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6.6 DETERMINATION OF METHOXYL 

Apparatus​

      The apparatus for methoxyl determination is diagrammatically in the accompanying figure. The boiling flask, A, is fitted with a capillary side-arm for the introduction of carbon dioxide or nitrogen and is connected to a column, B, which serves to separate aqueous hydriodic acid from the more volatile methyl iodide. The methyl iodide passes through water in a scrubber trap, C, and is finally absorbed in the bromine-acetic acid solution in an absorption tube, D. The carbon dioxide or nitrogen is introduced through a pressure-regulating device and connected to the apparatus by a small capillary containing small cotton pledget. (Note Avoid the use of organic solvents in cleaning this apparatus, since traces remaining may interfere with the determination.) This test is used also for ethoxyl determination with an 80-minute reaction time and a titrant equivalent of 0.751 mg of ethoxyl moiety (OC₂H₅).

      For greater convenience in use and cleaning, a ground-glass ball joint connects the two upright columns of the apparatus. The top of the scrubber C consists of a 35/20 ball joint, the upper half of which is connected to the side-arm leading into tube D. This permits taking the apparatus apart and facilitates adding the water to the trap. Also, it allows access to the loose inverted (10-mm) test-tube that serves as the trap over the inner tube of the scrubber.

Procedure

      Prepare the apparatus by disconnecting the ball joint and pouring water into trap C until it is half-full. Connect the two parts, using a minimal amount of a suitable silicone grease to seal the ball joint. Add 7 ml of acetic bromine TS to absorption tube D. Weigh the sample in a tared gelatin capsule, and add it to the boiling flask along with a few boiling chips or pieces of porous plate. Finally add 6 ml of hydriodic acid and attach the flask to the column, using a minimal amount of suitable silicone grease to seal the junction. Bubble carbon dioxide through the apparatus at the rate of two bubbles per second, place the boiling flask in an oil-bath or heating mantle heated to 150º, and continue the reaction for 40 minutes. Drain the contents of the absorption tube into a 500-ml conical flask containing 10 ml of a 25 per cent w/v solution of sodium acetate. Rinse the tube with water, adding the rinsings to the flask, and finally dilute with water to about 125 ml. Add formic acid, dropwise, with swirling until the reddish brown colour of the bromine is discharged, and then add 3 additional drops. A total of 12 to 15 drops usually is required. Allow to stand for 3 minutes, and add 15 ml of dilute sulfuric acid and 3 g of potassium iodide. Titrate immediately with 0.1 M sodium thiosulfate VS, using starch TS as indicator near the end of the titration. Perform a blank determination, including also a gelatin capsule, and make any necessary correction. Each ml of 0.1 M sodium thiosulfate is equivalent to 0.517 mg of methoxyl moiety (OCH₃).

Apparatus for Determination of Methoxyl Dimensions in mm