Question

How do you carry out Tollens' reagent test?

Answer 1

Here's how you do it…

Prepare the reagent

Prepare the reagent by adding `"NaOH"` solution dropwise to a solution of `"AgNO"_3` until a precipitate just forms; then add dropwise just enough dilute aqueous `"NH"_3` to dissolve the precipitate.

Do the test

To carry out the test, add a few drops of the unknown to the freshly prepared reagent in a clean test tube, and warm gently for a few minutes in a hot water bath.

The formation of a grey-black precipitate or a silver mirror on the inside of the test tube is a positive result.

Why it works

The Tollens reagent is a colourless solution of `"Ag"("NH"_3)_2^+` that is prepared in a two-step procedure.

1. `"NaOH(aq)"` is added to `"AgNO"_3("aq")`.

`"2AgNO"_3 + "2NaOH" → "Ag"_2"O" + "2NaNO"_3 + "H"_2"O"`

2. Aqueous `"NH"_3` is added dropwise until the precipitated silver oxide dissolves completely.

`"Ag"_2"O"+ "4NH"_3 + "H"_2"O" → "2Ag"("NH"_3)_2^+ + "2OH"^"-"`

3. The `"Ag"("NH"_3)_2^+` oxidizes aldehydes to the corresponding carboxylic acids, which are converted to carboxylate ions in the basic medium.

`"RCHO"color(white)(l) + "2Ag"("NH"_3)_2^+ + "3OH"^"-" → "RCOO"^"-" + "2Ag(s)" + "2NH"_3 + "2H"_2"O"`

If you are lucky, a silver mirror forms on the inside of the test tube; if you are unlucky, you get a grey-black precipitate of silver.

The Tollens reagent does not oxidize ketones, so the Tollens test distinguishes aldehydes from ketones.