Silver mirrored bottles

[andrewtretiakov]

Silver Mirrored Bottles

One of my favourite qualitative organic chemical reactions is the test for aldehyde functional group. Since I first did it when I was about 16 years old I was transfixed and mesmerized. I've been demonstrating it to others in all sorts of glass containers small and large since then.

2 L RBF silver mirror

Equipment:
Round bottom flask, graduated cylinders, plastic pipettes, glass stirring rods, beakers, glass petri dishes, electronic balance, rubber stopper, wash bottle filled with distilled water


Chemicals:
Concentrated nitric acid, HNO₃ (16 M)
Acetone (CH₃)₂CO
Concentrated Ammonia solution (15 M)
Glucose solution, C₆H₁₂O₆ (0.5 M)
Silver nitrate solution, AgNO₃ (0.1 M)
Potassium hydroxide solution, KOH (0.8 M)


Warning:
After the demonstration, the silver residues and remaining solution MUST be disposed of with plenty of water immediately after silvering the flask. A highly unstable primary or contact explosive, fulminating silver, may form as small black crystals if the solution is left to stand for some time. Please see section below WASTE DISPOSAL.


Procedure:
Cleaning Procedure:
Firstly, we need to thoroughly clean the RBF otherwise the silver metal won't 'stick' to the inside glass wall. To accomplish this wash the flask with soap, rinse with dist. water and drain. Add about 10 mL of concentrated (conc.) HNO₃ into the flask place a rubber stopper firmly and shake for a few minutes covering all the inner walls. Pour the nitric acid in a 1 L beaker containing baking soda dissolved in water to neutralise the acid. Rinse with distilled water. Then add acetone about the same volume, 10 mL and swirl for a few minutes. Pour the acetone out. The flask is now clean and ready to be silvered!

The test itself is very straight forward. First, we make the Tollen's reagent or ammoniacal silver nitrate solution by adding conc. ammonia dropwise to 250 mL of 0.1 M silver nitrate solution until a brown precipitate first forms and then re-dissolves to give a clear, colourless solution. The solution contains now the colourless complex ion [Ag(NH₃)₂]⁺

The pH is raised again by addition of 150 mL 0.8 M potassium hydroxide solution. A dark brown almost black precipitate of silver(I) oxide forms straight away which re-dissolves with further addition of conc. ammonia drop by drop to give a clear, colourless solution.

Now that we have prepared the Tollen's reagent we need an aldehyde source to test, in this case a soluble reducing sugar (glucose). The 100 mL 0.5 M glucose solution (freshly made) is poured in a 2 L round bottom flask together with the Tollen's reagent, a stopper is firmly placed and the flask swirled around to make sure the whole of the inner surface of the flask is wetted. The solution turns black at first but with further swirling a mirror surface starts to appear.

NOTE: To avoid a lot of swirling around keep the flask warm by adding hot water from a kettle a few minutes before performing the reaction. Once the Tollen's reagent is ready pour the hot water down the sink. The reaction should take place much faster now.

Waste Disposal:
It has come to my attention that simply washing off the products down the sink with excess water is NOT proper or standard procedure (harmful to bacteria and marine/aquatic life). Any unreacted silver solution can be recovered by adding conc. HCl to form silver chloride ppt. and the salt recovered by gravity filtration. Also, an excess of glucose solution will guarantee that all the silver solution was reduced to silver metal. Furthermore, I always recycle the silver metal inside the flasks by adding 50 mL of 1 M nitric acid. The silver nitrated can now be recovered from the acid solution by evaporation and the flask can then be reused.


Explanation:
Reactions:
Formation of Tollens’ Reagent:
2 AgNO₃(aq) + 2 KOH(aq) → Ag₂O(s) + H₂O(l) + 2 KNO₃(aq)

Ag₂O(s) + 4 NH₃(aq) + H₂O(l) → 2 [Ag(NH₃)₂]OH(aq) (Tollens’ reagent)

Reduction of Tollens’ Reagent:
R-CHO(aq) + 2 Ag(NH₃)₂OH(aq) → R-COONH₄(aq) + 2 Ag(s) + 3 NH₃(g) + H₂O(l)
(reducing sugar)

The process is called electroless plating because no electricity is employed. The aldehyde group is a weak reducing agent which reduces the complex silver ion to silver metal and gets deposited on the inside wall of the flask. A 2 µm (micrometers) silver metal layer sticks to the glass. The aldehyde group in glucose oxidises to carboxylate ion, that is gluconate. In contrast, a ketone such as propanone (acetone) does not react.

Justus von Liebig is regarded to be the first person to use the silvering process or electroless process in 1835. This process is still used today to manufacture mirrors and Christmas baubles and other ornaments.


Pictures:
The video showing the procedure:

Larger collection

Here is also a microscale version with reduced concentrations and amounts of chemicals and less risk:

Etched with HF acid

collection of silver mirrored bottles

[mgritsch]

Hey Andrew,
Thanks for contributing an „article“ for our collection I was quite surprised that this beautiful classic was not yet there.

Re the format for articles we have some „publication standards“ that would at least require a list of used equipment and chemicals (incl hazard information) and a section that describes the exact „how to“ (concrete amounts, concentrations, procedure etc.), no matter how „straight forward“ An explanation of the reaction mechanism would also be great.

If you could add this, I would of course help you out with the formatting.

P.s.:

the silver residues and remaining solution MUST be disposed of down the sink

Well although this might be the usual practice it is actually not a best practice Ag Ions are highly toxic to bacteria and can cause trouble in the waste water treatment. There should be a more professional way of dealing with the waste…

[aliquis]

@andrewtretiakov, regarding environmentally friendly disposal/recycling of silver salt waste:
If you add glucose in excess, all of the silver ions should precipitate out as metallic silver. This can simply be filtered off and the filtrate can then be disposed of down the drain without any problems. Gentle warming promotes complete conversion. If instead ascorbic acid is added afterwards, the qualitative conversion also works in the cold and much faster (therefore not well suited for the silvering process itself).
However, it is still advisable to process the silver suspension defused in this way promptly and not to collect and store it if bioavailable reducing agents such as sugars, vitamin C or hydroxycarboxylates were used, because unfortunately the germ-killing effect of silver hardly prevents fungal growth.
The filter residue of finely distributed silver is actually much too good to dispose of. It can also be collected and - when enough has been gathered - melted down into a regulus (use ashless filter paper!) and monetized if desired. Professional users can use it to produce silver nitrate again - if required and economically reasonable.
Silver salt waste can also be precipitated as chloride (avoid complexing re-dissolution through adding excess conc. HCl!) and then reduced to metallic silver with nascent hydrogen (e.g. by adding zinc granules to the acidic solution).
Mild acidification is a good way to avoid the formation of explosive nitrogenous decomposition products and to gain some time when further processing of waste containing silver salts is not immediately possible or desired.

[lemmi]

Yes, a very beautiful experiment! It would be nice to have more detailed information about the concentrations and volumes of the employed reactant solutions.

and swirled around to make sure the whole of the inner surface of the flask is wetted.

Does the swirling not affect the formation of the mirror layer?
I remember ther Glass surface has to ve cleaned very carefully bevore pouring in the reactant solutions

About a 2mm silver metal layer sticks to the glass.

Thus ist probably an typographic error, isn't it? The silver layer cannot reach a thickness of 2 mm. Perhaps 0,2 mm?

[Glaskocher]

2mm... ?
... two micrometers would be possible.

[andrewtretiakov]

Thank you all for the comments!
I'm fairly new to the forum so still making loads of mistakes and trying to find my way around and adjust to the templates.

I'll be editing the post so please be patient.

Thank you again,
A

[andrewtretiakov]

Thanks, I've been editing the article now so hopefully you'll get your answers.

No, it does not. The swirling is necessary to make sure the whole or as much as possible of the inner glass walls are covered by the silver metal which build on layer upon layer.
I remember ther Glass surface has to ve cleaned very carefully bevore pouring in the reactant solutions
Yes, you are correct. I added that section now. The inner glass needs to be extremely clean or else the silver won't stick to it. I even tried silvering plastic bottles with mixed results.

About a 2mm silver metal layer sticks to the glass.

Yes, sorry again. I made a mistake and I corrected it. Approx. 2 micrometres thick layer.

[mgritsch]

CLEANING PROCEDURE:
[...] Add about 10 mL of concentrated (conc.) HNO₃ into the flask place a rubber stopper firmly and shake for a few minutes

in your experience, ist this part of the cleaning procedure really necessary? Which kind of contamination are you targeting here?
As nitric acid is a bit difficult to come by for most users, do you see any alternatives?

[MarbsLab]

Conc. nitric acid is commonly used to clean glass. It does this by leeching the ions from within the surface of glass. From my experience, 37% hydrochloric acid should also work.

Cheers

Markus

[lemmi]

As nitric acid is a bit difficult to come by for most users, do you see any alternatives?

Römpp and Raaf in their classical " Chemische Experimente die gelingen" even recommend dichromate in conc. sulfuric acid!

I did this once in a test tube, using simple household detergent and hot water to clean the glass before carrying out the experiment. It worked perfectly.

Historical the silver mirrors were introduced by Justus Liebig. Initially the had to face some competition by the elder method of producing mirrors, which consisted in covering the glass by a layer of tin foil and pouring liquid mercury on top. After standing overnight, the excess mercury was pressed out of the amalgam formed by applying manual pressure, and the thus obtained layer of tin amalgam was covered with barnish. In some old buildings you can find these mirrors up top the present day. The have a somewhat greenish shine.

[mgritsch]

I did this once in a test tube, using simple household detergent and hot water to clean the glass before carrying out the experiment. It worked perfectly.

This also matches my experience although simple little test tubes might be much easier to work with than large vessels.

If the key issue ist the wettability of the glass surface to allow a perfect contact of the tollens reagent, I would rather think about using ethanolic KOH than an acid (after doing the usual degrease with acetone). Nothing makes for a better wettability than this reagent, it can even get rid of traces of silicone that are usually really hard to come by.

[aliquis]

Nitric acid is only necessary for re-dissolving the silver coating afterwards if required, or - where not available - a mixture of conc. HCl and alkali nitrate can also be used instead for that purpose.

For cleaning (degreasing) the inside of the glassware beforehand simple acetone is well suited.

[lemmi]

@andrew: thank you for re-editing the article!
The article will be reviewed by one of the moderators and then passed to the corresponding section. This will take some time.

[mgritsch]

How about some copper mirror?

[MarbsLab]

And now potassium!