SPECIAL TESTS II 



7. Methods Useful in the Examination of Vegetable Food 



Products 



1. Sublimation Test for Benzoic Acid. Place a drop or two 

 of the suspected liquid or semi-liquid food substance into a 

 deep watch crystal of i in. diameter. Place over it a clean dry 

 slide. Now hold the watch crystal over a flame (alcohol lamp 1 ) 

 until the substance (as wine, vinegar, catsup, jam, jelly, etc.), 

 comes to an active boil. The steam vapor, carrying with it 

 the benzoic acid, is condensed on the slide. Remove the slide 

 and set it aside until the condensed moisture has evaporated; very 

 moderate heat may be used to hasten evaporation. Examine 

 under the microscope, whereupon the benzoic acid crystals may 

 be seen, provided any were present. The test is delicate, very 

 reliable and very few substances interfere with it. It is very 

 pronounced in the presence of o.oi per cent, of benzoic acid. 



2. Sublimation Test for Salicylic Acid. Made like the benzoic 

 acid test. The crystal formation (plates) is very pronounced in 

 dilutions of i : 1000. After having examined the crystals under 

 the microscope, add a drop of weak solution of ferric chloride to 

 the crystals upon the slide, whereupon a blue coloration develops. 

 Boric acid is likewise deposited by sublimation, but the test is 

 not as satisfactory as those for benzoic acid and for salicylic acid. 



The sublimation test may also be extended to other crystalline 

 substances which undergo sublimation on exposure to heat. 



3. Curcuma Thread Test for Boric Acid. Boil 5 grams of 

 powdered curcuma in 10 cc. of alcohol. To the evaporated alco- 

 holic extract add a little soda and several cc. of 50 per cent, 

 alcohol. In this place paper (bast fiber), cotton or linen threads 

 and bring to a brisk boil for a few moments. Remove threads 

 and dry between blotting paper, lay them in a very weak solu- 

 tion of sulphuric acid and rinse in water. When dry the threads 

 should be a golden yellow. 



1 Alcohol lamp is preferable because the flame is small and yet the heating is more 

 quickly done. 



