ASTM D1318 Test Method for Sodium in Residual Fuel Oil (Flame Photometric Method)
9. Procedure
9.1 Preparation of Sample:
9.1.1 Weigh into a thoroughly clean, dry 100-mL platinum dish the appropriate size sample (7.3) with an accuracy of at least 1 part per 100 parts of sample (Note 2). Samples larger than 75 g require a second filling of the platinum dish; for such samples, obtain the sample weight from the difference between the initial and final weights of the sample bottle. Place the platinum dish containing the fuel oil on a silica triangle properly supported, and heat with a bunsen burner until the contents ignite and burn readily (Note 3). Continue heating with the burner in such a manner that the sample burns at a uniform and moderate rate and only ash and carbon remain after burning ceases. For samples larger than 75 g, cool the dish and fill it approximately two-thirds full with additional well-shaken sample, and burn as above.
NOTE 2 - Handle the platinum dish only with platinum-tip tongs and do not touch it with the fingers during the test. Carefully dust the bottom of the dish with a clean camel-hair brush before each weighing.
NOTE 3 - If sample contains an appreciable amount of water, as indicated by spattering when heated, add a few millilitres of ethyl-alcohol (95 %) (Warning - Flammable) or isopropyl alcohol (99 %) (Warning - Flammable) before heating. Include the alcohol in the blank determination.
9.1.2 Place the dish in a muffle furnace at no more than 200°C (Note 3). Slowly raise the temperature to 550 more or less 50°C. Leave the muffle door slightly ajar until only a little carbon remains in the dish; then close the door and continue the ignition until no carbon is visible.
NOTE 4 - The platinum dishes should be placed on silica plates or silica triangles on the floor of the muffle furnace. Particular care must be exercised to avoid contamination of the sample with particles from the roof, walls, and the door of the furnace. During the initial ignition, the opening of the muffle door must be carefully adjusted so that the air flow into the muffle is not excessive. Too great an air flow causes high temperatures in the burning carbon and also possible loss of ash from the dish.
9.1.3 Cool the dish to room temperature, add 1 mL of H2SO4 (1+1) and about 1 mL of HF to the platinum dish (Note 5), evaporate to dryness under a hood, and replace the dish in the muffle furnace at 550 more or less 50°C for 15 min.
NOTE 5 - Unless the silica is removed, low values will result from the occlusion of sodium in the insoluble residue.
9.1.4 Cool the platinum dish, wash down its sides with about 10 mL of water and, by means of a dropper, add 2 drops of H2SO4 (1+1). Heat the dish on a steam bath, to effect the solution of the sodium salts, until approximately 1 mL of liquid remains. Remove from the steam bath, add 5 to 10 mL of water, and filter the solution through an acid-washed filter paper or an acid-washed, sintered-glass filter into a 100-mL volumetric flask. Wash the dish and filter with water, collecting the wash water in the volumetric flask. Dilute to 100 mL with water and mix. If the ashed sample is known to contain less than 0.5 mg of sodium, make a proportionately smaller dilution to obtain 5 to 10 mg of sodium per litre in the final solution.
9.2 Photometric Measurement:
9.2.1 Put the flame photometer into operation, using the identical burner and instrument settings that were determined during calibration in accordance with 8.2. Vaporize the medium-range standard solution (8.2) and adjust the sensitivity control to give the same scale reading that was obtained when the calibration curve was prepared.
9.2.2 Atomize a portion of the sample solution prepared in 9.1.4 in the flame photometer. If it is in the optimum range for the instrument in use, proceed as directed in 9.2.3. If it is too concentrated, place a few millilitres in a graduated cylinder, dilute to a known volume with water, and atomize. If necessary, repeat this operation to determine the correct dilution. When this is established, make the proper accurate dilution by pipetting an aliquot from the original 100-mL volumetric flask into another volumetric flask and dilute to the mark with water.
9.2.3 Atomize the properly diluted sample solution, record the scale reading, and obtain the sodium concentration in milligrams per litre by reference to the standard curve prepared in 8.4. Also atomize standards just lower and higher than the unknown to check the validity of the calibration curve.
9.2.4 Perform a blank determination on all of the reagents, including the water, in accordance with the procedure just described.
