X] FOOD PRESERVATIVES 149 



23 Phloroglucol Method.^ 



To 10 cc. of milk or other liquid food under examination in a test tube add, by 

 means of a pipette, 2 cc. of phloroglucol reagent (1 gram of phloroglucol, 20 grams of 

 sodium hydroxid and water to make 100 cc), placing the end of the pipette on the 

 bottom of the tube in such a manner that the reagent will form a separate layer. 



If formaldehyde be present, a bright red coloration (not purple) forms at the 

 zone of contact. This solution gives a yellow color in the presence of some alde- 

 hydes, and, if it is used for the detection of aldehyde formed by the oxidation of 

 methyl alcohol after the destruction of ethyl aldehyde with hydrogen peroxid, an 

 orange yellow color will slowly appear when an insufficient amount of hydrogen 

 peroxid has been employed. On the other hand, if the excess of hydrogen 

 peroxid is not fully destroyed before the use of this reagent, a purple color develops 

 slowly. The clear, red color given by the use of this reagent forms quickly, and, 

 in the presence of but a small amount of formaldehyde fades rapidly. 



FLUORIDS. 



QUALITATIVE TESTS. 



24 Method I. —Modified Method of Blarez.^ 



Thoroughly mix the sample and boil 150 cc. (in the case of solid foods an aqueous 

 extract may be employed provided the fluorids are in a soluble form). Add to the 

 boiling liquid 5 cc. of 10% potassium sulphate solution and 10 cc. of 10% barium 

 acetate solution. Collect the precipitate in a compact mass (a centrifuge may be 

 used advantageously) and wash upon a small filter. Transfer to a platinum crucible 

 and ignite. 



Dip a carefully cleaned glass plate, while hot, in a mixture of equal parts of Car- 

 naiiba wax and paraffin and allow to cool. Make, with a sharp instrument, a dis- 

 tinctive mark through the wax, taking care not to scratch the surface of the glass. 



Add a few drops of concentrated sulphuric acid to the residue in the crucible 

 and cover with the waxed plate, having the mark nearly over the center and mak- 

 ing sure that the edge of the crucible is in close contact with it. Keep the top sur- 

 face of the plate cool by means of a suitable device and heat the crucible for an hour 

 at as high a temperature as practicable without melting the wax (an electric stove 

 gives the most satisfactory form of heat). 



If fluorids be present, a distinct etching will be apparent on the glass where it 

 was exposed. 



25 Method II. 



The preceding method may be varied by mixing a small amount of precipitated 

 silica with the precipitated barium fluorid and applying the method for the detection 

 of fluosilicates, under 27 or 28. 



This method is of value in the case of foods whose ash contains a considerable 

 amount of silica. Under these circumstances, concentrated sulphuric acid liber- 

 ates silicon fluorid, which would escape detection under 24. 



FLUOBORATES AND FLUOSILICATES. 



26 PREPARATION OF SAMPLE. 



Make about 200 grams of the sample alkaline with lime water, evaporate to dry- 

 ness, and incinerate. Extract the crude ash with water, to which sufficient acetic 



