgNO3 to the mixture, AgCl and AgI are
precipitated. Add to precipitate dil ammonia
solution and filter
Filterate Cl-
Confirmed by chromyl chloride test
Precipitate Yellow ppt. I-
45
Cyanogen anions
This group of anions, are 1- Cyanide (CN-) 2-
Thiocyanate (SCN-) 3- Ferrocyanide Fe(CN)64-
4- Ferricyanide Fe(CN)63-
All cyanide containing anions are highly
poisonous. In all experiments in which the gas is
likely to be evolved or those in which cyanides
are heated, should be carried out cautiously in
the fume cupboard.
I. General characters
1- Parent Acids
a) Hydrocyanic acid HCN
It's very poisonous. It's colorless volatile
liquid (B.P. 26.5oC). It has an odor of bitter
almonds. It is not stable in solution due the
formation of ammonium formate. Any dil. mineral
acid can replace HCN in its solution.
46
On passing CO2 to CN- solution HCN is produced
with HCO3-.
CN- CO2 H2O HCN HCO3-
b) Thiocyanic acid HSCN
It is colorless toxic liquid (B.P. 85oC) with
unpleasant odor. It is as strong as HCI but
unstable. It is soluble in ether after the
addition of HCI to an aqueous solution of
SCN-. On standing its aqueous solution is
decomposed to HCN and yellow solid polymer.
3 HCNS HCN H2N2C2S3
c) Ferrocyanic acid H4 FeCN)6
It's white crystalline solid. Its aqueous
solution is strongly acidic. The first two
protons are nearly completely ionized.
d) Ferricyanic acid H3 Fe(CN)6
It's browinish crystalline solid, soluble in
water to give strongly acids solution. The three
protons are nearly completely ionized.
47
2-Solubility
CN- All cyanides are water insoluble except
alkali metals (Na, K), ammonium salt,
alkaline earth metals ( Ba2, Sr2 and Ca2) and
mercuric cyanide. SCN- All thiocyanates are
water soluble except AgSCN, Hg2(SCN)2
Cu2 (SCN)2. Pb (SCN)2 as PbCI2 is sparingly
soluble in cold water, but soluble in
hot water. Ferro and Ferricyanides All are
insoluble in water except those of alkali metals,
ammonium salt and alkaline earth metals.
3-Complexing agent Cyanide ion has strong
tendency to the formation of complexes which may
be double cyanides or complex cyanides.
1- Argentocyanide complexes Double cyanides When
a ppt. is formed upon reacting CN- with Ag, at
first white turbidity is formed which is AgCN.
According to the medium, if CN- ions are present
in excess a soluble complex is formed.
AgCN CN- (Ag (CN)2)-
48
2- Complex cyanides Stable metallo-cyanogen
complexes can be formed by reacting FeSO4 with
CN- in alkaline medium to give stable
ferrocyanide complex. Similar complex is formed
with Fe3 to give ferricyanide. Therefore
Fe(CN)64- and Fe(CN)63- are considered to be
stable complexes from CN- ions. Also Co can
form stable complexes with CN-.
Fe2 6 CN- Fe(CN)64-
Fe3 6CN- Fe(CN)63-
When cyanides are heated with polysulphides
(NH4)2Sx or thiosulphate (S2O3--) they give
thiocyanate ion
CN- (NH4)2Sx (NH4)2Sx-1 SCN-
CN- S2O32- SO3-- SCN-
4-Oxidizing agent Ferricyanides has
oxidizing effect, they can oxidizes I- into I2
5-Reducing agent
Ferrocyanides has mild reducing effect, they can
be oxidized to ferricyanide by oxidizing agents,
such as MnO4-, NO3-, H2O2 and Cl2
49
II. General Reactions
1- Dry Reactions a- Action of dilute HCl
a) CN- HCN gas evolved with characteristic
bitter almond odor and can be tested by
1- Converting HCN evolved into SCN-, by exposing
the evolved HCN gas to a paper moistened with
ammonium polysulphide.The resulted SCN- can be
tested by adding dil. HCI and a drop of FeCI3
solution, a blood red color is produced.
2- By passing the evolved gas into AgNO3
solution, a white ppt. of AgCN is formed
insoluble in dil. HNO3, soluble in ammonia
solution.
HCN AgNO3 AgCN HNO3
AgCN 2NH3 (Ag(NH3)2)CN
3- Prussian blue test The evolved HCN gas is
passed into NaOH solution, add drops of FeSO4
solution, heat to boiling, the HCN is converted
into ferrocyanide which can be tested by adding
drops of FeCl3 solution to produce a prussian
blue ppt.
50
b) SCN- No reaction as SCN- is as strong as HCl
c) Ferrocyanide and Ferricyanide
With cold dil. HCI, no gases, but may be
precipitation of hydro ferrocyanic and
hydroferricyanic acid occur.
(Fe(CN)6)4- 4H H4(Fe(CN)6)
(Fe(CN)6)3- 3H H3(Fe(CN)6)
b- Action of conc. H2SO4
a) CN- All cyanides are decomposed on heating.

CN -2H H2O NH4 CO
b) CNS- Decomposition with evolution of carbonyl
sulphide, which burns with a blue
flame.
SCN- 4H 2SO4-- H2O NH4
2HSO4-COS
Carbonyl
Sulphide
51
c) Ferrocyanide and Ferricyanide
On heating with conc. H2SO4, CO will be
evolve which burns with a blue flame. SO2 is
produced in case of ferrocyanide.
(Fe(CN)6)4- 6H2O 22H 10 SO42-
Fe26NH4 10 HSO4- 6 CO ?
2Fe2 4H SO4-- SO2 2H2O 2Fe3
(Fe(CN)6)3- 6H2O 22H 10 SO42-
Fe3 6NH4 10 HSO4- 6CO ?
2- Wet Reactions
a- Silver nitrate solution
1- CN- SCN- form white ppts. of silver
cyanide and silver thiocyanate. AgCN is soluble
in excess CN-, ammonia solution, but insoluble in
dil. HNO3
Ag SCN- ? AgSCN
H
HCN AgCN
Ag CN- ? AgCN CN-
(Ag(CN)2)-
52
2- Ferro- and Ferricyanides Both Fe(CN)64-and
Fe(CN)63- react with AgNO3 solution with the
formation of a white ppt. and orange red ppt.,
respectively
4 Ag Fe(CN)64- ? Ag4Fe(CN)6
Insoluble
in dil. ammonia
Insoluble in dil. HNO3
3 Ag Fe(CN)63- ? Ag3Fe(CN)6
Orange
red ppt.
Insoluble in dil. HNO3
Soluble in dil. ammonia
The solubility of silver ferricyanide ppt. can be
used for the separation of ferrocyanide and
ferricyanide when present in a mixture. Oxidation
of the white ppt. of Ag4 Fe(CN)6 by warming
with few drops of conc. HNO3, leads to orange
red ppt. of Ag3 Fe(CN)6 which becomes soluble
in dil. ammonia solution.
b) Reaction with BaCI2 No observed reaction
53
c) Reaction with FeCI3 This reaction is very
important, since it is
diffrantiating reaction. The diluted
sample solution is added to a 1ml of FeCI3
reagent.
1- CN- iron (III) cyanide will be formed form
dil. solution as a ppt. which is
dissolved in excess cyanide forming ferricyanide.
3CN-
Fe3 3 CN- Fe (CN)3

Fe(CN)63-Ferricyanide
2- SCN- This reaction is specific for iron(III)
and SCN- in the absence of other
interfering ions.
A cold acidic solution of SCN- is treated with
FeCI3 reagent, a blood red color is produced
which is extractable with ether. The formed
color is subjected to have the following
structures
Fe3 SCN- Fe(SCN) or Fe(SCN)3 or
Fe(SCN)63-
In order to increase the sensitivity of the test
the following precautions must be done

1. Ensure the presence of iron in the Fe3 state.

54
2- Acidification of the medium (dil. HCI is
preferable). 3- Cooling of the solution befor
testing. 4- Removal of intreferring ions by
precipitation or complexation.
F-, PO43- , oxalate and tartrate bleach the
colour, therefore it must be absent F- for e.g,
reacts with iron to form stable complex.
6 F- Fe3 (FeF6)3- other ions
which react with SCN- e.g, Hg2 which form
unionized Hg (SCN)2 which is colorless. Iodides
also interferes by being oxidized by Fe3 into
the brown colour I2.
2I- 2Fe3 H I2 2Fe2
3- Ferro and Ferricyanides
A Prussian blue characteristic ppt. is formed
form acidic solution of Fe(CN)64-, which is
insoluble in dil. HCI, but soluble in alkali
hydroxide.
3Fe(CN)6)4- 4Fe3 Fe4Fe(CN)63
Prussian blue
In case of Ferricyanide, a brown color is formed
of the non-ionised ferricyanide
Fe3 Fe(CN)63- FeFe(CN)6
Brown color
This test can be used to differentiate between
ferro and ferricyanide
55
d) Reaction with FeSO4 reagent
1- CN- Cyanide forms with FeSO4 solution a
yellow brown ppt. at first which is
then form ferrocyanide, this reaction is enhanced
by heating or addition of alkali.
2CN- Fe2 Fe(CN)2 4CN- Fe(CN)64-

2- SCN- No reaction.
3- Ferri and Ferrocyanide Ferricyanide forms
with FeSO4 reagent a similar blue ppt.
(turnbulls blue), as that of Prussian blue, but
differ in the distribution of iron-different
oxidation state is varied.
Fe(CN)63 Fe2 Fe3 Fe(CN)64-
Turanbull's blue Prussian blue
Ferrocyanide forms white ppt. of ferrous
ferrocyanide.
2KFe Fe(CN)64- K2FeFe(CN)6
56
e- Reaction with CuSO4
To the sample solution, add CuSO4 reagent
dropwise.
1- CN- In acidic medium, CN- likes I-, reacts
with Cu which oxidizes CN- into
cyanogens (CN)2 or cyanate CNO- (in alkaline
medium).
Cu 2CN- Cu(CN)2 ?
Greenish yellow
2CU (CN)2 Oxid-red Cu2 (CN)2 ? (CN)2
white cyanide
cyanogen
Cu2(CN)2 ? 4CN- 2 (Cu (CN)3)2-
Excess cuprocyanide complex
Soluble
As a conclusion of this reaction, cupric ions
react with excess cyanide to form soluble
complex cuprocyanide and cyanogen.
2Cu 8CN- 2Cu (CN)32- (CN)2
In alkaline medium cyanogen is converted to CN-
cyanate CNO-.
(CN)2 2OH- CN- CNO- H2O
57
2- SCN- Thiocyanate reacts with CuSO4 reagent,
to form a green color which
changes into a black ppt Cu (SCN)2 with excess
CuSO4 reagent Cu (SCN)2
decomposes gradually to white cuprous
thiocyanate Cu2(SCN)2 and separation of
thioyanogen as a gummy mass
Cu SCN- Cu (SCN)2
2 Cu (SCN)2 ? unstable ? Cu2 (SCN)2
(SCN)2 decomposition
white gummy mass
3- Ferro and Ferricyanides Both ferro and
ferricyanides form brown and green ppts. of
copper ferro and copper ferricyanides,
respectively. Both ppts. are insol. in dil. acids
Fe(CN)64- 2Cu Cu2Fe(CN)6
Brown
2 Fe(CN)63- 3Cu Cu3Fe(CN)62

green
58
f- Reaction with Cobalt Nitrate To the sample
solution, add excess Co(NO3)2 reagent.
1- CN- A buff ppt., of cabaltous cyanide
dihydrate is formed, which is soluble
in excess CN- to form soluble complex,
cobaltocyanide
4CN-
Co2 2CN- 2H2O Co (CN)2. 2H2O

Co (CN)64-
soluble complex.
2- SCN- Vogel's Reaction The reaction of Co
with SCN- to produce a characteristic blue color
extractable with ether or amyl alcohol known as
vogel's reaction. Other cyanogen anions form
precipitates with Co (NO3)2 reagent.
Co2 4SCN- Co (SCN)42-
Extractable with ether (blue)
3- Ferro and Ferricyanide Both form greyish
green and red ppts. of cobalt ferrocyanide
and cobalt ferricyanide.
2 Co2 Fe(CN)64- Co2Fe(CN)6

greyish green
3 Co2 2Fe(CN)63- Co3Fe(CN)62

red ppt.
59
III. Special Tests

• 1- For Cyanides
• Prussian blue test This test is specific for CN-
  which can be converted
• into
  ferrocyanide and allowed to react with Fe3.

b) Iron thiocyanate This test for CN- depends on
the direct combination of alkali cyanides with
sulphur (ammonium polysulphide). A blood red
coloration is produced upon addition of FeCI3
reagent. This blood red color is extractable
with ether. This test is applicable to CN- in
presence of S2- or SO32- if SCN- is originally
present, the CN- must be isolated first by
precipitation e.g. as zinc cyanide.

• 2- For thiocyanate
• Reduction Test This reaction depends on the
  reduction of SCN- with
• metallic zinc and dil. acid into H2S and HCN
  which can tested for.

Zno 2H 2 (H) Zn2
2SCN- 4(H) 2 ? HCN ? H2S S--
b) Vogels reaction
60
3- For ferrocyanides As mild reducing
agents It can be oxidized to ferricyanide by
oxidising agents, such as, MnO4-, NO3-, H2O2 and
CI2.
2Fe(CN)64- CI2 2Fe(CN)63-
2CI-
4- For Ferricyanides As oxidizing agents For
example, Fe(CN)63- can oxidizes I- into a brown
colored I2 which identified by starch or CHCI3.
2Fe(CN)63- 2I- 2Fe(CN)64-I2
IV. Analysis of Mixtures
1- Mixture of CN-, SCN-, Fe(CN)64- Fe
(CN)63-
CN- must be tested at first, then removed from
the mixture. This is done depending on its
strong affinity to protons, low ionization and
volatility of HCN.
61
The following procedure could be applied. a-
Passing CO2 in the mixture solution using acetic
acid or NaHCO3 and heat, until no more HCN
evolved which can be confirmed by i- Passing in
AgNO3 solution acidified dil. HNO3 which gives a
white ppt. ii- Passing in NaOH, adding FeSO4
solution heating, followed by HCI then FeCI3
solution (Prussian blue).
b- To the remaining solution, after removal of
CN-, acidify with dil. HCI, cool and add FeCI3
solution and centrifuge
Deep blue ppt.
Centrifugate
Fe (CN)64-
blood red color extractable with ether SCN-
brown solution
SnCI2
blue ppt .
Fe (CN)63-
62
2- Mixture of SCN-, CI-, Br- and I-
SCN- is tested for by reacting with FeCI3, to
give blood red color which is extractable with
ether and removed. In presence of I-, I2 is also
formed which can be extracted with CHCI3 (Violet
color). The blue complex formed with Co2 can
also be used to detect and remove SCN- by
extraction with ether or amyl alcohol. The
halides are tested for in the usual way after the
removal of SCN-, since it interferes with their
precipitation. After testing for SCN-, it is
removed by igniting the mixture till no more
blackening or no odor of burnt sulphur is
observed. The residue will contain only CI-,
Br-, I-, and test for CI- by chromyl chloride
test for I- and Br-, carry out chlorine water
test.
63
Arsinic and phosphorous containing anions
This group of anions, are 1- Arsenate (AsO43-)
2- Arsenite (AsO33-) 3- Phosphate (PO43-)
I. General characters
1- Parent Acids
a) Orthoarsenic acid H3AsO4
Its aqueous solution is a moderately strong acid,
slightly weaker than phosphoric acid. It has the
tendency for condensation and formation of
pyroarsenic acid, H4As2O7, and meta-arsenic
acid, HASO3 by gentle heating.
-H2O
-H2O
2H3AsO4
H4As2O7
2HAsO3
H2O
H2O
(Orthoarsenic acid) (Pyro arsenic acid)
(Meta arsenic acid)
64
Arsenic acid and arsenate ion are mild oxidizing
agents. Three series of arsenates exist, the
primary arsenate H2AsO4-, the secondary arsenate
(HAsO42-) and the tertiary arsenate (AsO43-).
b) Arseneous acid H3AsO3
It exist in aqueous solutions, cannot be isolated
as such because of thermal decomposition to the
anhydride, As2O3, sometimes written as As4O6.
The oxide is slightly soluble in water yielding
ortho arsenious acid and meta arsenious acid.
4HAsO2 4H2O
4H3AsO3
As4O6 6H2O
(ortho arsenious acid) (meta arsenious acid)
Two series of salts of arsenites exist,
orthoarsenites H2AsO3-, meta arsenites AsO2-,
both respond similarly to different reactions.
Arsin-containing acids and salts are highly
poisonous
65
Reduction of As5 and As3 Pentavalent arsenic
salts, or anions containing, can be reduced first
to the trivalent arsenous, or the corresponding
anion containing it, and finally to the metalic
form.
As5 2e
As3 3e
Aso ?
The reduction can be made using reducing agents
with lower redox-potential e.g. saturated
solution of stannous chloride, a powerful
reducing agent in the presence of conc. HCI.
As5 Sn2
Sn4 As3
(H)
2AS3 3Sn2
3Sn4 2Aso
(OH-)
c) Orthophosphoric acid H3PO4
It is crystalline solid, its aqueous solution is
acidic ionises into
H3PO4 H H2PO4- dihydrogen
phosphate H2PO4- H HPO42- monohydrogen
phosphate HPO42- H PO43- tribasic
phosphate
66
The intermolecular loss of water from two
molecules of orthophosphoric acid, will give
pyrophosphoric acid (H4P2O7) and metaphosphoric
acids (HPO3).
Orthophosphoric acid forms three series of salts
in which one, two or three hydrogens are replaced
by metals, for example, NaH2PO4, Na2HPO4 and
Na3PO4. these salts are known respectively as
primary, secondary and tertiary
orthophosphates.The aqueous solution of the
primary salt is acid, that of the secondary is
slightly alkaline while in the case of the
tertiary salt, the solution is strongly
alkaline.
2-Solubility
All their salts are insoluble in water except
those of Na, K and NH4 beside the alkali
dihydrogen salts as Ba(H2AsO4)2
3- Redox-reaction with I2/I- Aresnate has
oxidizing effect and aresnite has reducing effect
Arsenate (AsO43-) ions oxidises iodide into
iodine but the redox reaction is reversible due
to the narrow difference in Eo values of the two
redox systems.
67
H
AsO43- 2H 2I-
AsO33- H2O I2
NaHCO3
Arsenate oxidise iodide into iodine in acid
medium, while arsenite (mild reducing agent)
reduces iodine into iodide in alkaline medium.
II. General Reactions
1- Dry Reactions a- Action of dilute HCl
No visible reaction, since phosphates, arsenates
and arsenite acid are non volatile.
b- Action of conc. HCl 1- PO43- no visible
reaction
2- AsO43-On hot arsenate ion oxidises HCI into
free CI2, while it will be reduced
to arsenite
2CI- AsO43- 4H CI2 ? AsO2- 2H2O
68
3- AsO33- Arsenite will react and vapour of
arsenious chloride is evolved.
AsO2- 3CI- 4H AsCI3 ? 2H2O
c- Action of conc. H2SO4 1- PO43- and AsO43- no
visible reaction
2- AsO33- Arsenite on heating, some reduction
to SO2 may occur.
2- Wet Reactions
a- Silver nitrate solution
3Ag PO43-
Ag3PO4 ? (yellow ppt)
3Ag AsO43-
Ag3AsO4 ? (chocolate ppt.)
3Ag AsO33-
Ag3AsO3 ? (yellow ppt.)
All the precipitates are soluble in dil. HNO3 due
to the fact that the corresponding acids
(phosphoric, arsenic and aresnious acids) are
weaker than nitric acid in the presence of which
they yield lower concentration of their ions
insufficient to precipitate their silver salts
69
All the precipitates are soluble in ammonia
solution, due to the formation of the complex ion
Ag (NH3)2, which yields lower concentration
of silver ions insufficient to precipitate their
silver salts.
3Ag 6NH3
3Ag(NH3)2
These precipitates are insoluble in acetic acid.

b) Reaction with BaCI2
White precipitates of the secondary salt (BaHPO4,
BaHAsO4, BaHAsO3) from neutral medium, or of the
more insoluble tertiary salt (Ba3(PO4)2,
Ba3(ASO4)2 or Ba3(AsO3)2) from ammoniacal or
dilute alkaline solutions. The precipitates are
soluble in dilute acids including acetic acid.
c) Reaction with Magensia Mixture
Magnesia mixture reagent is formed of MgCI2,
NH4CI and NH4OH Mg2, the precipitating ions,
NH4OH, to render the medium ammoniacal NH4CI,
to reduce OH- concentration by common ion effect
to be insufficient to ppt. Mg (OH)2. The
reagent solution form white crystalline
precipitate with phosphates and arsenates in
neutral or ammoniacal solution. The precipitate
is soluble in acetic acid and in mineral acids.
No precipitate is formed with arsenites.
70
PO43-Mg2 NH4 Mg (NH4) PO4
magnesium ammonium phosphate
AsO43- Mg2 NH4 Mg(NH4)AsO4
magnesium ammonium
arsenate
If the white precipitates are treated with AgNO3
(in acetic acid medium), that of the phosphate
will be transformed into yellow ppt. while that
of the arsenate into chocolate ppt. due to the
transformation to the less soluble Ag3PO4 and
Ag3AsO4 respectively.
d) Reaction with ammonium molybdate
The addition of a large excess (2-3ml) of this
reagent in conc. HNO3 to a small volume (0.5ml)
of the test solution acidified with HNO3 and heat
gradually, produces a canary yellow crystalline
precipitates of ammonium phosphomolybdate
(NH4)3PO4. 12MoO3 (on warming to 40oC) and of
ammonium arsnomolybdate (NH4)3 AsO4. 12MoO3 (on
boiling) in case of phosphates and arsenates
respectively. No precipitate is formed with
arsenites. The precipitates are soluble in
ammonia or alkali hydroxides, in excess
phosphates or arsenates respectively and on
boiling with ammonium acetate solution, insoluble
in HNO3. MoO3 produced from the action of acid on
ammonium molybdate.
71
(MoO42-) 2H
H2MoO4
MoO3 H2O
3 NH4 12 MoO3 PO43-
(NH4)3PO4.12MoO3
3NH4 12 MoO3 AsO43-
(NH4)3ASO4.12MoO3
Chloride and reducing agents, such as S2-,
SO32-,Fe(CN)64- and tartarates, seriously
affect the reaction, and should be destroyed
before carrying out the test.
e) Reaction with H2S
Acidify the test solution with dilute HCI and
pass H2S. No precipitate is formed in case of
phosphate. Aresnites, produce immediate yellow
ppt. of arsenious sulphide As2S3. The ppt. is
soluble in HNO3 and alkali hydroxides insoluble
in hot conc. HCI.
2HAsO2 3H2S As2S3 4H2O
72
Arsenates, not produce any immediate visible
change, but after prolonged passage of H2S,
yellow ppt. of AS2S3 is produced. It is evident
that the first action of H2S is to reduce the
arsenate into arsenite through the formation of
thioarsenate ion H2AsO3S- which decomposes slowly
arsenious acid and suphhur.
H2AsO4- H2S H2AsO3S- H2O
H2AsO3S- H HAsO2 H2O S ?
2HAsO2 3H2S As2S3 4H2O
If the acid concentration is high and the strean
of H2S is rapid, no preliminary reduction to
arsenite occurs and arsenic pentasulphide precipit
ate (As2S5) is produced.
2H2AsO4- 5H2S 2H As2S5 ? 8H2O
However, if the solution is heated under the same
conditions, mixture of As2S3 and As2S5 is
formed.
73
f) Reaction with CuSO4 solution
Phosphates and arsenates form bluish-green ppt.
of the cupric phosphate or arsenate, CuHPO4, or
CuHAO4, respectively. On adding an excess of
NaOH, the ppt. assumes a pale blue color but
dose not dissolve, and on boiling no red ppt. is
produced. The ppt. is soluble in mineral acids
and in ammonia. Aresnites from yellowish green
ppt. of copper arsenite CuHAsO3 from the sample
solution just alkaline with NaOH. The ppt. is
soluble in excess NaOH to give deep blue color of
CuO.HAsO2. On boiling red ppt. is formed due to
the reduction of CuO into cuprous oxide (Cu2O),
the arsenious acid is simultaneously
partially-oxidised to arsenic acid.
Cu2 AsO2- OH- CuHAsO3CuO.HAsO2
2CuO.HAsO2H2O Cu2O ? H3AsO4HAsO2
g) Uranyl acetate solution
Light yellow, gelatinous precipitate of uranyl
ammonium phosphate Uo2(NH4) PO4 or arsenate UO2
(NH4) AsO4 in case of phosphates and arsenates
repectively, in the presence of exce