Lecture Notes: Identification of Cations

Overview

• Discussed the identification of cations: Calcium, Aluminum, Lead, Zinc
• Mnemonic to remember: "Can Only Do Zinc"
• Testing for cations involves observing reactions with specific reagents

Reagents and Methods

Sodium Hydroxide (NaOH) Test

• Procedure:
  1. Add NaOH to the salt solution containing unknown cations
  2. Observe whether a precipitate forms

Results Interpretation:

• No Precipitate:

  • Heat the solution and test evolved gas with red litmus paper
  • Gas turns red litmus blue: Presence of Ammonium ion
  • White precipitate forms with hydrochloric acid: Formation of Ammonium Chloride

• Precipitate Forms:

  • White Precipitate: Possible presence of Calcium, Aluminum, Lead, or Zinc
  • Add more NaOH to see if precipitate dissolves:
    • Remains: Calcium ion present (confirmed with dilute H2SO4 to form Calcium Sulphate)
    • Dissolves: Possible Aluminum or Lead, confirmed with Potassium Iodide (KI)

Ammonium Hydroxide (NH4OH) Test

• Add NH4OH dropwise, then in excess
• Zinc Ion: Dissolves in excess ammonia
• Aluminum or Lead:
  • No precipitate with KI: Aluminum
  • Yellow precipitate with KI: Lead

Colored Precipitates in NaOH

• Blue Precipitate:
  • Copper ion presence, confirmed by excess NH4OH (dissolves)
• Green Precipitate:
  • Iron (Fe2+) ion, oxidizes to reddish-brown indicating Fe3+
  • Confirm with Potassium Ferricyanide for dark blue precipitate
• Reddish-brown Precipitate:
  • Iron (Fe3+) ion, confirmed with Potassium Thiocyanate

Summary

• Understanding and identifying cations involves systematic testing with chemicals like NaOH and NH4OH.
• Precipitates, their color, and behavior in excess reagent are key indicators.
• Confirmatory tests help to distinguish between cations with similar properties.
• Mnemonics and systematic approaches aid in remembering and performing these tests effectively.

Additional Notes

• Keep practicing the identification process for mastery in qualitative analysis in chemistry.
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