148 METHODS OF ANALYSIS [Chap. 



1 9 Leach Method. 



Mix about 5 cc. of the distillate, obtained under 16, with an equal volume of pure 

 milk in a porcelain casserole and add about 10 cc. of concentrated hydrochloric 

 acid, containing 1 cc. of 10% ferric chlorid solution, to each 500 cc. of acid. Heat 

 to 80°-90°C. directly over the gas flame, rotating the casserole to break up the curd. 

 A violet coloration indicates formaldehyde. 



Rimini Method.'' 



20 Phenylhydrazin Hydrochlorid and Sodium Nitro-prussid Test. 



This method may be applied directly to liquid foods, to an aqueous or alcoholic 

 extract of solid foods, or to the distillate prepared as directed in 16. In the case 

 of milk, apply the method directly. In the case of meat, comminute the sample, 

 extract with 2 volumes of hot water, and employ the expressed liquid for the test. 

 Heat fats above their melting point with 10 cc. of alcohol, shake thoroughly, cool, 

 filter through a moistened filter, and use the filtrate for the test. 



Dissolve a lump of phenylhydrazin hydrochlorid about the size of a pea in 3-5 

 cc. of the liquid to be tested, add 2-4 drops (not more) of a 5-10% sodium nitro- 

 prussid solution and 8-12 drops of an approximately 12% sodium hydroxid solution. 

 If formaldehyde is present, a green or blue color develops depending upon the 

 amount. When formaldehyde is present to the extent of more than 1 part in 70,000- 

 80,000 in the solution tested, a distinct green or bluish green reaction is obtained. 

 In more dilute solutions the green tint becomes less marked and a yellow tinge tend- 

 ing toward greenish brown develops. 



With this method acetaldehyde and benzaldehyde give a color varying from 

 red to brown, according to the strength of the solution. A reaction may there- 

 fore be obtained with these aldehydes similar to that obtained with formaldehyde 

 in solutions more dilute than 1 part in 70,033. The presence of acetaldehyde or 

 benzaldehyde together with formaldehyde gives a yellowish or yellowish green 

 tinge. The reaction for formaldehyde may therefore be masked by the presence 

 of other aldehydes, but is characteristic when a clear green color is obtained. 



21 Phenylhydrazin Hydrochlorid and Potassium Ferricyanid Test. 



Proceed as directed in 20, substituting a solution of potassium ferricyanid for 

 the sodium nitro-prussid. Formaldehyde gives a red color. Alcoholic extracts 

 from foods must be diluted with water to prevent the precipitation of potassium 

 ferricyanid. The test is not applicable in the presence of the coloring matter of 

 blood. 



22 Phenylhrjdrazin Hydrochlorid and Ferric Chlorid Test. 



Treat 15 cc. of milk or other liquid food or of the distillate, prepared as directed 

 under 16, with 1 cc. of a dilute phenylhydrazin hydrochlorid solution, then with 

 a few drops of dilute ferric chlorid solution and, finally, with concentrated hydro- 

 chloric acid. The presence of formaldehyde is indicated by the formation of a 

 red color, which changes after some time to orange yellow. 



Milk may be examined directly by this method, but more delicate tests may be 

 obtained from the distillate from milk or from milk serum. Acetaldehyde or benz- 

 aldehyde does not interfere with the reaction. 



