206 THE DETECTION OF ADDED SUBSTANCES. 



the aqueous portion is pink. This should be removed, just 

 acidified with very dilute acetic acid, boiled to expel ether, and 

 to a portion a drop of dilute ferric chloride solution is added, 

 and a violet coloration is developed in the presence of salicylic 

 acid, while benzoates give a butt precipitate insoluble in dilute 

 acetic acid. To confirm the presence of salicylic acid a portion 

 is tested with bromine water, a curdy yellowish precipitate is 

 produced by salicylic acid, and the characteristic smell of halogen 

 phenol derivatives developed ; another portion is evaporated to 

 dryness with strong nitric acid, and the residue taken up with a 

 few drops of water ; a yellow coloration is produced on adding 

 ammonia if salicylic acid be present. 



Hinks' Method. Ten to 20 grammes are treated with an 

 equal volume of concentrated hydrochloric acid, till the curd 

 is dissolved, and after cooling shaken with 25 c.c. of 1 part of 

 ether to 2 parts petroleum ether. The ethereal layer is separated 

 and 1 drop of 0-880 ammonia added ; if benzoic acid is present 

 a precipitate occurs in the ethereal layer. 



Five c.c. of water is now added, and the mixture shaken well, 

 and the water separated, heated on the water-bath to expel 

 ammonia, and tested as above for salicylic and benzoic acid. 

 If the extraction is repeated twice more, and the ethereal ex- 

 tracts washed three times with about 5 c.c. of water, the salicylic 

 acid and benzoic acids can be titrated by adding 5 c.c. of water, 



N 

 a little phenol-phthalein, and the --^ alkali till the water is just 



permanently pink. The titration may be checked by evapor- 

 ating and weighing the alkaline salts. 



These reactions are not absolutely characteristic of salicylic 

 acid, as phenol (carbolic acid) and other hydroxy-benzene deri- 

 vatives behave in a similar manner, but Self's test appears to be 

 characteristic. A little of the dry residue is moistened with a 

 cold mixture of equal parts of strong sulphuric acid, and 40 per 

 cent, formaldehyde, and a little ammonium vandate added on 

 the end of a glass rod. A Prussian blue colour indicates salicylic 

 acid. 



Jorissen's test consists in adding to the solution 5 drops of 

 a 10 per cent, solution of sodium nitrite, 5 drops of 50 per cent, 

 acetic acid, and 1 drop of 1 per cent, copper sulphate. The 

 solution is heated in a water-bath for 45 minutes, and salicylic 

 acid gives a red colour. 



Salicylic acid may be estimated by comparing the colour 

 given with ferric chloride with that given by known weights of 

 salicylic acid. 



Biernath destroys salicylic acid by heating the solution with 

 alkaline permanganate ; at the Government Laboratory 1 c.c. 



