FORMALDEHYDE 59 



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 

 produced. The test is a very delicate one and will detect 

 quantities of formaldehyde varying from I part in 1,000,000 

 to I part in 100,000. Acetic aldehyde gives no colour with 

 this reagent. 



The following test, due to Deniges,f is sensitive for formal- 

 dehyde, even in presence of acetic aldehyde up to 2 per cent ; 

 5 c.c. of an aqueous solution of formaldehyde are mixed with 

 1*2 c.c. of pure sulphuric .acid (sp. gr. I '66) and 5 c.c. of Schiff's 

 reagent. An intense violet colour having an absorption band 

 in the orange is produced. SchifFs reagent may be prepared 

 by adding a litre of O'OI per cent of solution of magenta to 

 20 c.c. of sodium hydrogen sulphite solution (sp. gr. I -3), and 

 after five minutes adding 20 c.c. of hydrochloric acid (sp. gr. 

 I-I8). 



KimpflinJ tested for formaldehyde in the leaf of Agave 

 mexicana by injecting into it, by means of a capillary tube, a 

 concentrated solution of sodium hydrogen sulphite, contain- 

 ing an excess of /methylamino-^ecresol. The presence of 

 formaldehyde was indicated by the formation of a red pre- 

 cipitate on exposure to light. The precipitate is best seen 



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

 tDenige"s: "Compt. rend.," 1910, 150, 529 

 JKimpflin: id., 1907, 144, 148. 



