62 ALDEHYDES AND ALCOHOLS 



of formaldehyde in a solution by adding a known excess of 

 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. 



^0 17 

 But ^0 c.c. N/2 ammonia contain — — X -^ = -255 gram 

 ■^ ' 1000 2 



NH3, 



and since from the equation 4NH3 (68) are equivalent to 



6CH2O (180) 



.-. -225 gram NH3 = -68 gram CHgO, 



.-. 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 :— 

 CH3CHO + HNaSOg = CHjCHOHSOsNa 



7. Aldehydes also form additive compounds with hydrogen 



cyanide ; these compounds are known as hydroxycyanides or 



cyanohydrins : — ■ 



CH3CHO + HCN = CHsCHOHCN 



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 : — 



CH3CHO + NH2OH = CH3CH : NOH + H2O 

 CH3CHO + CgHjNHNH, = CH3CH : N . NHCeHj + HjO 



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

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

 teristic crystalline form and melting-point, which may be 



