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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.
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