ALDEHYDES 85 



Rimini's Test. 



A small quantity (2 drops) of phenylhydrazine is added to the solution, 

 then a drop of dilute freshly prepared sodium nitroprusside solution and a 

 few drops of sodium hydroxide solution. A deep blue colour forms if form- 

 aldehyde be present ; the colour changes through green and brown to red. 



Schryver has modified this test and made it more sensitive : 2 c.c. of 

 a freshly prepared and filtered i per cent, solution of phenylhydrazine 

 hydrochloride are added to 10 c.c. of the solution of formaldehyde, then i c.c. 

 of a 5 per cent, solution of sodium ferricyanide and 5 c.c. of hydrochloric acid ; 

 a magenta colour is formed. This test will show the presence of i part oi 

 formaldehyde in 100,000 to 1,000,000 parts of solution. No colour is given 

 by acetaldehyde. 



ESTIMATION. 

 Formaldehyde. 



(a) By Converting into Hexamethylenetetramine. 



25 c.c. of normal ammonium hydroxide solution are placed in a 100 c.c. 

 strong bottle provided with a rubber stopper. A measured volume of the solution 

 (not containing above -5 gm. of formaldehyde) is added. The cork is securely 

 fastened by tying and the bottle is submerged in a cold water-bath which 

 is then heated to boiling for i hour, the bottle being kept under water the 

 whole time. The bottle is cooled, opened and the contents titrated with 

 standard acid until the methyl orange which is used as indicator first becomes 

 red. 



A series of bottles should be taken containing different amounts, or none,. 

 of the aldehyde solution. Allowing for the blank each c.c. of normal am- 

 monium hydroxide used corresponds to *o6oi gm. of formaldehyde. 



The estimation should be carried out in water. The formaldehyde is 

 therefore distilled from its original solution, e.g. milk, plant extracts, and the 

 distillate is used. 



(b) By Titrating with Iodine and Sodium Thiosulphate. 



A known volume of the solution (10 c.c.) is mixed with 25 c.c. of -iN iodine 

 solution, and sodium hydroxide is added drop by drop till the liquid becomes 

 clear yellow. The flask is closed for 10 minutes, dilute hydrochloric acid is 

 added, and the free iodine is titrated with -iN thiosulphate. 



2 atoms of iodine = i molecule of formaldehyde. 



Good results are not given by this method for aldehydes other than form- 

 aldehyde. 



Acetaldehyde. 



By Combination with Sulphite. 



The solution of sulphite is prepared by dissolving 12*6 gm. of sodium 

 sulphite in 400 c.c. of water, adding 100 c.c. of -iN sulphuric acid and diluting 

 to 1000 c.c. with the purest ethyl alcohol of 95 per cent. 



The volume of aldehyde solution, not containing more than 2 per cent, 

 of aldehyde, is placed in a 100 c.c, measuring flask. A known volume of 

 the sulphite solution is added and the mixture diluted to 100 c.c. with the 

 purest 50 per cent, alcohol. A blank with the reagents is carried out simul- 

 taneously. 



The flasks are kept at 50 for 4 hours, cooled and titrated with standard 

 iodine solution, using starch as indicator. 



Each c.c. of -iN iodine solution corresponds to -0032 gm. of SO 2 or -0022 

 gm. of acetaldehyde. 



