Qualitative Analysis

Invariably involves detection or identification of cation(s) and or anion(s) present in given test substance in a series of reactions.

Theoretical basis

• Mainly inorganic reactions.

 

Skills tested

• Observational
• Colour change detection
• Scientific deductive skills nasedon chemistry concepts
• Handling of apparatus

 

Preliminary Deductions

this is an informed guess about the possible identity of the test substance based on its physical appearance.

 

Guidelines to the identification of solids

Key Questions

• is it coloured?
• is it crystalline/ powder?

 

Nature of test substance	Preliminary Deduction
White cystalline solid	not likely to be a transition metal compound (usually coloured). not likely to be an oxide/carnonates (powders). likely to have water of crystallization.
White powder	not likely to be a transition metal compound likely to be an oxide/carnonates/slphates of non-trans element. manganese carbonate is almost white.
Coloured crystalline	likely to be a transition metal compound (nitrates /halides /some sulphates of trans metals)
Coloured powder	likely to be caronate of transition metal e.g CuCO3 is green powder. it can be a sulphate/ another compound of a trans metal which has lost its ater of crystallisation. red lead oxide, Pb3O4 (non-transition element).
Black powder	likely to be a metal sulphide/ oxide of a transition metal/ non most metal sulphides are black powders eg PbS

Guidelines to the identification of species in solutions

Key questions

• is it coloured or not?
• is it clear coloured or clear colourless?

Nature of test substance	Preliminary deduction
Clear colourless solution	no transition cation /oxoanion /complex present substance is soluble
A strangely coloured solution	likely to be a mixture of trans metal cation(s). if solution is a mixture of trans cations, 20 colours of two cations should be expected eg Fe3+(a) -redbrown and Cu2+(aq) -blue = grey green colour
A clear colured solution	a transition metal/ oxoanion/ complex is likely to be present. maybe mixed with a soluble compound of a non transition metal

 

The colours of aqueous ions

 

Ion	Colour
Cu2+	pale blue
Fe2+	light green
Fe3+	red brown
Cr3+	green
Mn2+	faint pink
MnO4–	purple/violet
CrO42-	yellow
Cr2O72-	Orange

Procedure

Tips and hints

• Follow instructions
• Do not put too much test reagent in the test tube
• initially add one dropat a time with shaking and careful observations.

 

Golden rule

• Before you add any reagent, always ask yourself the following question(s) first
• What i this test for?
• What does the reagent to be added detect?

 

Common Reagents and their uses

The commonly used reagents

dilute NaOH HCl(aq) dilute H₂SO₄ KI(aq) K₂Cr₂O₇(aq) BaCl(aq) NaHCO₃(solid & aq) Dilute NH₃
dilute HNO₃ dilute H₂O₂ CrO₄^2- AgNO₃ NaCO₃(solid & aq) Starch solution Na₂S₂O₃(aq)

 

Different Uses of reagents in Qualitative Analysis

Dilute H₂SO₄

• to decompose carbonates to produce CO₂(g)
• expect and test for CO₂(g) when suspected solid or liquid contain carbonates
• the sulphates in dilute H₂SO₄ can detect Ba²⁺(aq) & Pb²⁺(aq) which form insoluble sulphates.
• can be used to neutralise a basic solution.
• to identify CrO₄^2-(aq) and Cr₂O₇^2-(aq) whosecolors are characteristic in acid or alkaline solutions
• to liberate SO₂(g) form SO₃^2-(aq) and NO₂(g) from NO₂^–(aq)
• to provide an acidic medium for oxidizing agents eg MnO₄(a), CrO₄^2-(aq), H₂O₂ or Cr₂O₇^2-(aq)

 

Conc H₂SO₄

• used to detect salts of waeker acids or less volatile acids from whose salts they are displaced
• when conc H2SO4 is added to a sold test substance and the following observations and deductions are made;

Test	Observation(s)	Deduction(s)
add conc H2SO4	HCl(g) evolve	Cl– present
	Br2 & HBr(g) produced	Br–
	I2(g)	I– present
	O2(g) form a dark brown solid. A white residue remains	PbO2(s) present

Dilute HCl(aq)

• the Cl (aq) can be used to detect Pb²⁺ by forming a white ppt of PbCl²(s) which dissolved when the sln is warmed but reappears after being cooled
• same as dil H₂SO₄ in;

• decomposing carbonates.
• neutralizing base.
• acidifying some reactions

 

Concentrated HCl

• used to detect oxidizing agents eg MnO₄
• it is oxidised to Cl₂
• To detect presence of a basic oxide -the solid dissolves upon addition of con HCl

 

Dilute HNO₃

• can be used to decompose carbonates and neutralise bases
• it is usually more preferable as a neutralizer since nitrate present does not form an insoluble ppt witth any cations that might be in the test substance
• to dissolve an formed ppt ie the filtered residues before further tests on it.

 

Dilute sodium hydroxide NaOH(aq)

• precipitates metal cations (except those of group [1]) in form of hydroxides which are insoluble/soluble in water.
• M²⁺ + 2OH^2- → M(OH)₂(s)-ppt eg Cu²⁺ + OH-(aq) & Cu(OH)₂(s)-skyblue ppt
• some ppts dissolve upon adding excess NaOH eg Pb²⁺ and Zn²⁺
• as the complex formed dissolves;
• M²⁺ + 2OH^2- → M(OH)₂(s) + 2OH^– → M(OH)₄^2-(aq) eg Al(OH)₄^2-(aq), Zn(OH)₄^2-(aq) & Pb(OH)₄^2-(aq)
• Colour of ppt depens on the cation present it is a transition metal.
• Can be use to neutralizean acidic solution – no apparent observation
• Used to dsplace H₂(g) when metals eg Al or Zn are added NaOH(aq)
  eg 2Al(s) + 2NaOH(aq) + H₂O(l) → 2NaAl(OH)₃ + 3H₂(g).
• Also used to displace a weaker base eg ammonia in the test for nitrates.

 

Ammonia NH₃(aq)

• precipitates metal cations in form of metal hydrroxides justlike NaOH(aq)
• Ammonia partially dissociates ie has is very low concentration of OH(aq)
• NaOH(aq) fully dissociates ie has high concentration of OH(aq)
• used for the confirmatory test for Cu2+(aq)- a pale ppt whichdissovless in excess NH₃ to give a deep a blue sln
• used to distinguish between silver halides AgCl(s) dissolves in dil NH₃ while the AgBr(s) & AgI(s) do not.

 

Oxidizing Agents

• Common used oxidizing agents include;
• KMnO₄
• K₂CrO₇
• H₂O₂
• *usually requires acidic conditions eg adding sulphuric acid before carrying out the test except MnO₄ requires alkaline conditions.

 

i)KMnO₄(aq)- purple

• used to detect Fe²⁺(aq) in acidic medium.

Test	Observation	Notes
add dil H2SO4 followed by KMnO4(aq)	KMnO4 is decolourised	Fe2+ is oxidised to Fe3+ and MnO4–(aq) is reduced to Mn2+(aq)

ii)Oxidation of iodide ions

• I^– ions are oxidised to yellowish brown iodine(aq)
• MnO₄^–(aq) are reduced to faint pink Mn²⁺(aq)

 

iii)Oxidation of hydrogen peroxide

• MnO₄^–(aq) oxidize H₂O₂ to waterr and oxygen
• MnO₄^–(aq) are reduced to faint pink/colourless Mn²⁺(aq)

 

iv)Oxidation of Ethanol

• it is oxidized in alkaline medium to ethanoic acid
• MnO₄^–(aq) are decolourised

 

K₂Cr₂O₇(aq)-orange

• stabilized by an acidic environment.
• acts as an oxidizing agent, while it is reduced to green Cr³⁺
• like MnO₄^– ions, dichromate ions oxidize;

• Fe(ii) to Fe(iii)- dirty green observed due to mixture of green & yellowish brown;
• I^– ions to I₂;
• H₂O₂ to H₂O & O₂
• Dichromate ions oxidize sulphite ions to sulphate and nitrate ions to nitrate.
• Ethanol can be used to test for dichromate(vi) ions- the sln turns to green

 

Hydrogen peroxide(H₂O₂)- colourless

• can work in both acidified and alkaline medium.
• it is reduced to water and there is no O₂(g) that is liberated
• oxygen is liberated only when it is acting as a reducing agent

The table below shows the observations and deductions made when H₂O₂(aq) is added to a test substance containing some cations and anions;

Observations	Deductions	Notes
Solutions turns red-brown from pale green	Fe(ii)(aq) present	Fe(ii)(aq) is oxidized to Fe(iii)(aq)
Solutions turns dark brown & black-brown ppt from pale pink	Mn2+(aq) prsent	Oxidation of Mn2+ to MnO2(s)
Solution changes from colourless to orange-brown; a black ppt settles at the bottom after a few minutes	I–(aq) present	I– is oxidized to I2(s). black ppt are crystals of iodine(s). orange-brown sln consists of I3-(aq)

Reducing Agents

• Commonly used reducing agents include;
• KI-iodides;
• Sodium thiosulphate -thiosulphate ions

Potassium Iodide, KI

• I^–(aq) is oxidized to I₂(aq)-brown or I₂(s)-black ppt

 

Observations and deductions when KI is added to various test reagents

Observations	Deductions	Notes
Pale solution to brown ppt plus few white ppt	Cu2+ present	Cu2+ is reduced to Cu1+ white ppt is CuI(s)
a bright yellow ppt soluble in excess KI to give colourless sln	Pb2+ present	yellow PbI2 dissolves in excess KI bcz a complex PbI42- is formed
A dark brown sln from pale red brown sln; black ppt forms slowly	Fe3+	Fe(iii) to Fe(ii)
purple sln to faint pink sln	Mn2+ present	Mn7+ to Mn2+

Hydrogen Peroxide

• it can act as a reducing agent in some reactions
• oxygen gas is liberated, you should prepare to test for oxygen.
• examples include;

• Acidified hydrogen peroxide reduces Mn(7+) to Mn(2+)
• pH change may affect the behaviour of hydrogen peroxide in redox reactions eg;

• in acidic medium: Mn(7+) is reduced to Mn(2+)
• in alkaline medium: Mn(2+) is oxidized to Mn(4+) -dark brown ppt

Reactions Of Transition metal cations

Reaction of Cu²⁺

Cu²⁺(aq) (pale blue) 1ons can be obtained b
Dissolving a solub1e copper (II) salt in water, for example solid copper (II) sulphate (pale blue).
Dissolving black CuO in an acid such as sulphuric acid.

Test	Observation	Notes
(a) Add dil HCl and warm	Solution turns form light blue to yellow-green	warming drives out some water and thus increases the concentration of Cl ligands, resulting in the formation of yellow complex. Ggreenish-yellow is caused by mixing of blue of Cu2+ and yellow of [CuCl4]2
Add conc HCl	Solution turns from light blue to yellowish-green	Chlorine ligands replace water ligands from the aqua complex
Add NaOH(aq)	Pale blue ppt formed, insoluble in exess NaOH	Precipitation of Cu2+ by OH– ions
Add KI(aq) followed by;   (i) Na2S2O3   (ii) Starch solution	Solution turns pale blue to brown. white ppt is formed. Brown colour disappears, leaving a white solid   blue black colour appears	2Cu2+(aq) + 4I– → 2CuI(s) + I2(aq). CuI(white ppt) Brown iodine is reduced to colourless iodide by sodium thiosulphate   This is a test to confirm the presence of iodine
Add NH3(aq)	A blue ppt is formed soluble in xs ammonia to give a deep blue solution	Cu(OH)2(s) is first precipitated. The ppt dissolves in xs ammonia to form a deep blue complex

 

Reaction of Fe²⁺

Colour : pale green, almost colourless.

Test	Observation	Notes
Add H2O2   Then add dilute H2SO4(aq)   Followed by NaOH(aq)	Solution turns from light green to yellowish brown Intensity of yellowish-brwon colour increases   A red-brown ppt is formed upon addition of a few drops of NaOH(aq)	Fe2+(aq) is oxidized by hydrogen peroxide to Fe3+(aq) which is yellowish-brown The action of hydrogen peroxide as an oxidising agent is promoted by an acid   Fe3+(aq) from the oxidation of Fe2+(aq) by hydrogen peroxide is precipitated as Fe(OH)3(s), which is a red-brown solid.
Add NH3(aq)   Leave ppt in the air for a few minutes	A green ppt is formed   ppt changes from green to red-brown	Fe(OH)2(s) is produced. Fe(OH)2(s) oxidized by oxygen to red-brown Fe(OH)3(s)
Add NaOH(aq)	Pale green ppt is formed insoluble in xs. The ppt slowly turns brown in air	Fe3+ precipitated as Fe(OH)3(s).
Add dilute H2SO4 followed by KMnO4(aq), drop by drop	KMnO4 is decolourised. A yellowish-brown solution is formed	Oxidation of Fe3+ by KMnO4 to form a yellowish-green solution of Fe3+. The yellowish-brown colour observed is due to Fe3+.

 

Reaction of Fe³⁺

Colour : yellow brown to orange when unmixed with other colored cations

Test	Observation	Notes
Add solid NaCO3	Effervescence occurs, gas which turns limewater milky is produced. A coffee brown ppt is formed	Brown ppt is formed Fe(OH)3. Fe3+ is acidic, protonate water hence carbonate is dicomposed.
Add KI(sq) followed by either;   Strach solution   Na2S2O3(aq)	Solution turns deeper brown   a blue black colour appears   Brown colour disappears leaving a pale green solution	I– oxidized to brown iodine. meanwhile Fe3+ is reduced to pale green Fe2+ whose color is masked by that of iodine. The blue black colour is due to the formation of an iodine/starch complex   Brown iodine is reduced by thiosuphate ions to I– leaving Fe3+

 

Reaction of Cr³⁺

Its colour is green. A solution of Cr³⁺ is commonly provided in form of CrCl³(aq). In that case the solution is blue-violet rather than green this is caused by the environment of Cl^– and hydrolysis

Test	Observations	Notes
Add NH3	an immediate green ppt soluble in xs is formed	Cr(OH)3(s)
Add solid sodium carbonate	Effervescence, gas which tunrs limewater milky is produced.Solution turns to grey-green. A pake green ppt is also formed	Solution of Cr3+(aq) is acidic and so reacts witha carbonate to produce CO. Green ppt is Cr(OH)3(s)
Add NaOH(sq)	an immediate grey-green ppt is formed whichis soluble in xs NaOH to form a dark green solution	Grey-green ppt is Cr(OH)3(s). The ppt reacts with further xs OH– ions in a ligand exchange reaction forming a dark green complex [Cr(OH)6]3+
Add NaOH(aq) followed by 20Vol Hydrogen peroxide.	Solution truns yellow	Cr3+ oxidized to CrO42-

 

Reaction of Mn²⁺

Color : pale pink, almost colourless

Test	Observations	Notes
Add NaOH(aq)	An off-white ppt is formed insoluble in xs NaOH and rapidly turning brown on contact with air	Off-white ppt is Mn(OH)2(s). In air it is rapidly oxidized to brown Mn(OH)3(s) The brown color of the ppt continues to darken due to dehydration to form Mn2O3(s)
Add NH3   Fliter the solid and leave it in the air	off-white ppt is formed insoluble in xs NH3 and rapidly turning brown on contact with air   Brown colour of ppt becomes deeper	Off-white ppt is Mn(OH)2(s). In air it is rapidly oxidized to brown Mn(OH)3(s) The brown color of the ppt continues to darken due to dehydration to form Mn2O3(s)