i MILK 121 



the ether has evaporated spontaneously, the salicylic acid is 

 left in the form of colourless crystals which can be dissolved 

 in alcohol and tested with a drop of 1 per cent, ferric chloride 

 solution. The characteristic violet denotes salicylic acid. 



4. Benzoic Acid. 



E. Meissl 1 recommends the following method for the detec- 

 tion of this preservative : 250-500 c.c. of milk are made alkaline 

 with a few drops of lime- or baryta water, and then evaporated 

 to about a quarter of the bulk. Powdered gypsum is stirred 

 into the remaining liquid until a paste is formed, which is then 

 dried on the water-bath. The gypsum only serves to hasten 

 the drying, and powdered pumice stone or sand can be 

 used equally well. When the mass is dry, it is finely powdered 

 and moistened with dilute sulphuric acid and shaken out three 

 or four times with about twice the volume of 50 per cent, 

 alcohol, in which benzoic acid is easily soluble in the cold, the 

 fat only being dissolved to a very slight extent or not at all. 

 The acid alcoholic liquid from the various extractions, which 

 contains milk sugar and inorganic salts in addition to the 

 benzoic acid, is neutralised with baryta water and evaporated 

 to a small bulk. Dilute sulphuric acid is again added, and the 

 liquid shaken out with small quantities of ether. On evapora- 

 tion of the ether, the benzoic acid is left behind in almost pure 

 state, the only impurities being small quantities of fat or ash. 



The benzoic acid which is obtained is dissolved in a small 

 quantity of warm water, a drop of sodium acetate and neutral 

 ferric chloride added, and the red precipitate of benzoate of 

 iron indicates the presence of the acid. 



5. Formaldehyde. 



Fritzmann's reaction (p. 102) can be employed for the 

 detection of formalin if applied in the reverse order to that used 

 in testing for a nitrate. Ten c.c. of milk are taken, and a few 

 drops of very dilute nitric acid and also 10 c.c. of chemically 

 pure sulphuric acid (sp. gr. T815) are added. The presence of 

 formaldehyde in the milk is indicated by a violet-blue colora- 

 tion. A simpler and more reliable method is to use SchifFs 



1 Zeitxchrift filr aiialytische Chemie, 1882, Vol. XXI, p. 531. 



