I have used many silver mirror/Tollen's test labs. I have struggled with some and over the years I have found that this version is very reliable if the directions are followed carefully. Timing is very important too. If you choose to use glassware other than the ornaments, I will tell you that they must be very clean. Sometimes residues from other chemicals will interfere so that the silver does not adhere to the glass. As I said, I have used many versions. I have kept many of them in my paper files and on my computer. I found four different versions in my digital files and of course, none of them have author names on them. I cannot take credit for any of the details, but what I have written is a compilation of many versions. You can find kits available through the usual chemistry education vendors if you wish to go that route. If you are preparing your own silver nitrate solution, you should mix it within a couple of days of the activity.
Have students work in with one or two partners to create one silver plated ornament at a time. Helping each other is important to complete the ornament quickly quickly. Then the group can go through the steps again until all the students have their own.
Students can follow steps #1-3 at any lab counter. It is important that they keep all solutions labeled so they can keep track of them as they move to the fume hood.
Under the fume hood, students will add drops of concentration ammonium hydroxide per directed, add the KOH to the beaker, add more drops of ammonium hydroxide if required and then finally transfer the solution from the beaker into the ornament. Some students may opt to use a funnel for that last transfer.
Cover the opening with two pieces of parafilm and firmly press down while gently swirling. Within minutes, you will see a silver coating appear.
Be sure to follow the procedure carefully and you will have a beautiful finished product.
oxidation-reduction, Tollen's Test
15 minutes of prep time and 20 minutes of class time if you have multiple hoods available for students to use. I only have one hood, so I have another activity for students to complete while two groups rotate through the lab over two 60 minute class periods.
Clear glass ornament
3 - 150 mL beakers
10 mL graduated cylinder
2 - 50 mL graduated cylinder
2 - Disposable pipettes
Parafilm (two small pieces to cover opening of ornament) or stopper to fit the ornament (must be a good seal to avoid leaks)
Container to store and protect the ornament from breaking on the way home.
.25 M dextrose solution (10 mL per ornament)
.8 M potassium hydroxide (KOH) (15 mL per ornament)
.1 M silver nitrate solution (AgNO3) (30 mL per ornament)
Concentrated ammonium hydroxide (NH4OH) (10 mL per ornament)
Acetone or isopropyl alcohol (10 mL per ornament)
We will create a silver coating on the inside of a glass ornament by using the “silver mirror test” or “Tollen’s test”. We will mix AgNO3 (aq) with NH3 (aq) to produce a solution known as Tollen’s reagent. The Tollen’s test is used to qualitatively identify aldehydes. The reagent contains the silver diammine ion Ag(NH3)2+. Although this ion is a very weak oxidizing agent, it will oxidize the aldehyde function group (-CHO) of dextrose, a sugar, to a carboxylate ion (-COO-). As this oxidation occurs, silver is changed from Ag+ to solid silver which is deposited on the glass.
CH2OH(CHOH)4CHO + 2[Ag(NH3)2]+ + 3OH- → 2Ag(s) + CH2OH(CHOH)4COO- + 4NH3 + 2H2O
CH2OH(CHOH)4CHO = dextrose
[Ag(NH3)2]+ = silver diammine ion
PROCEDURE (Read the procedure through before proceeding. Timing is important. You do not want to have much time elapse between steps. The NH4OH is under the hood and only two beakers are available at any given time. It is important to be patient and take turns with classmates so that no one ends up waiting between steps. Work with a partner and help each other go through the steps twice so that you each have an ornament to take home. Have all of your materials close to the hood, so that you can get to them quickly. Once you are done adding chemicals, go elsewhere to shake the container.)
1. Set a clean glass ornament (with the metal hanger removed) on top of a 150 mL beaker. Add 10 mL of the .25 M dextrose solution to the ornament.
2. Measure 15 mL of .8 M potassium hydroxide (KOH) and set aside for step #5.
3. Add 30 mL of .1 M silver nitrate (AgNO3) solution to a 150-mL beaker.
4. While stirring, add concentrated ammonium hydroxide (NH4OH) dropwise to the silver nitrate solution in the beaker until the gray black silver hydroxide (AgOH) precipitate forms. Continue adding concentrated NH4OH dropwise with swirling until the silver diammine ionic complex (Ag(NH3)2+ forms and the solution JUST becomes clear and colorless.
5. Add the 15 mL of KOH measured in step #2. The silver hydroxide solid usually precipitates again, so add NH4OH dropwise with swirling until the solution clears.
6. Pour the contents of the beaker into the glass ornament. Cover the opening with two layers of parafilm (stretch to seal) or a rubber stopper. Put your finger over the parafilm (stopper) and swirl gently so the liquid contacts the entire inner surface of the ornament. Continue to swirl and within 5 minutes, the entire ornament will be coated with a silver mirror surface.
7. It is very important to pour the remaining liquid down the drain with plenty of water. Rinse the flask gently but thoroughly with water. Then rinse carefully with about 10 mL of acetone or isopropyl alcohol to help it dry quickly. You may insert the metal hanger and take it home. BE CAREFUL! I recommend putting it into a box for the trip home.
You can protect the inside of your flask from tarnishing by coating it with a clear varnish or paint.
1. What is an aldehyde?
2. What is the specific aldehyde used in this activity?
3. What does a “positive” Tollen’s test look like?
4. Is the change in oxidation of the silver in this activity an oxidation or a reduction?
5. What two reactants in the laboratory are used to produce the silver diammine ion?
6. Look at the reactants in net ionic equation listed. Which reactant in this activity provides the OH-?
Mix the required solutions a day before the activity. If preparing your own solutions from the solid reagents, fresh solutions will be most reliable.
As I said, I have used many versions. I have kept many of them in my paper files and on my computer. I found four different versions in my digital files and of course, none of them have author names on them. I cannot take credit for any of the details, but what I have written is a compilation of many versions. An internet search will yield many versions along with some YouTube videos.