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.
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