Two colorless liquids are poured into a 500 mL Florence flask - the flask is then stoppered and swirled. Over a period of a few minutes, the inside of the flask darkens and gradually becomes reflective as a mirror, much like a Christmas tree ornament.
The reaction used in this demonstration is Tollen's test, which is the reaction used in qualitative organic analysis to identify aldehydes. Silver metal is formed by the reduction of silver nitrate by an aldehyde.
Explanation of Experiment:
Tollen's test is usually performed in a small test tube. To increase the effectiveness of this demonstration, a Florence flask is mirrored. The mirror forms when a thin film of metallic silver deposits on the inner surface of the flask. When an aldehyde is combined with an ammonia complex of silver in a basic aqueous solution, the aldehyde slowly reduces the complex to silver metal. When the surface of the glass is clean and wet, the silver metal adheres to the glass, forming a highly reflective surface.
The reaction which takes place in this demonstration is:
CH2OH(CHOH)4CHO + 2 [Ag(NH3)2]+ + 3 OH- → 2 Ag + CH2OH(CHOH)4COO- + 4 NH3 + 2 H2O
The aldehyde functional group (-CHO) of dextrose is oxidized to an acid (-COO-). The silver(I) in the diaminesilver(I) ion is reduced to metallic silver. The metallic silver is deposited on the sides of the reaction vessel in the form of a "mirror." Other aldehydes such as benzaldehyde can also be used to produce a silver mirror. Dextrose is used in this demonstration because it is soluble in water. A more uniform mirror is produced with a water-soluble aldehyde.
Dextrose is a sugar, i.e. a soluble carbohydrate. Carbohydrates have a molecular formula of the form Cx(H2O)n. For dextrose, x=6 and n=6. Common table sugar is sucrose, a carbohydrate where x=12 and n=11. Carbohydrates contain either an aldehyde group or a ketone group. Those that contain an aldehyde group produce a positive Tollen's test, i.e. they reduce [Ag(NH3)2+] in basic solution to metallic silver. For this reason, they are called reducing sugars. Dextrose is a reducing sugar, while sucrose is not.
Because this process of plating a flask with silver is performed without an external electrochemical circuit, it is called electroless plating. It is most often used to produce a metal coating on a surface that does not conduct electricity.
30 mL 0.10 M silver nitrate, AgNO3 (To prepare 100 mL of solution, dissolve 1.7 g of AgNO3 in 60 mL of distilled water and dilute the resulting solution to 100 mL.)
ca. 3 mL concentrated (15 M) aqueous ammonia solution, NH3 (from a dropping bottle.)
15 mL 0.80 M sodium hydroxide, NaOH (To prepare 100 mL of solution, dissolve 3.2 g of NaOH in 60 mL of distilled water and dilute the resulting solution to 100 mL.
ca. 3 mL concentrated (16 M) nitric acid, HNO3 (from a dropping bottle.)
10 mL 0.5 M dextrose, C6H12O6 (To prepare 100 mL of solution, dissolve 9 g of dextrose in 60 mL of distilled water, and dilute the resulting solution to 100 mL.
Hot water to fill Florence flask.
100 mL beaker
500 or 1000 mL Florence flask.
This mixture should be prepared no more than one hour before use due to potential hazards. Pour 30 mL of 0.10 M AgNO3 into the 100 mL beaker. While stirring the solution, add drops of 15 M NH3 until the brown precipitate which forms initially has just dissolved. Add 15 mL of 0.80 M NaOH to this mixture. If a precipitate forms again, add drops of 15 M NH3 until it dissolves.
Pour 3 mL 0f 16 M HNO3 into the Florence flask and stopper it. Swirl the acid around to dampen the entire interior surface of the flask. Pour the acid from the flask, and flush it down the drain with water. Rinse the flask and stopper it.
Within ten minutes of presenting the demonstration, fill the flask with hot water to warm it.
Empty the water from the flask. Pour 10 mL of 0.5 M dextrose into the flask. Add the contents of the beaker, and stopper the flask. Swirl the flask continuously to cover its entire surface with a thin coating of the liquid. Within about a minute, the flask will begin to darken as a film of metallic silver forms on its inside surface. Continue to swirl the flask until the entire interior of the flask is covered with a film of silver, and the flask looks like a mirror. Pour the liquid from the flask, and flush it down the drain with water.
If mixtures of aqueous silver nitrate, ammonia, and sodium hydroxide are heated or allowed to stand for several hours, a highly explosive precipitate (silver nitride) may form. Therefore, the mixture should be freshly prepared for each demonstration. Furthermore, the mixture used to silver the flask should be flushed down the drain with water immediately after it is used.
Concentrated nitric acid is both a strong acid and a powerful oxidizing agent. Contact with the skin can result in severe burns. The vapor irritates the respiratory system, eyes, and other mucous membranes - concentrated nitric acid should be handled only in a well ventilated area.
Sodium hydroxide solutions can cause severe burns to the eyes, skin and mucous membranes. Silver nitrate is likewise irritating, and will stain the skin - these stains can be bleached by rinsing with an aqueous solution of sodium thiosulfate, (Na2S2O3) followed by water. If taken internally, silver nitrate can be toxic.
Concentrated ammonia can irritate the skin, and its vapors are harmful to the eyes and mucous membranes.
The solution used to silver the flask and any unused solutions should be flushed down the drain with water.
After the demonstration, the silver can be removed from the inside of the flask by dissolving it in 50 mL of 1 M nitric acid, and the flask may then be reused. Silver nitrate may be recovered from the acid solution by evaporating the acid.
- Organic Reactions
Type of Reaction:
- Shakhashiri, B.Z. (1992) Chemical Demonstrations - A Handbook for Teachers of Chemistry, vol.4, pp. 240-243.