Flame Test

Classification Of Anions

Methods available for the detection of anions are not as systematic as those used for the detection of cations. Furthermore anions are classified essentially on the basis of process employed.

Class A: Includes anions that are identified by volatile products obtained on treatment with acids.It is further divided into two sub groups.

(i) Gases evolved with dil HCl/ dil H₂SO₄.

(ii) Gases or acid vapours evolved with conc H₂SO₄

Class B: Includes anions that are identified by their reactions in solution. It is subdivided into two groups: (i) Precipitation reactions

(ii) Oxidation and reduction in solution

Class A (i): Anions which evolve gases on reaction with dil. HCl/dil. H₂SO₄.

It includes - CO₃2-_{, SO} 3 2-_{, S}2-_{, NO} 2 -_{, CH} 3COO -_{, S} 2O3 2- 1. Carbonate (CO₃2- _{) :}

(i) Dilute HCl : gives effervescence, due to the evolution of carbon dioxide CO₃2- _{+ 2H}+ _{ CO}

2 + H2O

The gas gives white turbidity with lime water and baryta water CO₂ + Ca2+ _{+ 2OH}- _{ CaCO}

3  + H2O

CO₂ + Ba2+ _{+ 2OH}- _{ BaCO}

3  + H2O

On prolonged passage of carbon dioxide in lime water, the turbidity slowly disappears due to the formation of soluble hydrogen carbonate.

CaCO₃  + CO₂ + H₂O  Ca(HCO₃)₂

The following tests performed with then aqueous salts solution.

(ii) Barium chloride or Calcium chloride solution: White ppt of barium or Calcium carbonate is obtained, which is soluble in mineral acid.

CO₃2- _{+ Ba}2+ _{ BaCO} 3 

CO₃2- _{+ Ca}2+ _{ CaCO} 3 

(iii) Silver nitrate solution : White ppt of silver carbonate is obtained. CO₃2- _{+ 2Ag}+ _{ Ag}

2CO3

The ppt so obtained is soluble in nitric acid and in ammonia, the ppt becomes yellow or brown on addition of excess reagent and same may also be happened if the mix is boiled, due to the formation of silver oxide

Ag₂CO₃  Ag₂O  + CO₂  2. Sulphites (SO₃2-_):

(i) Dilute HCl or Dilute H₂SO₄ : decomposes with the evolution of sulphur dioxide

SO₃2- _{+ 2H}+ _{ SO}

2 + H2O

The gas has a suffocating odour of burning sulphur.

(ii) Acidified potassium dichromate solution: The gas turns filter paper moistened with acidified potassium dichromate solution, green due to the formation of Cr3+_ions.

SO₂ + K₂Cr₂O₇ + H₂SO₄  K₂SO₄ + Cr₂(SO₄)₃ + H₂O green

(iii) Lime water : On passing the gas through lime water, a milky ppt is formed. SO₂ + Ca(OH)₂  CaSO₃  + H₂O

milky

Precipitate dissolves on prolonged passage of the gas, due to the formation of soluble hydrogen sulphite ions.

(iv) Barium chloride or Strontium chloride solution : Salt solutions gives white ppt of barium or strontium sulphite.

SO

₃2

Ba

2

BaSO

3  







SO

3

Sr

SrSO

2 2 3  







3. Sulphide (S-2_{) :}

(i) Dil HCl or Dil H₂SO₄ : A colourless gas with a smell of rotten eggs (H₂S) is evolved S2- _{+ 2H}+ _{ H}

2S

(ii) The gas turns lead acetate paper black

(CH₃COO)₂Pb + H₂S  PbS  + 2CH₃COOH black

(iii) Salt solution gives yellow ppt. with CdCl₂ Na₂S + CdCl₂  CdS + 2NaCl

yellow

(iv) Silver nitrate solution : black ppt. of silver sulphide insoluble in cold but soluble in hot dil nitric acid.

S2- _{+ 2Ag}+ _{ Ag} 2S 

(v) Sodium nitroprusside solution : Turns sodium nitroprusside solution purple Na₂S + Na₂[Fe(CN)₅NO]  Na₄[Fe(CN)₅NOS]

4. Nitrites (NO₂-_{) :}

(i) Dil HCl and Dil. H₂SO₄: Adding to solid nitrite in cold yield pale blue liquid (due to the presence

of free nitrous acid HNO₂ or its anhydride N₂O₃) & the evolution of brown fumes of nitrogen dioxide, the latter being largely produced by combination of nitric oxide with the oxygen of the air

NO₂- _{+ H}+ _{ HNO} 2

2HNO₂  H₂O + N₂O₃

3HNO₂  HNO₃ + 2NO + H₂O 2NO + O₂  2NO₂

Following tests performed with an aqueous salt solution. (ii) Silver nitrate solution : White crystalline ppt. is obtained

NO₂- _{+ Ag}+ _{ AgNO} 2

(iii) Turns acidified KI - starch paper blue 2KI + 2NO₂  2KNO₂ + I₂ 

Starch + I₂  Blue Colour

(iv) Brown ring test: When the nitrite solution is added carefully to a conc. solution of Iron(II)

sulphate acidified with dil acetic acid or with dilute sulphuric acid, a brown ring, due to the formation of [Fe,NO]SO₄ at the junction of the two liquids.

NO₂- _{+ CH}

3COOH  HNO2 + CH3COO -

3HNO₂  H₂O + HNO₃ + 2NO Fe2+ _{+ SO}

4

2- _{+ NO  [Fe, NO]SO} 4.

5. Acetate (CH₃COO-_{) :}

(i) Dilute Sulphuric Acid: Smell of vinegar

CH₃COO- _{+ H}+ _{ CH}

(ii) Iron (III) Chloride Solution: Gives deep - red colouration

CH₃COONa + FeCl₃  (CH₃COO)₃Fe + 3NaCl Brown colour

6. Thiosulphates (

S O

_{2 3}2):

(i) Dil Hydrochloric acid : Gives sulphur & sulphur di oxide S₂O₃2 _{+ 2H}+ _{ S  + SO}

2 + H2O

(ii) Iodine Solution : Decolourise due to formation of tetrathionate ion I₂ + 2S₂O₃2 _{ 2I}- _{+ S}

4O6 2-

(iii) Barium chloride solution : White ppt. of barium thiosulphate is formed S₂O₃2 _{+ Ba}2+ _{ BaS}

2O3 

But no ppt. is obtained with CaCl₂ solution.

(iv) Silver nitrate solution : Gives white ppt. of silver thiosulphate. S₂O₃2 _{+ 2Ag}+ _{ Ag}

2S2O3 

The ppt. is unstable, turning dark on standing, due to the formation of silver sulphide. Ag₂S₂O₃ + H₂O  Ag₂S + H₂SO₄

(v) Lead acetate or Lead nitrate solution : Gives white ppt.

 2 3 2O S + Pb2+ _{ PbS} 2O3 

On boiling it turns black due to the formation of PbS. PbS₂O₃  + H₂O  PbS  + 2H+ _{+ SO}

4 2-

Class A(ii): Gases or acid vapours evolved with conc. Sulphuric acid It includes - Cl-_{, Br}-_{, I}-_{, NO}

3 -_.

1. Chloride (Cl-_{) :}

(i) Conc. H₂SO₄ : decomposes with the evolution of HCl.

Cl- _{+ H}

2SO4  HCl + HSO4 

Gas so produced

1. Turns blue litmus paper red

2. Gives white fumes of NH₄Cl when a glass rod moistened with ammonia solution is brought to the mouth of test tube.

(ii) Manganese dioxide and conc. sulphuric acid: When a solid chloride is treated with MnO₂ and conc. H₂SO₄, yellowish green colour is obtained.

MnO₂ + 2H₂SO₄ + 2Cl- _{ Mn}2+ _{+ Cl}

2 + 2SO4 2- _{+ 2H}

2O

The following tests are performed with the salt solution.

(iii) Silver nitrate solution: White, curdy ppt. of AgCl insoluble in water & in dil nitric acid, but

soluble in dilute ammonia solution. Cl- _{+ Ag}+ _{ AgCl }

AgCl  + 2NH₃  [Ag(NH₃)₂]Cl Ag(NH₃)₂Cl + 2H+ _{ AgCl + 2NH}

4 +_.

(iv) Lead acetate solution: White ppt. of lead chloride is formed

2Cl- _{+ Pb}+2 _{ PbCl} 2 

(v) Chromyl chloride test : When a mix containing chloride ion is heated with K₂Cr₂O₇ and conc. H₂SO₄ orange red fumes of chromyl chloride (CrO₂Cl₂) are formed.

K₂Cr₂O₇ + 4NaCl + 6H₂SO₄  2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂  + 3H₂O orange – red fumes

Chlorides of mercury, owing to their slight ionization, do not respond to this test and only partial conversion to CrO₂Cl₂ occurs with the chlorides of lead, silver, antimony and tin.

When chromyl chloride vapours are passed into sodium hydroxide a yellow solution of sodium chromate is formed which when treated with lead acetate gives yellow ppt. of lead chromate.

CrO₂Cl₂ + 2NaOH  Na₂CrO₄ + 2HCl Yellow solution Na₂CrO₄ + (CH₃COO)₂ Pb  2CH₃COONa + PbCrO₄ 

(yellow ppt.) 2. Bromide (Br-₎

(i) Conc. H₂SO₄ : Gives reddish brown vapours of bromine.

2KBr + H₂SO₄  K₂SO₄ + 2HBr 2HBr + H₂SO₄  2H₂O + SO₂  + Br₂

(reddish brown)

(ii) Manganese dioxide and conc. sulphuric acid : When a mix of solid bromide, MnO₂ and conc. H₂SO₄ is heated reddish brown vapours of bromine are evolved.

2KBr + MnO₂ + 2H₂SO₄  Br₂  + K₂SO₄ + MnSO₄ + 2H₂O The following tests are performed with the salt solution.

(iii) Silver nitrate solution: A pale yellow ppt. of silver bromide is obtained. This ppt. is sparingly

soluble in dil but readily soluble in conc. ammonia solution and insoluble in dil. HNO₃. Br- _{+ Ag}+ _{ AgBr}

(iv) Lead acetate solution: White crystalline ppt. of lead bromide which is soluble in

boiling water.

2Br- _{+ Pb}+2 _{ PbBr} 2 

(v) Chlorine water: When this solution is added to a solution of bromide and chloroform free

br om i ne i s l i b er at ed , whi c h col our s the or ga ni c l a ye r

orange – red.

2KBr + Cl₂ (water)  2KCl + Br₂ Br₂ + Chloroform  Orange red colour

(vi) Potassium dichromate & conc. H₂SO₄: When a mix of solid bromide, K₂Cr₂O₇, and conc. H₂SO₄ is heated and passing the evolved vapours into water, a yellowish brown solution is obtained.

2KBr + K₂Cr₂O₇ + 7H₂SO₄  3Br₂  + Cr₂(SO₄)₃ + 4K₂SO₄ + 7H₂O. 3. Iodide (I-_{) :}

(i) Conc. H₂SO₄ : Gives violet vapours of iodine

2I- _{+ 2H}

2SO4  I2 + SO4 2- _{+ 2H}

2O + SO2 

violet vapours The following tests are performed with the salt solution.

(ii) Silver nitrate solution: Yellow, curdy ppt. of silver iodide AgI, very slightly soluble in conc.

ammonia solution and insoluble in dil nitric acid. I- _{+ Ag}+ _{ AgI}

(iii) Lead acetate solution: Yellow, curdy ppt. of lead iodide soluble in much hot water forming a

colourless solution & yielding golden yellow plates (spangles) on cooling. 2I- _{+ Pb}2+ _{ PbI}

2

(iv) Potassium dichromate & conc. sulphuric acid: Iodine is liberated

6I- _{+ Cr} 2O7 2- _{+ 2H} 2SO4  3I2  + 2Cr 3+ _{+ 7SO} 4 2- _{+ 7H} 2O.

(v) Chlorine water: Iodine is liberated, by the dropwise addition of chlorine water to iodide, and

on addition of CHCl₃ violet coloured organic layer is obtained. 2I- _{+ Cl}

2  I2 + 2Cl -

I₂ + chloroform  violet coloured layer.

(vi) Copper sulphate solution: Gives brown ppt. consisting of a mixture of copper (I) iodide &

i odi ne a nd on ad di ton of hyp o sol uti on b rown p pt c ha ng es t o

white ppt. 4I- _{+ 2Cu}2+ _{ 2CuI + I} 2 I₂ + 2S₂O₃2- _{ 2I}- _{+ S} 4O6 2-_.

v(ii) Mercury (II) chloride solution: Forms scarlet ppt. of HgI₂

2I- _{+ HgCl}

2  HgI2  + 2Cl -_.

This ppt. dissolves in excess of KI, forming tetraiodo mercurate (II) complex. HgI₂ + 2I- _{ [HgI}

4] 2-

4. Nitrate (NO₃- _{) :}

(i) Conc H₂SO₄ : Gives reddish - brown vapours of nitrogen dioxide

4NO₃- _{+ 2H}

2SO4  4NO2 + 2SO4 2- _{+ 2H}

2O + O2

The following tests are performed with the salt solution.

(ii) Brown ring test : When a freshly prepared solution of iron (II) sulphate is added to nitrate

solution & conc. H₂SO₄ is poured slowly down the side of the test - tube, a brown ring is obtained. 2NO₃- _{+ 4H}

2SO4 + 6Fe

2+ _{ 6Fe}3+ _{+2NO + 4SO} 4

- _{+ 4H} 2O

Fe2+ _{+ NO  [Fe(NO)]}2+

On shaking and warming the mix, the brown colour disappears, nitric oxide is evolved and a yellow solution of Iron(III) ions remains.

Action of heat : The result varies with the metal

1. Nitrates of sodium and potassium evolve oxygen (test with glowing splint) & leave solid nitrites (brown fumes with dilute acid)

2NaNO₃  2NaNO₂ + O₂.

2. Ammonium nitrate yields dinitrogen oxide & steam NH₄NO₃  N₂O  + 2H₂O.

3. Nitrates of the noble metals leave a residue of the metal and a mix of nitrogen dioxide and oxygen is evolved.

2AgNO₃  2Ag + 2NO₂  + O₂.

4. Nitrates of other metals, such as those of lead and copper, evolve oxygen and nitrogen dioxide and leave a residue of the oxide.

2Pb(NO₃)₂  2PbO + 4NO₂ + O₂. Class B (i) Precipitation reaction : SO₄2-

(ii) _{Oxidation and reduction in solution - CrO4}2 - _,Cr 2O7

2_{, MnO}

4 -

1. Sulphate (SO42-):

All sulphates except those of Ba, Pb, Sr are soluble in water. Sulphates of calcium and mercury(II) are slightly soluble. The following tests are performed with the salt solution.

(i) Barium chloride solution: White ppt. of barium sulphate BaSO₄ insoluble in warm dil. hydro- chloric acid and in dilute nitric acid, but moderately soluble in boiling, conc. hydrochloric acid.

SO₄2- _{+ Ba}2+ _{ BaSO} 4 

(ii) Mercury (II) nitrate solution : Gives yellow ppt. of basic mercury (II) sulphate. SO₄2- _{+ 3Hg}2+ _{+ 2H}

2O  HgSO4.2HgO  + 4H +

2. Chromate CrO₄2 -_{and Dichromate (Cr} 2O7

2_{) :}

Metallic chromates gives yellow solution when dissolved in water. In the presence of H+ _{chromates are}

converted into dichromates (orange-red solution). 2CrO₄2 + 2H+ _Cr 2O7 2 + H2O Cr₂O₇2 _{+ 2OH}- _2CrO 4 2 _{+ H} 2O

It may also be expressed as : 2CrO₄2_{+ 2H}+ _2HCrO 4 - _Cr 2O7 -2 _{+ H} 2O

(i) Barium chloride solution: Pale - yellow ppt. of barium chromate soluble in dilute mineral acids

but insoluble in water and acetic acid. CrO₄2 _{+ Ba}2+ _{ BaCrO}

4 

Dichromate ions also gives the same ppt. but due to the formation of strong acid precipitation is partial.

Cr2O7

2_{+ 2Ba}2+ _{+ H}

2O 2 BaCrO4  + 2H+

If sodium hydroxide or sodium acetate is added, precipitation becomes quantitative.

(ii) Silver nitrate solution: Brownish - red ppt. of silver chromate Ag2CrO4 which is soluble in dil.

nitric acid & in ammonia solution, but is insoluble in acetic acid. CrO42- + 2Ag+  Ag2CrO4 

2 Ag₂CrO₄ + 2H+ _{ 4 Ag}+ _{+ Cr} 2O7

2- _{+ H} 2O

Ag2CrO4  + 4NH3  2[Ag(NH3)2]+ + CrO42-

Ag₂CrO₄  + 2Cl- _{ 2AgCl + CrO} 4

2-

A reddish brown ppt. of silver dichromate Ag2Cr2O7 is formed with a conc. solution of a

dichromate.

Cr2O72- + 2Ag+  Ag2Cr2O7

(iii) Lead acetate solution: Yellow ppt. of lead chromate PbCrO₄ insoluble in acetic acid, but soluble in dil nitric acid

CrO₄2- _{+ Pb}2+ _{ PbCrO} 4 . 2PbCrO₄  + 2H+ _2Pb2+ _{+ Cr} 2O7 2- _{+ H} 2O.

(iv) H₂O₂: If an acidic solution of a chromate is treated with H₂O₂ a deep blue solution of chromium penta oxide is obtained.

CrO₄2- _{+ 2H}+ _{+ 2H}

CrO₅ is unstable and it decomposes yielding oxygen and a green solution of a Cr+3 _Salt.

In document 8 IIT JEE CHEMISTRY M 8 (Page 137-144)