Analytical Chemistry-ICSE-Class 10|Biswajit Das

1. Introduction to Analytical Chemistry

Analytical chemistry is the branch of chemistry that deals with the identification and quantitative determination of the components of a substance. In this chapter, we focus on the qualitative analysis of certain metal ions (cations) and their identification using specific chemical reagents, primarily ammonium hydroxide (NH4​OH) and sodium hydroxide (NaOH). The reactions often produce visible precipitates of different colors and properties.

2. Reactions of Ammonium Hydroxide (NH4​OH) with Metal Salt Solutions

Ammonium hydroxide is a weak base, so it provides a low concentration of hydroxide ions (OH−). This is useful for selectively precipitating certain metal hydroxides.

• Ferrous Salts (Fe2+): With ferrous sulfate solution, it forms a dirty green precipitate of ferrous hydroxide, which is insoluble in excess of the reagent.FeSO4​+2NH4​OH→(NH4​)2​SO4​+Fe(OH)2​↓ (Dirty green)
• Ferric Salts (Fe3+): With ferric chloride solution, it forms a reddish-brown precipitate of ferric hydroxide, which is insoluble in excess.FeCl3​+3NH4​OH→3NH4​Cl+Fe(OH)3​↓ (Reddish-brown)
• Copper Salts (Cu2+): With copper sulfate solution, it initially forms a pale blue precipitate of copper hydroxide. This precipitate is soluble in excess of ammonium hydroxide, forming a deep blue colored solution of a complex ion.CuSO4​+2NH4​OH→(NH4​)2​SO4​+Cu(OH)2​↓ (Pale blue)Cu(OH)2​+(NH4​)2​SO4​+2NH4​OH→[Cu(NH3​)4​]SO4​+4H2​O (Deep blue solution)
• Zinc Salts (Zn2+): With zinc sulfate solution, it forms a gelatinous white precipitate of zinc hydroxide. This precipitate is soluble in excess, forming a colorless solution.ZnSO4​+2NH4​OH→(NH4​)2​SO4​+Zn(OH)2​↓ (Gelatinous white)Zn(OH)2​+(NH4​)2​SO4​+2NH4​OH→[Zn(NH3​)4​]SO4​+4H2​O (Colorless solution)
• Lead Salts (Pb2+): With lead nitrate solution, it forms a chalky white precipitate of lead hydroxide, which is insoluble in excess.Pb(NO3​)2​+2NH4​OH→2NH4​NO3​+Pb(OH)2​↓ (Chalky white)

3. Reactions of Sodium Hydroxide (NaOH) with Metal Salt Solutions

Sodium hydroxide is a strong alkali, so it provides a high concentration of hydroxide ions (OH−).

• Ferrous Salts (Fe2+): Forms a dirty green precipitate of ferrous hydroxide, insoluble in excess.FeSO4​+2NaOH→Na2​SO4​+Fe(OH)2​↓ (Dirty green)
• Ferric Salts (Fe3+): Forms a reddish-brown precipitate of ferric hydroxide, insoluble in excess.FeCl3​+3NaOH→3NaCl+Fe(OH)3​↓ (Reddish-brown)
• Copper Salts (Cu2+): Forms a pale blue precipitate of copper hydroxide, insoluble in excess.CuSO4​+2NaOH→Na2​SO4​+Cu(OH)2​↓ (Pale blue)
• Zinc Salts (Zn2+): Forms a gelatinous white precipitate of zinc hydroxide. This precipitate is soluble in excess, forming a colorless solution.ZnSO4​+2NaOH→Na2​SO4​+Zn(OH)2​↓ (Gelatinous white)Zn(OH)2​+2NaOH→Na2​ZnO2​ (Colorless solution)
• Lead Salts (Pb2+): Forms a chalky white precipitate of lead hydroxide. This precipitate is soluble in excess, forming a colorless solution.Pb(NO3​)2​+2NaOH→2NaNO3​+Pb(OH)2​↓ (Chalky white)Pb(OH)2​+2NaOH→Na2​PbO2​ (Colorless solution)
• Calcium Salts (Ca2+): With calcium nitrate, it forms a white precipitate of calcium hydroxide, insoluble in excess.Ca(NO3​)2​+2NaOH→2NaNO3​+Ca(OH)2​↓ (White)

4. Special Tests for Cations

• Test for Copper (Cu2+): The formation of a deep blue solution with excess NH4​OH is a confirmatory test.
• Test for Ferric (Fe3+): The reddish-brown precipitate with NaOH or NH4​OH is a strong indicator. Another test involves the reaction with potassium ferrocyanide, which gives a deep blue precipitate.
• Test for Lead (Pb2+): Lead salts, when heated, produce a characteristic color change. For example, lead oxide is yellow when hot and white when cold.2Pb(NO3​)2​Δ​2PbO+4NO2​+O2​The reaction with potassium iodide produces a bright yellow precipitate of lead iodide, which is a common confirmatory test.Pb(NO3​)2​+2KI→2KNO3​+PbI2​↓ (Bright yellow)

5. Identification of Gases

Certain gases are identified by their characteristic properties or reactions.

• Ammonia (NH3​): Pungent smell. Forms dense white fumes with a glass rod dipped in concentrated HCl.
• Hydrogen Chloride (HCl): Pungent smell. Forms dense white fumes with a glass rod dipped in concentrated NH4​OH.
• Carbon Dioxide (CO2​): Colorless, odorless gas. Turns lime water milky due to the formation of calcium carbonate.
• Sulfur Dioxide (SO2​): Pungent, suffocating smell. Turns acidified potassium dichromate solution from orange to green.
• Hydrogen Sulfide (H2​S): Smell of rotten eggs. Turns lead acetate paper black.