Qualitative Inorganic Analysis - Weeblyhishamezzat.weebly.com/uploads/9/0/6/0/9060375/final_anion2012_1.pdf · Qualitative Inorganic Analysis Anions are divided into six groups: 1-

اسرة فرسان صيدلة pharmaza اسرة فرسان صيدلة

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

1. Carbonates and Bicarbonates group, CO32-, HCO3

-

CO32- HCO3

-

Parent acid Carbonic acid (H2CO3) is a very weak 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

H2CO3 ⇋ H+ + HCO3- ⇋ H+ + CO3

2-

Solubility Insoluble in water except (Na+, K+, NH4+) Soluble in water

1- Dry Reactions

a) dilute HCl

Decomposition with effervescence due to the evolution of CO2 gas, for

both CO3 2- and HCO3

-

CO3-- + 2H+ ⇋ CO2 + H2O

NaHCO3+ H+ ⇋ CO2 + H2O + Na+ # (Test for CO2 gas)

b) sulphuric acid

As HCl, eff. In addition the formation insoluble sulphate (with gp v metals)

BaCO3 +H2SO4 ⇋ ↓BaSO4 + ↑SO2 + H2O

2- wet reaction

a- with AgNO3

A white precipitate of silver carbonate is immediately formed.

CO3 -- +2Ag+ Ag2CO3

The ppt. is soluble in mineral acids (nitric acid) and in ammonia.

Ag2CO3 + 2H+ 2 Ag+ + CO2 + H2O

Ag2CO3+4NH3 2[Ag (NH3)2]+ + CO32-

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

Ag2CO3 𝐁𝐨𝐢𝐥𝐢𝐧𝐠 → Ag2O +CO2

b- with BaCl2,

CaCl2 and MgSO4:

White precipitates of BaCO3,

CaCO3 and MgCO3 will be obtained

with carbonate solution.

BaCl2 + NaCO3 BaCO3 + 2 NaCl

Ca++ + CO3 -- CaCO3

Mg++ + CO3 -- MgCO3

The precipitate is soluble in mineral

acids

No ppt. on cold since all HCO3- are

soluble in water.

III. Mixture of CO32-, HCO3

-: 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.


2

Separation: Add excess CaCl2 (BaCl2 or MgSO4) to a solution of the mixture CO32- /HCO3

- a white

ppt. indicates CO3--

, centrifuge or filter

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

# 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 turbidity indicates the presence of carbonates or bicarbonates.

CO2 + Ca(OH)

2 CaCO

3 + H

2O

CO2 + Ba (OH)

2 BaCO

3 + H

2O

With prolonged passage of CO2, the turbidity formed due to the insoluble carbonates, slowly

disappears as a result of the formation of soluble bicarbonate.

CaCO3 + CO

2 + H

2O

𝐁𝐨𝐢𝐥𝐢𝐧𝐠 → Ca (HCO3)2

Sulphur-containing anions This group of anions are;

1- Sulphide (S2-)

2- Sulphites (SO32-)

3- Thiosulphate (S2O32-)

4- Sulphates (SO42-)

5- Perasulphate (S2O82-).


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2. Sulphur-containing anions

Sulphide (S2-) Sulphites (SO32-) Thiosulphate (S2O3

2-) Sulphates (SO42-)

Parent acid Hydrogren sulphide or

Hydrosulphuric acid (H2S)

Gas with offensive rotten

egg odour and poisonous.

In solution it gives a weak

acid, which ionizes in two

steps;

H2S ⇋ H++ HS-

HS- ⇋ H++ S--

Sulphurous acid:(H2SO3)

H2O + SO2 𝐇𝐞𝐚𝐭 → H

2SO

3 ⇋

H++ HSO3

- ⇋ H

++ SO3

--

Thiosulphuric acid:

(H2S2O3)

Decomposes to give, H2O,

SO2 and S.

Sulphuric acid: (H2SO4)

(general properties #)

Reducing

agent:

Sulphides, sulphites and

thiosulphates are reducing

agents. They reduce

solutions of I2, KMnO4 and

K2Cr2O7

I2+S2- → 2I-+So

lodine (brown) Colourless

2KMnO4+ 5S2-+ 16H+ →

2Mn+++ 5 SO4--+ 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+

8MnO4-+ 5 S2O3

--+ 14H+

→8Mn+++10SO4--+7H2O

4Cr2O72-+ 3S2O3

2-+ 26H+ →

8 Cr3++6SO4--+ 13 H2O

------

1- Dry

Reactions

a- Action of

dilute HCl

H2S gas; evolved has rotten

egg odour,

1- blackening of filter paper

moistened with lead acetate

sol

S-- + 2H+ → H2S↑

H2S+Pb++ →PbS ↓ black

SO2 gas evolved has bunt

sulphur odor and turbid lime

water due to the formation

of the insoluble CaSO3

Ca (OH)2 +SO2 ⇋ CaSO3 +

H2O

SO2 has reducing character,

bleaches the brown color of

The solution becomes turbid

due to the liberated yellow

colloidal sulphur with

evolution of SO2 gas.

S2O3-- + 2H+

⇋ H2S2O3 ⇋

H2O + SO2 + So↓

No reaction with dil. HCl


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2- turns filter paper

moistened with cadmium

acetate solution, yellow

H2S + Cd++ →CdS ↓ Yellow

It bleaches the brown color

of l2 solution, changes the

pink color of acid KMnO4 into

colorless and changes the

orange color of acid K2Cr2O7

into green.

H2S + l2 ⇋ 2l- + 2H+ +So

5H2S + 6H+ +2 MnO4- ⇋

2Mn++ + 8H2O + 5So

3H2S + 8H+ + Cr2O7-- ⇋ 2Cr3+

+ 7H2O + 3So

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

Complexing agent:

Thiosulphate form

complex with Fe3+

Fe3++ 2S2O3-- → (Fe(S2O3)2)-

purple color

2- Wet

Reactions

Reaction

with BaCl2:

No visible reaction White ppt. of BaSO3 is

formed which is soluble in dil.

HCl.

Ba+++SO32- =BaSO3

No ppt. in dilute solution, but

a ppt. is formed from very

concentrated solution

A white ppt. of BaSO4 is

formed which is insoluble in

dil. HCl, even upon boiling.

Ba+++ SO4-- = BaSO4 ↓ White

Reaction

with AgNO3

a black ppt. of Ag2S soluble

in hot dil. HNO3, insoluble in

ammonia an

2 Ag++ S-- ⇋ Ag2S black

A white crystalline ppt. of

Ag2SO3, which on boiling with

water undergoes self redox

with the production of grey

ppt. of metallic silver.

2 Ag++ SO32-⇋ Ag2SO3

2 Ag2SO3 boil→2 Ago +

Ag2SO4 ↓ + SO2 ↑

White ppt. which changes its

color on standing to yellow,

brown and finally black, due

to the formation of Ag2S.

2 Ag+ + S2O3--⇋ Ag2 S2O3

Ag2S2O3+ H2O ⇋ Ag2S +

H2SO4

No ppt. in dil solution, but a

ppt. may be formed in a very

concentrated solution.

Reaction

with FeCl3

a black ppt. of Fe2S3 is

formed which is soluble in

dil. HNO3

2Fe3++ 3S-- → Fe2S3

A drak red color of ferric

sulphite is produced on cold.

2Fe3++ SO3-- → Fe2(SO3)3

A purple color of complex

disappears on boiling

Fe3++ 2S2O32- → (Fe(S2O3)2)-

2 S2O3--+ 2Fe3+ ⇋ 2Fe+++

S4O6--

do not react with FeCl3


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Reaction

with lead

acetate:

A black ppt. of PbS is

produced

Pb+++ S-- → PbS

A with ppt. soluble in cold

HNO3. On boiling oxidation

to PbSO4.

SO3--+ Pb++ →PbSO3

A white ppt. is soluble in cold

HNO3, on boiling a black ppt.

of PbS is formed.

Pb+++S2O3-- →PbS2O3

A white ppt. lead suphate,

which is insoluble in cold dil.

mineral acids, but soluble in

ammonium acetate and

hydroxide solutions

Special

Tests

Cadmium carbonate test

S--+ CdCO3 → CdS + CO32-

Canary yellow ppt.

Zinc nitroprusside test :

Add ZnSO4 + K4[Fe (CN)6] +

1% sodium nitroprusside

solution. salmon-colored ppt.

of zinc nitroprusside

is formed Zn (Fe(CN)5 NO).

Formation of thiocyanate :

By boiling with KCN + NaOH,

Cool, acidify and add FeCI3, a

blood red color of ferric

thiocyanate complex

is produced.

S2O3--+ CN- OH- →SCN-+ SO3

-

Fe3++ SCN- Cool →Fe(SCN)2+

Hepar’s test

Sulpate is reduced by

carbon to sulphide

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


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# 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 𝐇𝐞𝐚𝐭 → H2O + SO2 + [O]

It is reduced to SO2, while with active reducing agents it may be reduced to So or H2S.

2. 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


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3. Halides

Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-) Therefore the order of stronger halogen acid is from HI HBr HCl HF.

Parent Acids: HF:coloress fuming highly

corrosive and itching liquid

HCl Colorless gas with

irritating odor, fumes in moist

air,

HBr Colorless gas with

irritating odor, fumes in

moist air

HI Colorless gas

with irritating

odor, fumes

strongly in moist

air,

Dry Reactions

a- dilute HCl

Hydrochloric acid shows no reaction .This reaction can differentiate carbonate and sulphur group from halides.

b- conc. H2SO4 2X-+ H2SO4 = 2 HX + SO42- X = may be CI-, I-, Br- and F-

HF colorless fumes, the glass

rod acquire oily appearance

due to the formation of silicic

acid and hydrofluorosilicic

acid.

2 F-+ H2SO4 → 2H F + SO4—

4HF+SiO2→SiF4+ 2H2O

3 SiF4+ 3H2O → H2 SiO3+ 2 H2

SiF6

silicic acid & hydrofluoro

silicic acid

HCI gas is evolved

2CI-+H2SO4 → 2 HCI + SO4--

identified by :

Formation of white fumes of

NH4CI when a glass rod

moistened with ammonium

hydroxide

NH4OH + HCI →

NH4CI + H2O

A mixture of HBr and

Br2 have characteristic

brown color

2 Br-+ H2SO4 → 2 HBr +

SO4--

2 HBr + H2SO4

→ Br2 + SO2+ 2 H2O

I2 which appears as

violet fumes. I2 can

be detected by

exposing the evolved

gas to paper

moistened with

starch solution, it

changes into blue.

2I-+ H2SO4 →2 HI +

SO42-

2HI + H2SO4 → I2 +

SO2 + 2H2O

Conc.H2SO4 & MnO2: 2X- + 4H++ MnO2 ⇋ Mn+++ 2H2O +X2 X = may be CI-, Br- and I-

The free halogen, (X2) could be detected by: 1- Bleaching of a moistened colored litmus paper.

2- Suffocating, and irritating odor. 3- Characteristic color of Br2 (brown), I2 (violet) and CI2 gas

4- I2 changes starch paper into blue, Br2 turns it orange. 5- CI2 and Br2 change a starch – KI into blue due to

the oxidation of I- to I2 produce a blue adsorption complex.

CI2+ 2KI → 2KCI + I2 Br2+ 2KI → 2KBr + I2


8

2- Wet Reactions

a- Reaction with

AgNO3:

No

precipitate

A white curdy ppt. insoluble in

nitric acid, soluble in KCN and

dil. ammonia solution to give the

ammine complex.

AgCI + 2NH3 → [Ag(NH3)2]CI

Silver ammine chloride

A curdy, pale yellow

precipitate , sparingly

soluble in dilute, but

readily soluble in conc.

ammonia solution

AgBr + 2 NH3 →

[Ag(NH3)2]+ + Br-

A curdy yellow ppt.

insoluble in dil. ammonia

but very slightly soluble

in conc. ammonia

solution.

Ag++ I - → AgI

b- Reaction with BaCI2

solution:

The white gelatinous

BaF2 ppt. is partially

soluble in dil. HCI or

HNO3

Ba+++ 2F- → BaF2

No ppt. is formed

c- Reaction with

FeCI3:

a white crystalline ppt.

of the complex salt,

which is sparingly

soluble in water

Fe3++ 6 F- → [FeF6]3-

CI -and Br -: do not react with FeCI3 Reacts with FeCI3, due

to its strong reducing

action with the

liberation of I2.

Reaction with lead

acetate

F-, Cl- and Br- form a white ppt with lead acetate, sparingly soluble in cold more soluble

in hot water, crystallize on cooling

forms a bright yellow

ppt of PbI2 which is

soluble in hot water

and crystallizes on

cooling as golden

spangles

Chlorine water test Chloride and Fluoride do not react with chlorine water 2Br-+ CI2 → Br2+ 2CI-

Br2+ CI2 →2 BrCI (yellow)

Br2+ CI2 (excess) + 2H2O

→2HOBr+2HCI Colorless

2I- + CI2 → I2+ 2CI-

I2+ 5CI2 (excess) +

6H2O → 2

HIO3+10HCI


9

Special Tests Boron fluoride test:

fluoride + borax and conc.

H2SO4.→ boronfluoride gas.

Na2B4O7 + H2SO4+ 5H2O →

4H3BO3 +Na2SO4

2NaF + H2SO4 → 2HF +

Na2SO4 H3BO3+3HF→ BF3+

3H2O

Chromyl chloride test #:

The solid chloride + powder

potassium dichromate in a tube, +

conc. sulphuric acid. The deep red

vapors of chromyl chloride CrO2CI2

are passed into sodium hydroxide

solution.→

Yellow color;

4CI-+ Cr2O7--+ 6H+ cond.→ 2CrO2 Cl2

+ 3H2O

CrO2CI2 + 4OH- → CrO4-- + 2CI-

+ 2H2O

I-

A) 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) reacts with Cu++ forming a whit ppt.

of Cu2I2,

2Cu+++4I- ⇋ Cu2I2 +I2

# N.B.(for Chromyl chloride test)

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)


10

Pharmaza ------ اسرة فرسان صيدلة

11

4. Cyanogen anions

Cyanide (CN-) Thiocyanate (SCN-)

Ferrocyanide

[Fe(CN)6]4-

Ferricyanide

[Fe(CN)6]3- 1-Parent Acids: Hydrocyanic acid :HCN has an odor of

bitter almonds.

On passing CO2 to CN- solution HCN is

produced with HCO3-

CN-+ CO2+ H2O → HCN + HCO3-

Thiocyanic acid: HSCN colorless

toxic liquid

On standing its aqueous solution is

decomposed to HCN and yellow

solid polymer.

3 HCNS → HCN + H2N2C2S3

H4[FeCN)6] white

crystalline solid.

H3 [Fe(CN)6]

browinish

crystalline solid.

Complexing

agent:

Cyanide ion has strong tendency to the formation of complexes which

1- Argentocyanide complexes: Double cyanides

CN- + Ag+, at first white turbidity is formed which is AgCN, 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.

Fe2++ 6 CN- → [Fe(CN)6]4- and 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-+ S2O32- → SO3

--+ SCN-

4-Oxidizing

agent:

Ferricyanides has oxidizing effect, they can oxidizes I- into I2


12

5-Reducing

agent:

Ferrocyanides has mild reducing effect, they can be oxidized to ferricyanide by oxidizing agents, such as MnO4-, NO3

-

, H2O2 and Cl2

2. Dry

Reactions

a- dilute HCl

HCN gas evolved with characteristic bitter almond odor and can be tested:

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.

No

reaction

as SCN-

is as

strong

as HCl

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)

conc. H2SO4: decomposed on

heating

CN- +2H++ H2O →

NH4+ +CO

SCN-+ 4H++ 2SO4--+

H2O → NH4++ 2HSO4

- +

COS Carbonyl Sulphide

On heating, 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- → Fe2++6NH4

++ 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:

CN- & SCN- : form white ppts. 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

Both [Fe(CN)6]4-and [Fe(CN)6]3- form white ppt. and orange red ppt.,

respectively

4 Ag++ [Fe(CN)6]4- → Ag4[Fe(CN)6]

Insoluble in dil. Ammonia and dil. HNO3

3 Ag++ [Fe(CN)6]3- → Ag3[Fe(CN)6] Orange red ppt.

Insoluble in dil. HNO3 and Soluble in dil. Ammonia


13

reaction with

FeCI3:

iron (III) cyanide dissolved in excess

cyanide forming ferricyanide.

Fe3++ 3CN- → Fe (CN)33CN-→

[Fe(CN)6]3- . Ferricyanide

blood red color

Fe3++ SCN- →

[Fe(SCN)]++

or Fe(SCN)3

or [Fe(SCN)6]3-

Prussian blue ppt.

insoluble in dil. HCI, but

soluble in NaOH

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

→ Fe4[Fe(CN)6] Prussian

blue

brown color is formed

Fe3++ [Fe(CN)6]3- →

Fe[Fe(CN)6] Brown color

This test used to differentiate

between ferro and

ferricyanide

d) Reaction with

FeSO4 reagent:

yellow brown ppt. at first which is

then form ferrocyanide,

2CN-+ Fe2+ →Fe(CN)2 4CN-→

[Fe(CN)6]4-

No reaction.

[Fe(CN)6]3++Fe2+ → Fe3++ [Fe(CN)6]4-

Turanbull's blue Prussian blue

Ferrocyanide forms white ppt.

2K++Fe+++ [Fe(CN)6]4- → K2Fe[Fe(CN)6]

Reaction with

CuSO4

form soluble complex cuprocyanide and cyanogen in acid

medium

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

Cu+++ SCN- →Cu (SCN)2 green

2 Cu (SCN)2 unstable → Cu2 (SCN)2+ (SCN)2

decomposition white gummy mass

[Fe(CN)6]4-+ 2Cu++ → Cu2[Fe(CN)6]

Brown

2 [Fe(CN)6]3-+ 3Cu++ → Cu3[Fe(CN)6]2

green

Reaction with

Cobalt Nitrate:

Co2++ 2CN-+ 2H2O → Co (CN)2.

2H2O 4CN-→ [Co

(CN)6]4- soluble

complex.

Vogel's Reaction

characteristic blue color

extractable with ether or amyl alcohol

Co2++ 4SCN- →[Co (SCN)4]2-

Extractable with ether (blue)

2Co2++ [Fe(CN)6]4- → Co2[Fe(CN)6]

greyish green

3 Co2++ 2[Fe(CN)6]3- → Co3[Fe(CN)6]2

red ppt.

Special Tests

Prussian blue test Vogel's Reaction

As mild reducing agents

2[Fe(CN)6]4-+ CI2 →

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

As oxidizing agents

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

→ 2[Fe(CN)6]4-+I2


14

IV. Analysis of Mixtures

1- Mixture of CN-, SCN-, [Fe(CN)6]4- & [Fe (CN)6]3-

a- Passing CO2 in the mixture solution using acetic acid or NaHCO3

and heat, until no more HCN evolved.

To the remaining solution, after removal of CN-, acidify with dil.

HCI, cool and add FeCI3 solution and centrifuge

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.


15

5. Arsinic and phosphorous containing anions

Arsenate (AsO43-) Arsenite (AsO3

3-) Phosphate (PO43-)

1- Parent Acids: Orthoarsenic acid :H3AsO4 Arseneous acid :H3AsO3 Orthophosphoric acid :H3PO4

1- Dry Reactions

a- dilute HCl

On hot arsenate ion oxidises HCI into free

CI2, while it will be reduced to arsenite

2CI-+ AsO43-+ 4H+ ⇋ CI2 +AsO2

- + 2H2O

Arsenite will react and vapour of

arsenious chloride is evolved

AsO2-+ 3CI- + 4H+ ⇋ AsCI3 + 2H2O

no visible reaction

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.

2- Wet Reactions

a- Silver nitrate

solution:

All the precipitates are soluble in ammonia

solution, due to the formation of the complex ion

[Ag (NH3)2]+,

Reaction with

Magensia Mixture:

reagent is formed

of MgCI2, NH4CI

and NH4OH

The reagent solution form white crystallineprecipitate 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]


16

Reaction with

ammonium

molybdate

reagent + conc. HNO3 + test solution heat gradually, = 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.

Reaction with H2S yellow ppt. of arsenious sulphide

As2S3.

The ppt. is soluble in HNO3 and alkali

hydroxides insoluble in hot conc. HCI.

2H2AsO4-+ 5H2S +2H+ → As2S5

+8H2O

not immediate, but after prolonged

passage of H2S, yellow ppt. of AS2S3 is produced.

No precipitate

Reaction with

CuSO4 solution:

green ppt. of the cupric arsenate, or

CuHAO4,

The ppt. is soluble in mineral acids and

in ammonia.

yellowish green ppt. of copper arsenite CuHAsO3

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

Bluish ppt of

CuHPO4,

The ppt. is soluble in

mineral acids and in

ammonia.

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


17

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

Special Tests

Potassium iodide test

a) Bettendorf's test:

test solution + 4ml of conc. HCI, and 1 ml of

stannous chloride = dark brown and finally black

ppt. of arsenic is formed.

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

b) Iodine test:

NaHCO3 solution + sample solution.+ few drops

of I2 solution. The brown color of I2 disappears

immediately due to the reducing effect of

arsenite. this test can be used to distinguish

arsenite from arsenate or phosphate.

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:

Magnesium test:

It depends on reduction

of the stable phosphates

into phosphide (P3-),

PO43++ 4Mg (heat) →

4MgO + P3-

P3-+ 3H2O → PH3 +

3OH-


18


19

6. Nitrogen- containing anions

Nitrate (NO3-) Nitrite (NO2

-)

Parent Acids Nitric acid :HNO3 decomposes (NO2).

4HNO3 → 4NO2 + O2 +2H2O

Nitrous acid :HNO2

2HNO2 → NO + NO2 + H2O

Dry Reactions

a- Action of dilute

HCl

No reaction 2NO2-+ 2H+ → 2HNO2 →NO + NO2 +H2O

2NO + O2 → 2NO2

Action of conc. H2SO4 Nitric acid is formed and some of it decomposed with evolution of brown

fumes of NO2.

NO3-+ H+ → HNO3

4HNO3 → 4NO2 + O2 + 2H2O

As in dil HCl

Wet Reactions

with Ag2SO4 solution

No ppt.

White crystalline ppt. of AgNO2.

NO2-+ Ag+ → AgNO2

with KI solution: No reaction I2 is liberated give blue color to the starch.

2NO2-+ 2I-+ 4H+ → 2NO + I2+ 2H2O

with Fe SO4 solution. (Brown Ring Test):Acidify the test solution (5ml) with dil. H2SO4, add (1ml) freshly prepared

FeSO4 solution. 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)

3Fe2++ NO3-+ 4H+ → 3Fe3++ NO + 2H2O Fe2++ NO2

-+ 2H+ → Fe3++ NO + H2O Fe2++ NO → [Fe (NO)]2+

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).

1- Permanganate test:

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

2- Urea test:

CO (NH2)2+ 2HNO2 → 2N2 + CO2 +3H2O


20

IV. Analysis of Mixtures

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

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Qualitative Inorganic Analysis - Weeblyhishamezzat.weebly.com/uploads/9/0/6/0/9060375/final_anion2012_1.pdf · Qualitative Inorganic Analysis Anions are divided into six groups: 1-