Schiffs Test for Aldehydes

Preparation Sign in at www.cengage.com/login to read the MSDSs for the chemicals used or produced in this procedure. 

If Schiff s reagent is not available as a stock solution, prepare it by dissolving 0.01 g of p-rosaniline hydrochloride in 10 mL of water, adding 0.4 mL of saturated aqueous sodium bisulfite, and allowing the resulting solution to stand for 1 h. Then add 0.2 mL of concentrated hydrochloric acid to complete the preparation. 

Place 1 mL of Schiff s reagent in a small test tube and add 1 drop of the unknown. Compare any color changes with those observed when a known aldehyde and a known ketone are subjected to the same test. 

Neutralize the acidic solutions by adding solid sodium carbonate and pour the resulting solution into a container for hazardous aqueous waste. 

Another method for distinguishing between aldehydes and ketones is Tollens s test. A positive test indicates the presence of an aldehyde function, whereas no reaction occurs with ketones. Tollens s reagent consists of silver-ammonia complex, Ag(NH3)2, in an ammonia solution. This reagent oxidizes both aliphatic and aromatic aldehydes to the corresponding carboxylic acids silver ion is reduced to elemental silver, which is deposited as a silver mirror on the glass wall of a clean test tube. Thus, the formation of the silver mirror or of a precipitate is considered a positive test. Equation 25.12 shows the reaction that occurs. 

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See also in sourсe #XX -- [ , ]

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