210 THE DETECTION OF ADDED SUBSTANCES. 



chloric acid, it appears that this explanation is not correct ; it 

 is probably due to traces of alkali dissolved from the glass. 



The following tests are a selection from the many which have 

 been devised : 



(1) To the distillate add one drop of a dilute aqueous solution 

 of phenol, and pour in some strong sulphuric acid down the 

 sides of the tube. In the presence of formaldehyde a bright 

 crimson zone appears at the junction of the two liquids. This 

 test, which is also due to Hehner, is as delicate as the test pre- 

 viously described, and has the further advantage that it is 

 obtained by formaldehyde solutions of all strengths. If there 

 is more than one part of formaldehyde per 100,000 a white 

 turbidity appears in the solution above the sulphuric acid, 

 while in strong solutions a white or pinkish curdy precipitate is 

 obtained. Many hydroxy-derivatives of benzene, such as salicylic 

 acid, resorcinol, and pyrogallol may be substituted for phenol. 

 Quinol, however, gives not a red colour, but an orange-yellow 

 one. Acetaldehyde gives an orange-yellow colour with phenol 

 and sulphuric acid. 



(2) Mix the distillate with strong sulphuric acid, and sprinkle 

 a little morphine on the surface ; a violet colour is produced 

 in the presence of formaldehyde. 



(3) To a decigramme of diphenylamine add 2 c.c. of strong 

 hydrochloric acid, and pour some of the distillate into the warm 

 solution. In the presence of formaldehyde, a white turbidity 

 or precipitate is obtained, on further warming if necessary. The 

 precipitate on prolonged boiling turns green. This test, like 

 the last, is characteristic of formaldehyde, but is not of such 

 great delicacy as the former ones, and may not be obtained with 

 milk containing only a small amount. 



(4) Heat some of the milk for thirty minutes on the water 

 bath with a little sulphuric acid and a drop of dimethylaniline ; 

 filter ; render alkaline with caustic soda ; and boil till the smell 

 of dimethylaniline has disappeared. Filter ; moisten the filter 

 paper with acetic acid, and sprinkle lead peroxide on it. A blue 

 colour is developed if formaldehyde is present. 



(5) To the distillate add a 3 per cent, solution of aniline. 

 Formaldehyde produces a white precipitate, which is dissolved 

 on boiling, but is deposited again on cooling. 



(6) To 5 c.c. of the distillate add 1-5 c.c. of a 2 per cent, solution 

 of phenylhydrazine hydrochloride, 4 drops ferric chloride solu- 

 tion, and 12 drops sulphuric acid. A rose or dark red colour 

 is produced in the presence of formaldehyde. 



A preservative containing a nitrite in addition to formaldehyde 

 has been put on the market ; the nitrite masks the formaldehyde 

 reactions, but Monier- Williams has pointed out that if this is 



