MILK PRESERVATIVES 195 



alcohol added, and the mixture boiled. On igniting the 

 alcohol vapour, a green colour will be noticed indicative 

 of the formation of a volatile boron compound. 



Detection of Formaldehyde. — One drop of dilute 

 HNO3 is added to 10 c.c. of the milk from sample II. 

 above, and a few c.c. of concentrated HgSO^ poured 

 down the side of the test-tube. 



A violet-blue ring will be found at the junction 

 of the two liquids. This colour reaction between 

 formaldehyde and nitric acid in the presence of 

 sulphuric acid does not take place except in the 

 presence of proteins, and is therefore not given in 

 aqueous solutions. 



SchifTs reagent (rosaniline decolorised by SOg) 

 may also be used for the detection of formaldehyde in 

 milk. The milk is curdled with dilute HgSO^ and the 

 filtrate added to Schiff's reagent. The formation of 

 a pink colour shows presence of formaldehyde. 



Detection of Hydrogen Peroxide. — Ten c.c. of the 

 milk that has been treated as in III. above are placed 

 in a test-tube, and a few drops of an alcoholic solution 

 of benzidine added. A blue coloration shows the 

 presence of HgOg. 



Detection of Salicylic Acid. — A few c.c. of the milk 

 are treated with sufficient mercuric nitrate solution to 

 precipitate all the casein, and the mixture filtered. The 

 filtrate is then shaken up with ether in a large test- 

 tube, the ethereal solution poured off, divided into 

 two parts, and the ether allowed to evaporate. The 

 residue is treated in the one case with one drop of 

 neutral dilute ferric chloride, when a violet coloration 

 shows presence of salicylic acid. To the other residue 

 is added a little bromine water, when a yellowish 

 precipitate of tribromo - salicylic acid confirms the 

 presence of salicylic acid. 



