FOODS 81 



commercial sulphuric acid is then run down the side of the 

 test tube, so that it forms a distinct layer at the bottom of 

 the tube. Pure milk, free of formaldehyde, gives a greenish 

 ring, but when formaldehyde is present a violet ring is 

 formed. Pure sulphuric will not give the test, but will do 

 so after the addition of a few drops of ferric chloride. The 

 test, therefore, can be varied thus. To the dilute milk add 

 the ferric chloride, and then some strong sulphuric, when 

 the milk becomes a reddish-purple colour ; but carbolic 

 and salicylic acids, if present, would give confusion colours. 

 (Hehner's test reacts with i part in 200,000, but fails with 

 milks containing overo -5 per cent). 



(2) Add a drop of carbolic acid to the dilute milk, and 

 repeat Hehner's test, using pure H 2 SO 4 , when a red ring 

 indicates the presence of formaldehyde. 



(3) Jorissen's Test. To 10 c.c. of milk in a tube are added 

 several drops of a 10 per cent aqueous solution of phloro- 

 glucinol, the mixture shaken, and a few drops of NaOH or 

 KOH added. Normal milk gives no reaction, but a milk 

 containing as little as i part of formalin in 20,000 gives a 

 fleshy-pink coloration. 



(4) Distil 100 c.c. of the milk, and use the distillate for 

 these tests. 



(a). Repeat test number (2) above. Detects i in 200,000. 



(b). Schiff's Magenta Test. Take a very dilute solution 

 of magenta (fuchsin or nitrate of rosanilin), and decolorize 

 with sulphurous acid, adding drop by drop. Add a few 

 c.c. of distillate, and watch for some minutes (or set 

 aside). Traces of formaldehyde bring back the colour 

 slowly. The test is also said to be got in the nitrate of 

 curdled milk. 



(c). Tatten-Thomson Test. To 20 c.c. of distillate add 

 5 to 10 drops of a reagent made by adding ammonia to a 

 2 per cent solution of silver nitrate until the precipitate 

 first formed just dissolves. Set aside in a dark place for 

 twenty-four hours. Darkening almost to blackness is due 

 to formaldehyde, but a slight browning may be disregarded 

 (often seen in silver solutions). 



Formalin is a 40 per cent solution of formic aldehyde 

 (CH 2 O) in water. Two to three drops keep a pint of milk 

 fresh for 3 to 4 days, and 0-05 per cent preserves milk for 



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