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)-sky-blue 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 neutralize an acidic solution – no apparent observation
• Used to displace 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 hydroxides just like 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 which dissovless in excess NH₃ to give a deep a blue solution
• used to distinguish between silver halides AgCl(s) dissolves in dil NH₃ while the AgBr(s) & AgI(s) do not.