Tollens Reaction - Silver Mirror Test for Aldehydes

Demonstration Experiment on Video

Tollens Reaction - Silver Mirror Test

Objective: Test for Aldehydes

Peter Keusch

German version

Chemicals:
glucose
0.1 M AgNO₃
aqueous ammonia 25 %

Apparatus and glass wares:
hot plate
thermometer
glass stirring rod
2 beakers 40 mL
beaker 200 mL
volumetric pipette 5 mL
volumetric 10 mL
pipette ball
measuring cylinder 100 mL
crystallizing dish (for water bath)
florence flask 500 mL

Hazards and safety precautions:

Concentrated ammonia solution is extremely damaging to eyes. Even contact with dilute ammonia solution can lead to serious eye damage Harmful if swallowed or inhaled and in contact with skin.

Good quality safety glasses with side protection against splashes. The experiment should be performed in a portable fume hood giving all-round visibility!

The Tollens reagent should be prepared fresh for each procedure. Un-reacted reagent must be destroyed with dilute nitric acid because it forms an explosive precipitate (probably Ag₃N) on standing.

Experimental procedure:

5 mL of ammonia are added to 150 ml of aqueous AgNO₃ while stirring. The precipitate formed is dissolved upon the addition of another 5 mL portion of ammonia. The solution is mixed with aqueous glucose (4 g glucose in 10 mL of dist. water). The mixture is poured into a round bottom flask, that is immersed in a water bath (70°C).

Result:

Within 4 minutes the inside surface of the flask will be coated with a reflective silver mirror.

Video clip (Download RealPlayer .rm file)

Discussion:

Ag⁺ ions react with OH ^- of aqueous ammonia to produce a brown precipitate of AgOH which is dissolved on addition of an excess of aqueous ammonia under the formation of
[Ag(NH₃)₂]⁺. The silver diamine complex is reduced by glucose to metallic silver. Glucose is oxidized to gluconic acid.

equation

Fig. 1: Redox reaction

The Tollens' test is important in carbohydrate chemistry, for proof of structure. The test is specific for reducing sugars. Fructose is also capable of reducing Tollens' solution, and is thus classified a "reducing sugar". Under basic conditions fructose is converted into glucose.

Note:
The silver that is adhered to the flask dissolves readily in a small amount of conc. nitric acid.

Index of Lecture Experiments