Tollen's Test Silver Mirror
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely
Categories: Organic Chemistry
Alternative titles: Identification of Aldehydes
Summary
Aldehydes reduce silver ions in Tollen’s reagent to metallic silver, forming a reflective silver mirror on the inside of a glass container. This classic test is used to identify aldehydes and reducing sugars.
Procedure
Add 5 mL of 10% silver nitrate solution to a clean flask.
Add 1 mL of 10% sodium hydroxide solution.
Add concentrated ammonia drop by drop until the brown silver oxide just dissolves, leaving a clear or slightly dark solution.
Introduce 7 mL of 1% glucose solution and swirl gently.
Within minutes, a silver mirror will form on the inner walls of the flask.
Links
Variations
Use different aldehydes instead of glucose to compare reactivity.
Perform the test in a test tube instead of a flask for quicker observation.
Demonstrate with reducing sugars (such as fructose or lactose) to show biological relevance.
Safety Precautions
Wear gloves to prevent skin staining from silver nitrate.
Avoid contact of silver nitrate with skin or clothing, as it leaves permanent stains.
Work in a well-ventilated area to minimize inhalation of ammonia vapors.
Use concentrated nitric acid carefully to remove the silver mirror after the experiment.
Dispose of silver waste according to local hazardous waste protocols.
Questions to Consider
Why does Tollen’s reagent react with aldehydes but not most ketones? (Aldehydes are more easily oxidized than ketones.)
What role does ammonia play in preparing Tollen’s reagent? (It dissolves the silver oxide, forming the reactive diamminesilver(I) complex.)
Could this test distinguish between glucose and sucrose? (Yes, glucose is a reducing sugar and gives a positive test, while sucrose does not.)