66 ALDEHYDES AND ALCOHOLS 



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 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. 1-66) and 5 c.c. of Schiff's 

 reagent. An intense violet colour having an absorption band 

 in the orange is produced. Schiff's 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. 1*3), and 



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

 t Deniges : " Compt. rend.," 1910, 150, 529. 



