D1840_10

The user may use alternative procedure B if preferred. For recommended techniques, refer to practices E169. Check carefully sections on handling and cleaning of cells and glassware, instrument adjustments, and method of absorbance measurement. Prepare three dilutions of the sample as follows. If the sample is more volatile than isooctane, add 10 to 15 milliliters of spectroscopic isooctane to a clean, dry, glass-stoppered, 25-milliliter volumetric flask. Weight out approximately 1 gram of sample in the flask. Dilute to volume with spectroscopic solvent and mix thoroughly. If the sample is less volatile than isooctane, weigh out approximately 1 gram of sample in the flask. Dilute to volume with spectroscopic solvent and mix thoroughly. Pipette 5 milliliters of the first dilution into a 50-milliliter glass-stoppered volumetric flask. Dilute to volume with spectroscopic isooctane and mix thoroughly. Dilute 5 milliliters of the second dilution to 50 milliliters in the same manner as section 10.2.2. Measure and record the absorbance of the spectroscopic isooctane-filled sample cell as compared to the spectroscopic isooctane-filled solvent cell. Transfer portions of the final dilution into the sample cell of the spectrophotometer. Cover the cells immediately to prevent transfer of aromatic hydrocarbons from the sample cell to the solvent cell. Check the windows of the absorption cells and make certain they are clean. Measure the absorbance as recommended in practices E169. Record the absorbance of the sample as compared to spectroscopic isooctane at 285 nanometers. The dilution of the sample should be controlled so that absorbance readings fall within a range of 0.2 to 0.8 for maximum reproducibility of results. To accomplish this, it may be necessary to use an alternative third dilution than the one specified in section 10.2.3, such as 10 milliliters of the second dilution to 25 milliliters with solvent.