Diamminesilver (I)/Tollens’ Reagent (Makin’ mirrors)
6th July 2006
Inspired by yesterday’s entry, which wasn’t really a molecule proper, today’s entry is about the diamminesilver (I) complex, better known as Tollens’ Reagent. In practice, this is usually generated by taking a solution of silver (I) nitrate and adding a drop of NaOH solution. This generates some Ag2O, or silver (I) oxide. Addition of aqueous ammonia will dissolve the silver oxide, generating the diamminesilver (I) complex. Why do we care?
As it turns out, silver (I) was happier being silver (0) (metallic silver, that is) all along. This means that it is pretty easy to get it to accept an electron from something. That is, silver (I) is easily reduced (accepts an electron easily) and is thus a good oxidizer (Oxidizers are always reduced in the process of oxidizing something else - that is, they accept an electron. The converse is true of reducing agents.) This is why silver (I) nitrate stains your skin so easily.
The diammine complex is also a mild oxidizer. Traditionally, it was used as a qualitative test for aldehydes. These were very important in the past before things like NMR supplanted such tests. The silver complex oxidizes the aldehyde to a carboxylic acid. The classic example is glucose:
As I’ve alluded to, you could say that the silver (I) oxidizes the glucose, or the glucose reduces the silver (I). Sugars with aldehyde ends like this are called reducing sugars. Sucrose (table sugar), for example, isn’t.
Finally, my favorite part about Tollens’ reagent is the silver it deposits. If you do it just right, you get a beautiful silver mirror deposited on the reaction vessel. Here is a photo, and here is a movie.
July 10th, 2006 at 4:13 am
Aw, shoot, so I can’t make mirrors just by dumping silver into lye? ‘Cos I have some silver (which is worth like $11/oz these days) and some lye, but no nitric acid, alas. :-/
July 10th, 2006 at 10:15 pm
Silver nitrate should be available commercially to Joe Sixpack. “Clear,” unscented cleaning “ammonia” is usually a pure solution of NH3 in water. There may be detergents. The generics are probably the most likely to be pure ammonia. Be very careful with the Ag(NH3)2+ complex, it becomes explosive after some (tens of?) hours. You could probably precipitate the silver as AgCl by adding salt, or you could just flush the whole mess down the drain, which might not be quite kosher these days, I’m never sure. Note that sucrose won’t work. Fructose will, but glucose will probably work better. You probably know about the little diabetic glucose/dextrose tablets, or I bet it’s at the grocery store. Acid treatment of sucrose will yield invert sugar, which is the partial glucose/fructose hydrolysis product of sucrose. Careful of chloride ion if you use HCl as your acid, since AgCl is almost entirely insoluble. Cheap artificial vanilla contains plenty of aldehyde, which will presumably work.