chem anions

Qualitative Inorganic Analysis

Analytical chemistry could be divided into three main parts;

qualitative, quantitative and applied.

Qualitative analysis deals with detection and identification of

different substances singly or in a mixture.

This part deals with the qualitative analysis of anions,

which could be defined as the negatively charged fragments

of salt or compound. Alternatively anions refer to acid radical.

An example is Nacl:

NaCl Na + + Cl-

Cation Anion

Basic radical Acid radical

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

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+ + HCO3- H+ + CO3

2-

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

This is a type of displacement reaction in which stronger acid

liberates the very weak carbonic acid, which spontaneously

decomposes to CO2 & H2O.

NaHCO3+ H+ CO2 + H2O + Na+

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

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

Ag2CO3+4NH3 2[Ag (NH3)2]+ + CO3

2-

The precipitate becomes yellow or brown if the mixture is boiled.

Ag2CO3 Ag2O +CO2

boiling

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

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

CO3

2- /HCO3- a white ppt. indicates CO3

-- , centrifuge or filter

Contrifugate White ppt.

May be HCO3- CaCO3

2-

H+

CO2 + H2O

Confirmatory test: 1) Boil

2) Add ammonia

solution white ppt.

Ca (HCO3)2 + 2 NH3 CaCO3+ (NH4)2 CO3

Sulphur-containing anions

This group of anions, are;

1- Sulphide (S2-)

2- Sulphites (SO32-)

3- Thiosulphate (S2O32-)

4- Sulphates (SO42-)

5- Perasulphate (S2O82-).


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.

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:

H2SO3 H++ HSO3- H++ SO3

-- H2O + SO2

heat

Acid sulphite

)3O2S2Thiosulphuric acid: (H -c

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 S2O3

2-.

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)

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

2-Solubility:

All Na+, K+ and NH4+ salts of sulphur containing anions are soluble

in water.

, ++soluble except those of Ca-Other sulphides are in Sulphides :

Ba++, & Sr2+ dissolve due to hydrolysis.

soluble.-Other sulphites are all in Sulphites:

salts ++& Ba +2, Hg++, Pb+are soluble, Ag-23O2Most S Thiosulphates:

are slightly soluble.

. ++and Sr ++, Ba++All sulphates are soluble except Pb Sulphates:

Ca++ & Mg++ salts are slightly soluble.

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.

I2+S2- 2I-+So

lodine (brown) Colourless

H+

2KMnO4+ 5S2-+ 16H+ 2Mn+++ 5SO4--+ 8H2O +2K+

I2+SO32-+H2O SO4

2-+2I-+2H+

2 MnO4-+ 5 SO3

--+ 6H+ 2Mn+++ 5SO4--+ 3H2O

Cr2O7--+ 3SO3

2-+ 8H+ 2Cr3++ 3SO4--+4H2O

I2+2S2O3-- H+ S4O6

2-+2I- Tetrathionate

Fe3++2S2O32-

S4O6

2-+Fe2+

H+

8MnO4-+ 5 S2O3

--+ 14H+ 8Mn+++10SO4--+7H2O

4Cr2O72-+ 3S2O3

2-+ 26H+ 8 Cr3++6SO4--+ 13 H2O


1- Dry Reactions


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

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

H2S+Pb++ PbS black

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

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.

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

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

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-

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.

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.

Pb+++S2O3-- 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+)

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 + CO3

2-

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.

2- Sulphite: SO32-

Zinc nitroprusside test :

Add to cold saturated ZnSO4 solution, equal volume of K4[Fe (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 Na5[Fe(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+

4- Sulphate: SO42-

Hepar’s 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

IV. Analysis of Mixtures

1- Mixture of S2-, SO32-, S2O3

2- 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-, S2O3

2- , & SO42-

Solution + CdCO3

Yellow ppt

S2- Centrifugate

+ BaCI2

Centrifugate

S2O32-

SO2 + So

HCl Heat

White ppt

BaSO3+BaSO4

HCl

White PPt

SO42-

Centrifugate SO32-

confirm by reducing character

2- Mixture of CO32- and SO3

2- 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 S2O3

2- 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

Halides


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.


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.

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.


1- Dry Reactions


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.

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. 2 F-+ H2SO4 2H F + SO4

--

4HF + SiO2 SiF4+ 2H2O

glass

3 SiF4+ 3H2O H2 SiO3+ 2 H2 SiF6

silicic acid hydrofluoro silicic acid

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

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-

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.

Ag++ CI- AgCI

AgCI + 2NH3 [Ag(NH3)2]CI

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)2]

3-+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

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(NH3)2]

+ Ag++ 2NH3

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-

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- [FeF6]3-

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.

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:

Iodide react first with chlorine water before bromide as it has more

reducing character.

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.

Chloride and Fluoride do not react with chlorine water .

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-

Br2+ CI2 2 BrCI (yellow)

Br2+ CI2 (excess) + 2H2O 2HOBr+2HCI

Br2+ 5CI2 (excess) + 6H2O 2 HBrO3+10HCI

bromine monochloride

hypobromous acid

bromic acid

Colorless

2I- + CI2 I2+ 2CI-

I2+ 5CI2 (excess) + 6H2O 2 HIO3+10HCI

iodic acid

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 4H3BO3+Na2SO4

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:

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

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)

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


1- Mixture of F-, Cl-, Br- and I- :

a) 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.

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-

Cyanogen anions


1- Cyanide (CN-)

2- Thiocyanate (SCN-)

3- Ferrocyanide [Fe(CN)6]4-

4- Ferricyanide [Fe(CN)6]3-

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.


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.

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.

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)

-

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)6]4- and [Fe(CN)6]

3- are considered to be stable complexes

from CN- ions. Also Co++ can form stable complexes with CN-.

Fe2++ 6 CN- [Fe(CN)6]4-

Fe3++ 6CN- [Fe(CN)6]

3-

When cyanides are heated with polysulphides (NH4)2Sx or thiosulphate (S2O3--)

they give thiocyanate ion

CN-+ (NH4)2Sx (NH4)2Sx-1+ SCN-

CN-+ S2O3

2- 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


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.

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

On heating with conc. H2SO4, CO will be evolve which burns with a blue

flame. SO2 is produced in case of ferrocyanide.

c) Ferrocyanide and Ferricyanide:

2Fe2++ 4H++ SO4-- SO2+ 2H2O + 2Fe3+

(Fe(CN)6)4-+ 6H2O +22H++ 10 SO4

2- Fe2++6NH4++ 10 HSO4

-+ 6 CO

(Fe(CN)6)3-+ 6H2O + 22H++ 10 SO4

2- 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

Ag++ CN- AgCN CN- (Ag(CN)2)-

H+ HCN+ AgCN

2- Ferro- and Ferricyanides:

Both [Fe(CN)6]4-and [Fe(CN)6]

3- react with AgNO3 solution with the

formation of a white ppt. and orange red ppt., respectively

4 Ag++ [Fe(CN)6]4- Ag4[Fe(CN)6]

Insoluble in dil. ammonia

Insoluble in dil. HNO3

3 Ag++ [Fe(CN)6]3- Ag3[Fe(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

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.

Fe3++ 3 CN- Fe (CN)3

[Fe(CN)6]3-

Ferricyanide

3CN-

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)6]3-

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.

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)6]4-, which is insoluble in dil. HCI, but soluble in alkali hydroxide.

3[Fe(CN)6)4-+ 4Fe3+ Fe4[Fe(CN)6]3

Prussian blue

In case of Ferricyanide, a brown color is formed of the non-ionised ferricyanide

Fe3++ [Fe(CN)6]3- Fe[Fe(CN)6] Brown color

This test can be used to differentiate between ferro and ferricyanide

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)6]4-

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)6]3++ Fe2+ Fe3++ [Fe(CN)6]

4-

Turanbull's blue Prussian blue

Ferrocyanide forms white ppt. of ferrous ferrocyanide.

2K++Fe+++ [Fe(CN)6]4- K2Fe[Fe(CN)6]

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- 2[Cu (CN)3]

2-+ (CN)2

In alkaline medium cyanogen is converted to CN- & cyanate CNO-.

(CN)2+ 2OH- CN-+ CNO-+ H2O

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)6]4-+ 2Cu++ Cu2[Fe(CN)6]

Brown

2 [Fe(CN)6]3-+ 3Cu++ Cu3[Fe(CN)6]2

green

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

Co2++ 2CN-+ 2H2O Co (CN)2. 2H2O

[Co (CN)6]4-

soluble complex

4CN-

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)4]2-

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)6]

4- Co2[Fe(CN)6]

greyish green 3 Co2++ 2[Fe(CN)6]

3- Co3[Fe(CN)6]2

red ppt.

III. Special Tests

1- For Cyanides:

a) 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:

a) 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) Vogel’s reaction

3- For ferrocyanides:

As mild reducing agents:

It can be oxidized to ferricyanide by oxidising agents, such as, MnO4-,

NO3-, H2O2 and CI2.

2[Fe(CN)6]4-+ CI2 2[Fe(CN)6]

3-+ 2CI-

4- For Ferricyanides:

As oxidizing agents:

For example, [Fe(CN)6]3- can oxidizes I- into a brown colored I2 which

identified by starch or CHCI3.


2[Fe(CN)6]3-+ 2I- 2[Fe(CN)6]

4-+I2

1- Mixture of CN-, SCN-, [Fe(CN)6]4- & [Fe (CN)6]

3-

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.

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.

[Fe (CN)6]4-

Centrifugate

blood red color

extractable with

ether SCN-

brown solution

SnCI2

blue ppt

[Fe (CN)6]3-

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.

Arsinic and phosphorous containing anions


1- Arsenate (AsO43-)

2- Arsenite (AsO33-)

3- Phosphate (PO43-)


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.

2H3AsO4 H4As2O7 2HAsO3

H2O

H2O

-H2O -H2O

(Orthoarsenic acid) (Pyro arsenic acid) (Meta arsenic acid)

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 (AsO4

3-).

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.

As4O6+ 6H2O

4H3AsO3

4HAsO2+ 4H2O

(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]**

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+

2AS3++ 3Sn2+ 3Sn4++ 2Aso (H+)

(OH-)

c) Orthophosphoric acid

H3PO4

It is crystalline solid, its aqueous solution is acidic & ionises into:

H3PO4 H++ H2PO4

- [dihydrogen phosphate]

H2PO4- H++ HPO4

2- [monohydrogen phosphate]

HPO42- H++ PO4

3- [tribasic phosphate]

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

AsO43- +2H++ 2I- AsO3

3- + H2O +I2 H

NaHCO3

Arsenate oxidise iodide into iodine in acid medium, while arsenite

(mild reducing agent) reduces iodine into iodide in alkaline medium.


1- Dry Reactions


No visible reaction, since phosphates, arsenates and arsenite acid are

non volatile.

b- Action of conc. HCl 1- PO4

3- : 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

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 AsO4

3-: 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

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 3[Ag(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.

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.

(MoO42-)+ 2H+ H2MoO4

3 NH4++ 12 MoO3+ PO4

3-

3NH4+ + 12 MoO3+ AsO4

3- (NH4)3ASO4.12MoO3

MoO3+ H2O

(NH4)3PO4.12MoO3

Chloride and reducing agents, such as S2-, SO32-,[Fe(CN)6]

4- 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

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

precipitate (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.

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- CuHAsO3=[CuO.HAsO2]

2[CuO.HAsO2]+H2O Cu2O + H3AsO4+HAsO2

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 excess ammonium acetate.

The precipitate is soluble in mineral acids, but insoluble in acetic acid.

This test provides an excellent method of distinction of phosphate and

arsenate from arsenite, which does not give a precipitate with the reagent.

PO43-+ UO2

2++ NH+4 UO2(NH4)PO4

AsO4

3-+ UO22++ NH4

+ UO2(NH4) AsO4

III. Special Tests

1- For phosphate:

Magnesium test

It depends on reduction of the stable phosphates into phosphide (P3-),

by mixing with magnesium powder and heat in an ignition tube. Moisten the

cold mass with water, phosphine gas (PH3) is produced which has

unpleasant odor and is inflammable.

PO43++ 4Mg (heat) 4MgO + P3-

P3-+ 3H2O PH3 + 3OH-

2- For arsenate: Potassium iodide test: To the test solution (2ml) add 1 ml of chloroform,

3ml KI solution and 5ml conc. HCI. Shake vigorously

and allow to settle, a violet color of free iodine

appears in the organic layer.

AsO43-+ 2I-+ 4H+ ASO2

-+ I2+ 2H2O HCO3

-

H

The test can be used for the detection of arsenate in presence of

phosphate and arsenite (in absence of other oxidizing agents).

3- For arsenite:

a) Iodine test:

Add 0.5 ml of saturated NaHCO3 solution to 3 ml of the sample solution.

Add few drops of I2 solution. The brown color of I2 disappears immediately

due to the reducing effect of arsenite. This reaction is the reverse of that

for arsenate. In absence of other reducing agents this test can be used to

distinguish arsenite from arsenate or phosphate.

b) Bettendorf's test:

A few drops of the test solution are added to 4ml of conc. HCI, and 1 ml

of saturated stannous chloride solution is added. The solution is gently

warmed; it becomes drak brown and finally black ppt. of arsenic is formed.

Strong reducing agents as SnCI2 reduce arsenite in presence of conc. HCI

to elemental arsenic.

3 Sn2++ 8H++ 2AsO2- (heat) 2As +3Sn4++ 4H2O

This test is also positive with arsenates, being first reduced into arsenites.

However, the test can be made use of to establish the presence of

arsenic-containing anions.

c) Marsh's reaction: [ for small amounts of arsenic.]

In acidic solution arsenic (III) and (V) compounds are reduced by

hydrogen to the poisonous hydrogen arsenide gas (H3As) with garlic like

odor which when heated dissociates to elementary arsenic and hydrogen:

AsO33-+ 3Zno+ 9H+ H3As + 3Zn2++ 3H2O

(heat) 2H3As 2Aso + 3H2


1- Mixture of arsenite and arsenate :

Ammoniacal solution of the mixture + magnesia mixture &

filter

White PPt.

Mg(NH4)AsO4

Wash with dil Ammonia

Solution+ AgNO3 acidified

With acetic acid

Chocolate brown

ppt. of Ag3AsO4

Aresnate

Filtrate

1- Acidify with dil. HCI & Pass H2S

immediate yellow ppt. of As2S3 Arsenite

(or) 2- Add 5-7ml of 30%H2O2 soI.+ magnesia

mixture drop by drop (10ml) with

stirring a white crystalline ppt.

of Mg(NH4) AsO4 produced by

Oxidation of arsenite.

(or) 3- Addition of NaHCO3 sat. sol.+few

drops of I2 sol. The brown colour of I2

disappears arsenite

2- Mixture of arsenite and phosphate:

1- With magnesia mixture ( as the mixture of AsO33- and AsO4

3-) with the

only exception that when the ppt. of Mg (NH4) PO4 treated with AgNO3

acidified with acetic acid, yellow ppt. of Ag3 PO4 is produced.

OR

2- Pass H2S in the solution of the mixture acidified with dil. HCI, immediate

yellow ppt. of As2S3 indicates AsO33- filter. Drive off the excess H2S by

boiling and test for phosphate by the general test (amm. molybdate).

3- Mixture of arsenate and phosphate:

Dissolve in conc. HCI (10 ml), boil, pass H2S for 5 minutes.

Dilute with 25 ml H2O & filter.

Yellow ppt.

of As2S5

Arsenate

Filtrate, evaporate to dryness, dissolve

in conc. HNO3

add ammonium molybdate

& warm canary yellow ppt.

Phosphate.

4- Mixture of arsenite, arsenate and phosphate:

Ammoniacal solution + magnesia mixture & filter

White ppt of

Mg (NH4) PO4

Mg (NH4) AsO4

Wash with dil ammonia

Solution. Dissolve in conc. HCI.

Boil & pass H2S; proceed exactly

As mixture of PO43- and AsO4

3-

Filtrate

test for AsO33-

as in mixture (1)

Nitrogen- containing anions


1- Nitrate (NO3-)

2- Nitrite (NO2-)


1- Parent Acids:

a) Nitric acid

HNO3

Colorless liquid (B.P. 83OC), decomposes on aging to nitrogen dioxide

(NO2). Its solution in water are strongly acidic.

4HNO3 4NO2 + O2 +2H2O

b) Nitrous acid

HNO2

The pure acid has never been isolated, due to its thermal instability.

2HNO2 NO + NO2 + H2O

However addition of a strong acid to a solid nitrite or its solution in the cold

yields a transient pale-blue liquid (due to the presence of free HNO2 acid or

its anhydride, N2O3) and the evolution of brown fumes of NO2.

2-Solubility: All nitrates are soluble in water. Also all nitrites are soluble in water

except AgNO2 which is slightly soluble

3- Redox-reaction : The nitrate ion contains in its highest oxidation state of + 5, thus reacts

only as oxidizing agent, while nitrite ion contains nitrogen which has

oxidation number + 3, it can therefore act either as a reducing or

as oxidizing agent.


1- Dry Reactions

a- Action of dilute HCl No reaction case of nitrates, with nitrites, brown fumes of nitrogen

dioxide NO2 evolve and a transient pale blue liquid.

2NO2-+ 2H+ 2HNO2 NO + NO2 +H2O

2NO + O2 2NO2

H+ ions from dil. acids (including acetic acid) displace nitrous acid from its

salts. The acid spontaneously decomposes to colorless monoxide NO &

brownish NO2 gases. The brown fumes intensify when getting in contact

the atmosphere due to combination of NO with O2 of air.

b- Action of conc. H2SO4

Nitrate: Nitric acid is formed and some of it decomposed with evolution

of brown fumes of NO2 with characteristic odor.

NO3-+ H+ HNO3

4HNO3 4NO2 + O2 + 2H2O

When copper turnings are added, and the mixture heated to boiling, the

brown fumes of NO2 are increased due to the reduction of HNO3 by Cuo metal

which is oxidized to Cu2+ ions, which imparts a blue color to the solution.

2NO3-+ 4H++ Cuo 2NO2 + Cu2++ 2H2O

Nitrite: The reaction is the same as with dil HCI, but it takes place with

considerable violence. On adding Cuo metal, the same occurs as

with nitrates.

2- Wet Reactions

a) Reaction with Ag2SO4 solution:-

Nitrate: No ppt.

Nitrite: White crystalline ppt. of AgNO2 form concentrated solutions.

NO2-+ Ag+ AgNO2

b) Reaction with BaCI2 solution:

No precipitate is formed with either NO3- nor NO2

-

c) Reaction with KI solution:

Acidify the test solution (3 ml) dil. H2SO4, then add Kl solution and few

drops starch solution.

Nitrate: No reaction.

Nitrite: I2 is liberated imparting blue color to the starch.

2NO2-+ 2I-+ 4H+ 2NO + I2+ 2H2O

d) Reaction with Fe SO4 solution. (Brown Ring Test):

Acidify the test solution (5ml) with dil. H2SO4, add (1ml) freshly prepared

FeSO4 solution.

Nitrate: No visible change in case of using only dil. H2SO4, but on adding

conc. H2SO4 cautiously down the sides of the test tube, a brown ring is

formed at the interface.

Nitrite: Brown colour in the whole solution if FeSO4 solution is not

cautiously added or a brown ring at the junction of the two liquids,

if cautiously added.

FeSO4 reduces nitrate or nitrite ions to nitrogen monoxide, NO; nitrate ion

is not reduced except in solutions containing a high H+ ion concentration,

that is conc. H2SO4. The excess Fe2+ ions then combines with the NO

produced to form the unstable brownish-black complex ion [Fe (NO)]2+,

readily decomposed by heat.

3Fe2++ NO3-+ 4H+ 3Fe3++ NO + 2H2O

Fe2++ NO2-+ 2H+ Fe3++ NO + H2O

Fe2++ NO [Fe (NO)]2+

This test differentiates NO3- ion from NO2

- ion, since the latter gives

the brown ring in presence of dil. H2SO4 or even acetic acid, while NO3-

ion dose not form the ring except in presence of conc. H2SO4.

(NO2-, I- and Br- ions will interfere)

III. Special Tests

1- For Nitrate:

Ammonia test

If solution of NO3

- is boiled with Zno or Alo metals and NaOH solution,

NH3 will be evolved which can identified by its odor or with red litmus

paper (nitrites interfere).

NO3-+ 4Zno+ 7 OH- 3NH3 + 4[ZnO2]

2-+ 2H2O

zincate ions

3NO3

-+ 8Alo+ 5OH-+ 2H2O 3NH3 + 8 [AlO2]-

In acidic solution ( CH3COOH), nitrate can be reduced with Zno to nitrite.

2- For Nitrite:

a) Permanganate test

When a dilute potassium permanganate solution is added to an acid

solution of nitrite, its pink color is bleached. In this test, the

permanganate is reduced by the nitrite into colorless manganous

Salt and the nitrite is oxidized into nitrate.

2MnO4-+ 5NO2

-+ 6H+ 2Mn2++ 5NO3-+ 3H2O

Pink colorless

b) Urea test: When a solution of a nitrite is treated with urea and the mixture

acidified with dilute HCl, the nitrite is decomposed, and N2 and CO2 are

evolved. CO (NH2)2+ 2HNO2 2N2 + CO2 +3H2O

c) Ammonium Chloride test:

By boiling a solution of a nitrite with excess of the solid reagent,

N2 is evoled and the nitrite is completely destroyed.

NO2-+ NH+

4 N2 + 2H2O

d) Thiourea test:

When a dil. acetic acid solution of a nitrite is treated with a little thiourea,

N2 is evolved and thiocyanic acid is produced. The latter may be identified

by the red color produced with dil. HCl and FeCl3 solution.

CS (NH2)2+ HNO2 N2+ H++ CNS-+ 2H2O

N.B: Thiocyanates and iodides interfere, and if present must be removed

either with Ag2SO4 (solid) or dil AgNO3 solution.


1- Mixture of Nitrate and Nitrite : Nitrite can be tested for in presence of nitrate (by treatment with dil HCI,

KI, KMnO4, FeSO4 in dil. H2SO4); and by the special tests for nitrite.

Nitrate cannot be tested for in presence of nitrite, since nitrite gives all

the reactions of nitrate (conc. H2SO4, brown-ring test and ammonia test).

Therefore nitrite be removed before testing for nitrate by:-

1- Decomposition of NO2- through its brown complex with FeSO4 formed in

dil. H2SO4 or acetic acid by heat and shaking.

[Fe (NO)]2+ heat NO + Fe2+

2- Decomposition of NO2- through its reduction to nitrogen by boiling with

NH4CI or warming with urea and few drops of dilute H2SO4 or warming with

little sulphamic acid.

HO.SO2. NH2+ HNO2 N2 + H2SO4+ H2O

2- Mixture of Nitrate and Bromide or / and lodide:

Br- and I- can be detected in presence of NO3- by chlorine water test.

NO3- can be detected in presence of Br- and I- by the ammonia test.

On the other, the brown ring test for nitrates cannot be applied in the

presence of Br- and I-, since the liberation of free halogen with conc. H2SO4

will obscure the brown ring due to NO3-

Therefore Br- and I- must be first removed by either:

1- Addition of saturated solution of silver sulphate, where AgBr and AgI

are precipitated and then filtered off, the excess Ag+ is precipitated

with Na2CO3

OR 2- Adding potassium persulphate and dil. H2SO4 and warming to about

80oC. The halogen is removed by boiling or extraction with organic solvent.

S2O82-+ 2Br- Br2+ 2SO4

2-

S2O82-+ 2I- I2 + 2SO4

2-

The Halide-free solution is tested for NO3-

by the brown ring test

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