FOOD PRESERVATIVES. 



181 



The same difficulty may often be avoided, and in fact the extraction with 

 gasoline of the dry residue from the ether extraction may sometimes be obviated, 

 by precipitating before extraction with ferric chlorid or calcium chlorid, making 

 alkaline, and filtering. By this means tannin is entirely separated from the 

 product and other substances whose color masks the salicylic-acid reaction are 

 often removed. 



2. Benzole Acid, 

 (a) QUALITATIVE DETECTION. 



Separate benzoic acid as directed for salicylic acid. If benzoic acid be 

 present in considerable quantity, it will crystallize from the evaporated ether 

 in shining leaflets with characteristic odor on heating. Dissolve the residue in 

 hot water, divide into two portions, and test by the following methods : 



(1) FIRST METHOD. 



Make the residue alkaline with ammonium hydroxid, expel the excess of 

 ammonia by evaporation, take up the residue with water, and add a few drops 

 of a neutral 0.5 per cent solution of ferric chlorid. The presence of benzoic 

 acid will be indicated by the formation of a brownish-colored precipitate of 

 ferric benzoate. 



(2) SECOND METHOD (MOHLEB'S METHOD). 



Evaporate to dryness and treat the residue with 2 or 3 cc of strong sulphuric 

 acid. 6 Heat until white fumes appear, organic matter is charred, and benzoic 

 acid is converted into sulpho-benzoic acid. Then add a few crystals of potas- 

 sium nitrate which causes the formation of meta-dinitrobenzoic acid. When 

 cool dilute the acid with water and add ammonium hydroxid in excess. Then 

 cool the mixture, transfer to a test tube, and add a drop or two of fresh, color- 

 less ammonium sulphid so that the solutions do not mix. The nitrocompound 

 is converted into ammonium meta-diamidobenzoic acid, which possesses a red 

 color. This reaction takes place immediately and is seen at the surface of 

 the liquid without stirring. 



(b) QUANTITATIVE ESTIMATION. 



Evaporate the ether extract obtained as directed under salicylic acid to dry- 

 ness, thoroughly dry in a sulphuric acid desiccator (preferably in vacuum) and 

 sublime under a watch glass cooled with a piece of ice or a condenser, the lower 

 end of which is closed with a piece of rubber dam. Or the ether extract (or 

 its solution in gasoline) may be transferred into the tube a, as shown in the 

 accompanying figure, the ether or gasoline removed by a gentle current of air, 

 the tube placed In a vacuum desiccator until its contents are thoroughly dry, 

 and the residue sublimed at the temperature of 250 C., the sublimate being 

 collected in tube &. 



During the sublimation, air is drawn very slowly through the apparatus (a 

 wash bottle is used to gauge the speed of the current) to insure the volatilized 

 benzoic acid passing into tube b. The joint between the two tubes is preferably 

 made by means of a cork stopper. The most satisfactory results are obtained 

 by placing the tube a inside of an oven the temperature of which is raised grad- 



Mohler, Bui. soc. chim., Paris, 1890, 8 (3) : 414. 



6 If this is the only method employed, the sulphuric acid may be added directly to the 

 residue left on the evaporation of the ether. 



