Tollen's Silver Mirror Test

Introduction:

This classic demonstration comes from an old qualitative chemical test. Tollen discovered that silver would precipitate from a basic solution of silver nitrate and ammonia, when in the presence of a reducing sugar. What's more, the silver precipiate would form on the inside surface of the glass container, forming a perfect mirror. For a while, mirrors were produced industrially by this method, but it was later discovered that mirrors could be produced more cheaply and efficiently by simply spraying metal coatings onto the back of a glass surface. This could be done with cheaper metals than silver, which also produced mirrors that didn't corrode as easily. Tollen's test also eventually lost popularity because Tollen's reagent was somewhat unstable, and other quantitave analysis procedures were developed.

Preparation:

1. Gloves and eye protection should be worn throughout this experiment. Concentrated nitric acid will cause burns that are recognized by a yellow discoloration of the skin, which will later peel. Silver nitrate is an oxidizer and causes burns and dark stains. Potassium hydroxide has a slipperly feeling to it, but is a strong base and causes burns. Concentrated ammonia is an irritant to the eyes, skin, and nasal passageways. Glucose is not harmful, and can be purchased at a pharmacy as a diabetic supply.
2. Clean a 125mL flask with a soap brush and rinse it well. Rinse the flask with a few milliters of concentrated nitric acid (HNO₃), in order to get a perfectly smooth and clean surface, which is important for a good mirror product. Rinse the flask once more with water.
3. Add distilled water to 2.12g of crystalline silver nitrate (AgNO₃) to make 121.5mL of an aqueous silver nitrate solution.
4. Add distilled water to .91g of potassium hydroxide (KOH) pellets to make 20mL of an aqueous potassium hydroxide solution.
5. Do the following in a fume hood, as nitrate solutions can sometimes evolve NO₂, a poisonous gas. This gas is recognized by an exothermic reaction and the evolution of a brown, choking gas. Add concentrated aqueous ammonia (NH₄OH) dropwise to the silver nitrate solution. A brown precipitate will form, and continue adding the ammonia dropwise until the precipitate clears. The result is a clear solution.
6. To this solution, add the potassium hydroxide solution. A dark brown precipitate will form, but will clear up when you add more concentrated aqueous ammonia dropwise. If a dark brown precipitate does not form when you add the potassium hydroxide solution, just add one drop of concentrated ammonia. This result is Tollen's Reagent.
7. The fume hood is no longer necessary. Add 5g of glucose (C₆H₁₂O₆), the reducing sugar, to 113mL distilled water. When dissolved, the result is a 4.4% aqueous glucose solution.

Demonstration:

1. Add 15mL of Tollen's Reagent to the clean 125mL flask.
2. Add 2.25mL of the 4.4% glucose solution to the flask.
3. Stopper the flask quickly with a cheap, disposable stopper. Silver nitrate is an oxidizer, and it will destroy the stopper. I recomment Parafilm.
4. Swirl the flask gently, but so that the entire inside is wetted, for about two minutes. Initially, the solution will turn brown, and then the mirror will begin to form on the inside. When the mirror is fully formed, unstopper the flask and pour the spent solution onto water.

Cleanup:

Before allowing people to handle the silver mirror, rinse it gently but well with water. Once it is rinsed, it is simply a coating of silver on glass, and is thus harmless. To get the silver coating off the inside of the flask, add a few drops of concentrated nitric acid, under the fume hood. This will immediately dissolve the silver coating, as well as clean the inside of the flask for the next demonstration. This acidic solution can be poured down the sink if it is done with copious amounts of water. Keep in mind that nitric acid attacks almost everything, including copper pipes, so it must be very dilute. The spent basic solution can also be poured down the sink with water.