184 NORMAL MILK : ITS ADULTERATIONS, ETC. 



with a little amyl alcohol, the colour will pass to the amyl 

 alcohol. 



For the detection of benzoic and salicyhc acids the milk is made 

 alkaline with sodium carbonate and the casein precipitated by 

 heating on a water bath with one-tenth of the volume of 10 per 

 cent, calcium chloride solution ; after coohng, the filtrate is 

 neutrahsed and the proteins removed as in Eitthausen's method 

 (p. 126), The filtrate from this is acidified and extracted with 

 a mixture of ether and petroleum ether, and the solvent washed 

 with water, and finally a small quantity of water and a drop 

 or two of phenolphthalein added, and dilute caustic soda dropped 

 in with constant shaking till the aqueous portion is pink. This 

 should be removed, 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 sahcyhc acid, while benzoates 

 give a buff precipitate insoluble in dilute acetic acid. To confirm 

 the presence of sahcylic acid a portion is tested with bromine 

 water, a curdy yellowish precipitate is produced by sahcylic 

 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. 



These reactions are not absolutely characteristic of salicylic 

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

 vatives behave in a similar manner. 



Lintner gives the following test : — Boil with a few drops of 

 10 per cent, mercuric nitrate solution, add 2 or 3 drops of dilute 

 sulphuric acid, and a very small quantity of 1 per cent, sodium 

 nitrite solution, avoiding excess. A red colour indicates the 

 presence of salicylic acid. This test will detect 1 part in 

 500,000. 



Benzoic acid gives the reactions below ; if sahcyhc acid is 

 present a little bromine water is added, and a turbidity or pre- 

 cipitate will be produced ; bromine water should be added till 

 all the sahcylic acid is precipitated, and the precipitate removed 

 by filtration. The excess of bromine should be boiled off, and 

 the following tests applied : — 



(a) Add a few pieces of magnesium and hydrochloric acid 

 till gas begins to be evolved ; benzoates are reduced to benzal- 

 dehyde, which. has a characteristic smell. 



(b) Evaporate a little of the solution with soda-lime, and 

 ignite in a current of inert gas (nitrogen formed by passing 

 air through alkaline pyrogallol serves) ; benzoates are reduced 

 to benzene (characteristic smell), which may be collected in a 

 mixture of nitric and sulphuric acids, which form nitro-benzene 



