Appendix VIII C. Oxygen-flask Combustion

Unless otherwise prescribed the combustion flask is a conical flask of at least 500 mL capacity of borosilicate glass with a ground-glass stopper fitted with a suitable carrier for the sample, for example in platinum or platinum-iridium.

Finely grind the substance to be examined, place the prescribed quantity in the centre of a piece of filter paper measuring about 30 mm by 40 mm provided with a small strip about 10 mm wide and 30 mm long. If paper impregnated with lithium carbonate is prescribed, moisten the centre of the paper with a saturated solution of lithium carbonate R and dry in an oven before use. Envelop the substance to be examined in the paper and place it in the sample carrier. Introduce into the flask water R or the prescribed solution designed to absorb the combustion products, displace the air with oxygen by means of a tube having its end just above the liquid, moisten the neck of the flask with water R and close with its stopper. Ignite the paper strip by suitable means with the usual precautions. Keep the flask firmly closed during the combustion. Shake the flask vigorously to completely dissolve the combustion products. Cool and after about 5 min, unless otherwise prescribed, carefully unstopper the flask. Wash the ground parts and the walls of the flask, as well as the sample carrier, with water R. Combine the combustion products and the washings and proceed as prescribed in the monograph.

For Iodine

Burn the specified quantity of the substance being examined in the prescribed manner using a mixture of 10 mL of water and 2 mL of 1m sodium hydroxide as the absorbing liquid. When the process is complete, add to the flask an excess of acetic bromine solution (between 5 and 10 mL) and allow to stand for 2 minutes. Remove the excess of bromine by the addition of formic acid (about 0.5 to 1 mL), rinse the sides of the flask with water and displace any bromine vapour above the liquid with a current of air. Add 1 g of potassium iodide and titrate with 0.02m sodium thiosulfate VS using starch mucilage, added towards the end of the titration, as indicator. Each mL of 0.02m sodium thiosulfate VS is equivalent to 0.4230 mg of I.