DETECTION OF BENZOIC ACID. 1409 



flask and subjected to distillation.* The first portions of the distillate 

 are used in the detection of sulphurous acid as directed hereafter. 

 The last portion is employed in the detection of benzoic acid according 

 to one of the following methods: 



MOHLER'S METHOD. b 



The filtrate described above is neutralized with sodium hydroxid, 

 evaporated to dry ness, treated with 2 or 3 cc of strong sulphuric 

 acid, and heated until white fumes appear. By this means benzoic 

 acid is converted into sulphobenzoic acid. A few crystals of potassium 

 nitrate are added and the heating continued until the solution is almost 

 or quite colorless. This causes the formation of meta-dinitro-benzoic 

 acid. When cool the acid is diluted with water, ammonia added in 

 excess, and the mixture transferred to a test tube. A drop or two 

 of ammonium sulphid is now added, taking care that the liquids shall 

 not mix. The nitro compound is converted into ammonium meta- 

 diamido-benzoate, which possesses a peculiar red color. This reaction 

 takes place immediately, and is seen at the surface of the liquid 

 without stirring. Salicylic acid will sometimes give the same reaction, 

 but only after waiting some minutes. The benzoic acid must first be 

 separated in a state of approximate purity before this test can be 

 applied. Half a milligram of the acid can be detected in the absence 

 of interfering bodies. This reaction is also given by saccharin. This 

 reaction is very delicate, and all but distinct and characteristic tests 

 should be disregarded. It is well to confirm it by one of the other 

 methods described, using a larger quantity of the sample. 



OTHER METHODS. 



The presence of benzoic acid ma} 7 be confirmed by neutralizing the 

 aqueous solution of the extracted benzoic acid with sodium hydroxid, 

 evaporating to a very small volume, arid acidifying with sulphuric 

 acid, when the presence of a large amount of benzoic acid is indicated 

 by the formation of a white flocculent precipitate. 



The concentrated solution of the sodium salt may be further tested 

 by making it exactly neutral and adding a drop of a dilute ferric chlo- 

 rid solution, when ferric benzoate is precipitated in the presence of a 

 large amount of benzoic acid. The appearance of ferric benzoate is 

 markedly different from that of ferric hydroxid in that it is almost 

 white when viewed by transmitted light, whereas ferric hydroxid has 

 a brown color under the same conditions. 



a lf it is desired the filtrate may be made slightly akaline with sodium hydroxid, 

 the fat extracted with ether or petroleum ether, and the aqueous solution acidified 

 with sulphuric acid and extracted with ether as directed under salicylic acid. The 

 ether extract is then allowed to evaporate spontaneously and benzoic acid detected 

 by one of the methods described. 



b Bul. soc. chini., 1890, (3) 3, 414. 



