ESTIMATION OF FORMALDEHYDE 145 



2HCHO + KOH = CH.,OH + HCOOK 



Potassium formate 



5, Aldehydes react with ammonia to form additive 

 compounds ; thus acetic aldehyde undergoes the following 

 reaction : — 



CH3CHO + NH, = CH.,CHOHNH„ 

 Acetic aldehyde Aldehyde ammonia 



Here again formaldehyde behaves differently; if ammonia is 

 added to a formaldehyde solution, it is neutralized quantitatively 

 according to the equation 



6CH3O + 4NH., = (CH2)6N4 + 6H2O 

 Formaldehyde Hexamethylene tetramine 



with the formation of a crystalline solid which is used in 

 medicine under the name of urotropine. 



The reaction can be employed for estimating * the amount 

 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. 



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

 ^ ' 1000 2 ^^^ 



NH3 



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



6CH20(i8o) 



.-. -255 gram NH3 = -68 gram CH2O 



.•, 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 



* For another method of estimating formaldehyde by weighing the mercury 

 produced by the reduction of an alkaline solution of mercuric sulphite, see Feder : 

 " Archiv d. Pharm.," 1907, 245, 25. 



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