There are three common tests for aldehydes. The first is the Tollens test, the second is the Schiffs test and the third is a test using Brady's Reagent (2,4-dinitrophenylhydrazine). The Brady's test test for the carbonyl group, so it can give a positive test for ketones as well.
Tollens Test
A 1 molar solution of silver nitrate is mixed with sodium hydroxide solution, in equal volumes. A brown precipitate is formed. Add ammonia, drop by drop, until the precipitate just dissolves. A few drops of the aldehyde are added to approximately 3cm3of the reagent in a test tube, and warm carefully in a water bath. A silver mirror is formed around the inside of the tube.
Schiffs Test
Place 2ml of Schiff's reagent in a test tube and add 2 drops of the aldehyde. Shake the tube gently, and observe the colour that is developed. Aldehydes react with Schiff's reagent to form a complex which has a wine-purple colour.
Brady's Reagent
Add a solution of 1 or 2 drops of Aldehyde in 2ml of 95% ethanol, to 3ml of Brady's Reagent. Shake vigorously and allow the solution to stand for about 20 minutes. An orange preciptate formed is a positive test
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