ALDEHYDES 55 



standardized ammonia solution, and after some time titrating 

 back the excess of ammonia by means of standard acid, using 

 litmus as indicator. 



Thus, for example, if 25 c.c. of the formaldehyde solution, 

 after shaking with 50 c.c. of N/2 ammonia, required for 

 neutralization 20 c.c. N/2 hydrochloric acid, then the amount 

 of ammonia used up by the formaldehyde would be 50- 20 = 

 30 c.c. 



But 30 c.c. N/2 ammonia contain ~ x = -255 gram 



NH 8 



and since from the equation 4NH 3 (68) are equivalent to 



6CH 2 O(i8o) 



' *255 gram NH 3 = '68 gram CH 2 O 

 .-. 25 c.c. of the solution contained 0*68 gram formaldehyde. 



6. With sodium bisulphite aldehydes form crystalline 

 addition compounds which, being sparingly soluble in water, 

 can be used for isolating aldehydes from mixtures. 



Thus if some saturated sodium bisulphite solution be added 

 to a fairly strong solution of aldehyde and the mixture shaken 

 vigorously, a rise in temperature takes place accompanied by 

 the formation of a white crystalline precipitate : 



CH 3 CHO + HNaSO 3 = CH 3 CHOHSO 3 Na 



7. Aldehydes also form additive compounds with hydrogen 

 cyanide ; these compounds are known as hydroxycyanides or 

 cyanohydrins : 



CH 3 CHO + HCN = CH 3 CHOHCN 



Acetic aldehyde cyanohydrin 



8. Aldehydes form crystalline compounds with hydro- 

 xylamine, phenylhydrazine, and semicarbazide ; in all cases 

 water is split off between the two reacting substances. 



CH 3 CHO + NH 2 OH = CH 3 CH : NOH + H 2 O 

 CH 3 CHO + C 6 H 5 NHNH 2 = CH*CH : N NHC 6 H 5 + H 2 O 



The resulting compounds, which are known as oximes, hydra- 

 zones or semi-carbazones, are usually substances with a char- 

 acteristic crystalline form and melting point, which may be 

 employed for the identification of the corresponding aldehydes. 

 The use of phenylhydrazine for the identification of the sugars 

 has already been described. 



