S4 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Tests. 



Aldehydes are easily further oxidised into the corresponding fatty 

 -acids containing the same number of carbon atoms and they conse- 

 quently behave as reducing agents. 



Reduction of Metallic Oxides in Alkaline Solution. 



(a) Stiver. 



An ammoniacal solution of silver hydroxide is prepared by adding 

 dilute ammonia to silver nitrate until the precipitate first formed 

 just re-dissolves. Some dilute aldehyde solution is added and the 

 mixture is placed in a cold water-bath and heated to the boiling- 

 point. A mirror of metallic silver forms on the glass. 



A very sensitive reagent may be prepared by mixing equal volumes of 

 10 per cent, silver nitrate and sodium hydroxide and then adding ammonia 

 drop by drop till the silver hydroxide dissolves. 



A mirror is formed immediately if the solution contains i per cent, of 

 acetaldehyde, in 30 seconds if i per thousand ; a yellow-brown mirror forms 

 in 5 minutes if i per 10,000 be present. 



() Copper. 



Dilute aldehyde solution reduces Periling' s solution x on warming 

 with the formation of cuprous oxide. 



Action of Sodium Hydroxide. 



Except with formaldehyde, benzaldehyde and a few other alde- 

 hydes, caustic soda solution decomposes dilute aldehyde solutions on 

 warming. Yellow to brownish-red resins which rise to the surface 

 aldehyde resin are formed. The liquid has usually a peculiar smell. 

 Aldehyde resin is insoluble in water, but soluble in alcohol and ether. 



Formaldehyde is converted into methyl alcohol and formic acid. 



Oxidation. 



Aldehydes are converted into the corresponding acid on warm- 

 ing their solutions with potassium bichromate and dilute sulphuric 

 .acid, the solution becoming green. 



The aldehyde may be identified by preparing the acid by oxidation. 



Schiffs Test. 



A solution of magenta, or fuchsin, is decolorised by bubbling 

 sulphur dioxide through it. On adding the dilute aldehyde solution 

 the purple-red colour returns. 



Numerous other sensitive tests have been described for aldehydes, 

 especially formaldehyde. The following one has been used more particularly 

 in testing for formaldehyde in distillates from plant leaves, etc. 



1 Fehling's solution consists of copper sulphate, caustic soda and Rochelle salt (sodium 

 potassium tartrate). On adding caustic soda to copper sulphate a blue precipitate of 

 cupric hydrate Cu(OH) 2 is formed, which turns black on boiling. The presence of the 

 Rochelle salt keeps the Cu(OH) 2 in solution forming a deep blue solution. This solution 

 does not keep, so that it must be freshly made for each experiment. For this purpose two 

 solutions are therefore kept. The one contains the copper sulphate, the other the Rochelle 

 salt and caustic soda. When required for use, equal parts of each are mixed together, and 

 this forms the reagent. 



