Silver Mirror in a Flask

Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves
Performers Required: 1
Equipment	Chemicals
1 L Florence flask with stopper	Silver nitrate (AgNO3)
Three 500 mL bottles (one amber glass, labelled Solution A, two plastic, labelled Solutions B and C, respectively)	Potassium hydroxide (KOH)
One plastic dropper bottle for NH4OH solution	Sucrose (C12H22O11)
One 1 L plastic bottle for waste neutralization, labelled “Neutralization Solution ”	Nitric acid (HNO3), 16 M
10 and 50 mL graduated cylinders	Ethanol (EtOH), ≥95% solution
3 mL plastic pipette	Ammonium hydroxide (NH4OH), 6 M
Wash bottle filled with deionized water	Hydrochloric acid (HCl), 1 M
Filtration apparatus (for recycling silver)	Ascorbic acid (C6H8O6)

Procedure:

1.) Preparation of solutions:

Solution A: Dissolve 5 g AgNO₃ in 200 mL deionized H₂O (0.15 M Ag(NO₃). Store in a 250 mL amber glass bottle.

Solution B: Dissolve 18 g KOH in 400 mL deionized H₂O (0.8 M KOH). Store in a 500-mL plastic bottle.

Solution C: Dissolve 40 g sucrose, 1.75 mL HNO₃, and 50 mL EtOH in 400 mL deionized water. Boil this solution in a beaker on a hot plate for 30 minutes before use, or allow it to age for one month at room temperature. Store in a 500-mL plastic bottle.

Waste neutralization solution: Fill 1/4th with water and 1 mL conc. HCl

2.) Make sure that the flask to be silvered is absolutely clean, otherwise the mirror will not deposit properly.

3.) Add 20 mL of Solution A (AgNO₃) to the flask, and then titrate the solution with 6 M NH₄OH while swirling the flask constantly. This will initially form a brown precipitate, but upon further addition the precipitate will clear. Stop adding NH₄OH as soon as the precipitate clears.

4.) Add 10 mL of Solution B (KOH) to the flask; this will produce a cream-colored precipitate. Titrate again with NH₄OH, but this time stop the addition just before the precipitate clears (see Notes).

5.) Add 1 mL of Solution C (sucrose) and quickly stopper the flask and begin swirling the solution vigorously, constantly coating all of the interior surfaces. Make sure to hold the stopper in place with one hand while shaking the flask. The flask will start to become darker, and then appear black, but continued swirling will result in the deposition of a shiny silver mirror on the interior surface of the flask.

6.) After 1-2 minutes of shaking, the silver is completely deposited. Remove the stopper and pour the solution into the 1-L bottle with the waste neutralization solution, which will result in a cloudy white precipitate of silver chloride (AgCl) mixed with some gray metallic silver (Ag) that did not deposit on the walls. Use the wash bottle to rinse the inside of the flask thoroughly (4-5 times), with all rinseates being collected in the waste neutralization bottle.

Reduction of AgCl with Dextrose

1. Dissolve 0.54 g (30% excess) NaOH in 25 mL water. Add 1 g AgCl then 0.735 g (15% excess) dextrose. Cover with watch glass and heat to boiling for +15 min
2. Allow clear, tea-colored solution to cool. Filter silver pellets with Buchner funnel and filter paper. Wash with water then test filtrate for remaining Ag by by adding salt water (NaCl). If ppt, Ag remains. If no ppt, check pH < 10 then dispose into sink.
3. Dissolve with minimum quantity of HNO₃ (a small amount may not dissolve) then boil off the HNO₃. Dissolve AgNO₃ in water and filter out undissolved solids ( probably AgCl) with a fine membrane filter. KEEP FILTRATE.
4. Wrap beaker with filtrate in foil to minimize photoreduction. Boil off water from filtrate to get solid AgNO₃.

Clean-up: After the show, the mirror can be removed from the flask by dissolving it with HNO₃. This must only be done inside a chemical fume hood, as it generates highly toxic NO₂ gas. Once the silver is dissolved out, allow the nitric acid solution to evaporate, leaving crystals of AgNO₃ that may be reused in future demonstrations.

Hazards: HCl, HNO₃, KOH, and NH₄OH are highly corrosive, and will cause chemical burns on contact. NH₄OH also gives off noxious NH₃ fumes, and should be handled with care. AgNO₃ solutions will stain skin/clothing/anything dark brown or black. If allowed to evaporate, basic solutions containing NH₄OH and AgNO₃ can produce black crystals of Ag₃N, a highly-unstable, shock-sensitive explosive compound. Always pour the spent silvering solution and flask rinsings into the waste neutralization container, as this prevents the formation of Ag₃N. When the waste neutralization container is full, let solids settle then decant the liquid into the sink.

Principle: This demonstration uses the chemistry of Tollens’ test for aldehydes to produce a silver mirror on the inside of a glass flask. AgNO₃ reacts with NH₄OH in basic solutions to form diamminesilver cations ([Ag(NH₃)₂]⁺), which can be reduced by aldehydes or α-hydroxyketones to form metallic silver (Ag). The sucrose is slowly hydrolyzed in acidic solution (or quickly at elevated temperature) to produce glucose and fructose, which have aldehyde and α-hydroxyketone functional groups, respectively, in their fleeting open-chain conformations. These sugars reduce the diamminesilver into metallic silver, some of which gets deposited on the surface as a silver mirror. The remaining silvering solution is added to a bottle containing HCl, which will react with any Ag⁺ ions in solution to form the notoriously insoluble compound AgCl. By doing this, the possibility of forming explosive compounds is completely eliminated, and all of the silver is reclaimed as a solid that can be recycled for future use.

Notes: The first titration can be done up to one hour before the start of the demonstration. This decreases titration time. The second titration step requires a bit of practice to know when it is complete. As the titration proceeds the precipitate will begin to dissolve slowly, and the color change will not be apparent until the last few drops of the titration. Over-titrating is not the end of the demonstration, but it will cause less of the silver to precipitate out in the mirror. The final titration can be sped up by increasing the volume of solution C. Cold weather will adversely affect the speed of this reaction.The titration can be reversed by adding more of Solution A. Once all of Solution A is used up, a large portion of the silver can be reclaimed from the waste neutralization solution; the solid Ag and AgCl are collected by filtration and washed with water to remove any excess Cl^-, and then redissolved in NH₄OH. Add to this solution an excess of ascorbic acid (C₆H₈O₆), which will cause all of the silver to immediately precipitate out as a gray flocculant powder without depositing on the walls of the vessel. The solid Ag is then collected by filtration and washed with water again, and can be converted back to AgNO₃ using the method described in the Clean-up section, taking appropriate precautions. If the silver mirror is intended to be kept in the flask for any length of time, it is best to cover the silver with an impermeable barrier (varnish, nail polish, paint, etc.) to prevent tarnishing and protect the mirror from scratches.