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6.2 OXYGEN FLASK COMBUSTION​

      The oxygen flask combustion procedure is provided as the preparatory step in the determination of bromine, chlorine, fluorine, iodine, selenium and sulfur in some Pharmacopoeial articles. Combustion of the material under test (usually organic) yields water-soluble inorganic products, which are determined for specific elements directed for the individual monograph or general appendix.

      The caution statement given under Procedure of Combustion covers minimum safety precautions only, and serves to emphasize the need for exceptional care throughout.

Apparatus 

      The apparatus consists of a heavy-welled conical, deeply lipped or cupped 500-ml flask (unless a larger flask is specified), fitted with a ground-glass stopper to which is fused a test sample carrier consisting of heavygauge platinum wire and a piece of welded platinum gauze measuring about 1.5 cm × 2 cm, or of a platinum basket or cylinder made of platinum woven gauze.

Preparation of Sample

      For solid samples Accurately weigh the quantity of the sample specified in the individual monograph on the centre of halide-free filter paper measuring about 3 cm square, illustrated in the figure. Wrap the sample carefully along the dotted line without scattering, and place the parcel in a sample carrier, leaving its fuse-strip outside. (Note Pulverizable substances should be finely ground and thoroughly mixed before the specified quantity is weighed.) Ointment should be enclosed in grease-proof paper before wrapping in filter paper.

      For liquid samples Liquid substances are weighed in tared capsules, polycarbonate capsules being used for liquids in volumes not exceeding 200 μl, and gelatin capsules being satisfactory for use for larger volumes. (Note Gelatin capsules may contain significant amounts of combined halide or sulfur. If such capsules are used, perform a blank determination, and make any necessary correction.) Place the sample, together with a filter paper fuse-strip, in the platinum gauze sample carrier.

Procedure of Combustion

      Place the specified absorbing liquid in the flask, moisten the joint of the stopper with water, and flush the air from the flask with a stream of rapidly flowing oxygen, swirling the liquid to favour its taking up oxygen. (Note Saturation of the liquid with oxygen is essential for the successful performance of the combustion procedure.) Ignite the fuse-strip by suitable means. If the strip is ignited outside the flask, immediately plunge the sample holder into the flask, carefully tilt the flask so that the absorbing liquid makes a seal around the stopper as illustrated in the figure, and hold the stopper firmly in place. If the ignition is carried out in a closed system, the inversion of the flask may be omitted. After combustion is complete, shake the flask vigorously, and allow to stand for not less than 10 minutes with intermittent shaking. Then proceed as directed in the individual monograph or general appendix.