CHAPTER XIII. 



THE DETECTION OP ADDED SUBSTANCES. 



Preservatives. The detection and estimation of boric acid 

 have already been described (p. 109). 



Some idea as to whether boric acid or borax has been added 

 can be obtained by applying the turmeric test (1) to a solution 

 of ash of milk in water, and (2) to a solution of the ash in dilute 

 hydrochloric acid. If test (1) gives no reaction, while test (2) 

 gives a strong reaction, borax has been added ; if test (2) gives 

 a reaction no stronger than that obtained by test (1), boric acid 

 has been used ; while if test (1) gives a reaction, while test (2) 

 gives a stronger reaction, a mixture of the two is probable. 

 These tests are far from absolute, owing to the difficulty of 

 judging the strength of a reaction, and, further, owing to the 

 fact that the ash of milk is usually feebly alkaline, which would 

 cause some of the boric acid to be reckoned as borax. Occasion- 

 ally, the ash of milk is acid, and some of the borax would then 

 appear as boric acid. Nothing more than rough approximate 

 results are claimed for this method. 



Farrington has shown that when boric acid is added to milk 

 its acidity to phenol- phthalein is four times as great as its acidity 

 in aqueous solution ; if a milk is found to have a high acidity, 

 say 40, and does not smell or taste sour or curdle on boiling, it 

 is very probable that boric acid is present. 



Salicylic acid may be detected in the nitrate produced by 

 adding mercuric nitrate to milk ; if much salicylic acid be present 

 this will acquire a red colour after some time, and when shaken 

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

 alcohol. 



Revis' Method. For the detection of benzoic and salicylic 

 acids the milk or cream 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 cooling the filtrate is neutralised and the proteins removed 

 as in Ritthausen's method (p. 173). 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 phenol-phthalein added, 

 and dilute caustic soda dropped in with constant shaking till 



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