Compound Interest: Making silver mirrors using chemistry

This week I was lucky enough to be sent a silvered bottle, shown in the graphic, by Andres Tretiakov. If you’re a chemist you’ll already be familiar with the chemical reaction used to produce this effect, but if you’re not you might be wondering how it’s accomplished. Though sadly it’s not one that can be easily carried out at home, this graphic gives an insight into the chemistry involved!

The silver that eventually coats the bottle originally comes from a solution of silver nitrate. In order to arrive at the perfect silver mirror, a few other reagents are necessary: ammonia and sodium hydroxide. The ammonia is necessary to react with the silver nitrate to produce something called ‘Tollens’ reagent’.

Tollens’ reagent is a clear solution of a diamminesilver(I) complex, often more simply referred to as ammoniacal silver nitrate. This reagent is commonly used in organic chemistry classes as a means to identify aldehyde compounds. When an aldehyde is added to Tollens’ reagent, the aldehyde is oxidised to a carboxylic acid, and simultaneously the diamminesilver(I) ion present in Tollens’ reagent is reduced to form metallic silver, producing the silver mirror effect. Commonly, dextrose (a form of glucose) is used.

Chemistry teachers will attest that the effect is easier described than accomplished. If just silver nitrate is used (without ammonia) then the silver forms more quickly, and often a suspension of silver is formed rather than coating the surface of the glass, leading to a black and cloudy appearance. This is why ammonia is used to form the diamminesilver(I) ion before reaction with an aldehyde – this ion is harder to reduce than silver ions, resulting in a slower, more controlled production of silver during the reaction. Clean glassware is also a necessity for a good silver mirror effect.

Another complication with this reaction is that the Tollens’ reagent must be prepared fresh, as it cannot be stored. This is due to the fact that, on standing, shock-sensitive silver nitride can be formed from the solution, and this can be an explosion risk. The risk is very real – in 2002 a school in the UK made up Tollens’ reagent too far in advance and in too high a quantity, leading to an explosion in which two students were injured. There are several good resources online that detail how the reaction can be carried out safely in a laboratory setting – some are provided in the resources linked below.

This reaction isn’t just used in chemistry classrooms – at one time, a variant of it was used to create mirrors. Justus von Liebig is credited with creating the process to make silver mirrors using the reduction of silver nitrate in 1835, and though different processes are used in the present day, it’s an interesting historical application of the same chemistry.

Edit 07-09-2017: The initial version of the graphic posted to this page had an incorrect caption under D-gluconic acid. This has now been corrected.

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