16 



17. CARBON DIOXID. 

 See Bulletin 65, p. 95, 1902. 



18. PRESERVATIVES. 



Add 5 cc of dilute sulphuric acid to 100 cc of beer from which the alcohol has 

 been driven off, and shake out with an equal volume of a mixture of ether and ben- 

 zene. Pour off the supernatent liquid and test for salicylic acid, benzoic acid, and 

 saccharin in the usual manner. It is well to reserve 2 or 3 cc of the liquid in 

 a test tube for the salicylic-acid test before exaporating the remainder to dryness in 

 a small porcelain evaporating dish. 



(A) SULPHITES AND SULPHUROUS ACIDS. 



Place 25 cc of a solution of potassium hydroxid containing 56 grams per liter in a 

 flask of approximately 200 cc capacity. Introduce 50 cc of beer by means of a 

 pipette, mix with the potassium hydroxid, and allow the mixture to stand for fifteen 

 minutes with occasional agitation. Add 10 cc of 1-3 sulphuric acid and a few cubic 

 centimeters of starch solution, and titrate the mixture with a fiftieth-normal iodin 

 solution. Introduce the iodin solution as rapidly as possible and continue the addi- 

 tion until the blue color will last several minutes. One cubic centimeter of fiftieth- 

 normal iodin solution is equivalent to 0.00064 gram of sulphur dioxid. The number 

 of cubic centimeters of the iodin solution employed, multiplied by 0.00128, gives the 

 weight of the total sulphur dioxid expressed in grams per 100 cc. 



(B) FLUORIDS (METHOD OF BLAREZ). 



Thoroughly mix the sample and heat 150 cc to boiling. Add to the boiling liquid 

 5 cc of a 10 per cent solution of of barium acetate. Collect the precipitate in a com- 

 pact mass, using to advantage a centrifuge, wash upon a small filter and dry in the 

 oven. Transfer to a platinum crucible, first breaking up the dry precipitate and then 

 adding the filter ash to crucible. Prepare a glass plate (preferably of the thin variety 

 commonly used for lantern-slide covers) as follows: First thoroughly clean and pol- 

 ish and coat on one side by carefully dipping while hot in a mixture of equal parts of 

 Carnaiiba wax and paraffin. Near the middle of the plate make a small cross or other 

 distinctive mark through the wax with a sharp instrument, such as a pointed piece of 

 wood or ivory, which will remove the wax and expose the glass without scratching 

 the latter. Add a few drops of concentrated sulphuric acid to the residue in the cru- 

 cible and cover with the waxed plate, havmg the mark nearly over the center and 

 making sure that the crucible is firmly imbedded in the wax. Place in close contact 

 with the top or unwaxed surface of the plate a cooling device consisting of a glass, cylin- 

 der the bottom of which is a thin sheet of pure rubber. Keep the cylinder filled with 

 ice water so that the wax does not melt. Heat the bottom of the crucible gently over 

 a low flame or on an electric stove for an hour. Remove the glass plate and indicate 

 the location of the distinguishing mark on the unwaxed surface of the plate by 

 means of gummed strips of paper, melt off the wax by heat or a jet of steam and 

 thoroughly clean the glass with a soft cloth. A distinct etching will be apparent on 

 the glass where it was exposed if a fluorid be present. 



[Cir. 33] 



