How to separate lead ions from a mixture of aluminium ions and zinc ions

 In a qualitative analysis of ions like lead, zinc ions and aluminium ions, these ions show similar reactions with dilute sodium hydroxide solution. They produce white precipitates with a few drops of dilute sodium hydroxide solution, which are soluble in excess dilute sodium hydroxide solution.

 In this article, you will learn how to separate lead ions from a mixture of aluminium ions and zinc ions. Before I go ahead, let me list some reagents that are necessary for the tests:

• Aqueous ammonia
• Dilute hydrochloric acid
• Potassium dichromate solution
• Potassium iodide solution

Step one: separation of zinc ions from the mixture

To the solution of the unknown compounds, add a few drops of aqueous ammonia, and the formation of white gelatinous precipitate that dissolves in excess aqueous ammonia removes lead ions and aluminium ions as an insoluble compound in the excess aqueous ammonia while zinc remains in the solution.

Step two: separation of aluminium ions from the mixture

Filter the solution from step one above, add dilute hydrochloric acid to the residue, aluminium ions dissolve and lead ions will form a white precipitate which is soluble when hot and insoluble when cool. Note: The insoluble Lead ions are known to be soluble when heated.

Step three: separation and identification of the lead ions

Filter the solution from step two above, add potassium dichromate solution to the residue and heat gently. A yellow precipitate formed, and this yellow insoluble substance confirms the presence of lead ions in the mixture. Or add potassium iodide solution to the residue and gently heat the mixture. A yellow precipitate formed also shows that lead ions are present.