DETECTION OF FORMALDEHYDE 149 



Its peculiar behaviour towards ammonia, resulting in the 

 formation of hexamethylene tetramine, has already been 

 mentioned ; this substance, which is used under the name of 

 urotropine, is a crystalline base which dissolves in hot or cold 

 water ; with bromine it forms an additive compound — tetra- 

 bromo-hexamethylene tetramine (CH2),.N^Br4— which has been 

 used for detecting small quantities of formaldehyde in solu- 

 tion (see p. 150). 



Formaldehyde also reacts with ammonium salts as well as 



with free ammonia, as follows : — 



6CH2O + 4NH4CI = (CH2)6N, + 6H2O + 4HCI 

 Hexamethylene tetramine 



This reaction has been made use of as a means of estimating 

 ammonium salts in solution by titrating the amount of free 

 acid liberated according to the above equation on adding suffi- 

 cient formaldehyde to a solution containing ammonium salts. 

 For this purpose both the formaldehyde solution and the solu- 

 tion to be analysed must be previously neutralized, if necessary. 

 An excess of the neutralized formaldehyde solution is then 

 added to a known volume of the solution containing the 

 ammonium salts, and after thoroughly shaking for one or two 

 minutes the amount of acid set free is determined by titration 

 with standard caustic soda, using methyl orange as indicator ; 

 the amount of ammonia can be calculated from the fact that 

 each 36-5 grams of hydrochloric acid liberated correspond to 

 17 grams of ammonia. 



The reactions most suitable for characterizing small quan- 

 tities of formaldehyde are as follows : — 



Rimini's test consists in adding 2 drops of phenylhydra- 

 zine hydrochloride, 2 drops of sodium nitroprusside solution, 

 and I c.c. of sodium hydroxide to i c.c. of the liquid to be 

 tested. A blue colour is formed, which changes rapidly 

 through green and brown to red. Schryver* has modified 

 this test and made it much more sensitive ; he recommends 

 the following method: to 10 c.c. of the liquid to be tested 

 add 2 c.c. of a i per cent solution of phenylhydrazine hydro- 

 chloride freshly made up and filtered ; then add i c.c. of a 5 

 per cent solution of sodium ferricyanide, also freshly made up, 

 and 5 c.c. of hydrochloric acid ; a brilliant magenta colour is 

 * Schryver: " Proc, Roy. Soc, Lond.." B., 1910, 82, 226. 



