ASTM D4046 Test Method for Alkyl Nitrate in Diesel Fuels by Spectrophotometry
8. Calibration and Standardization
8.1 Carefully measure 1.0, 3.0, 5.0, 8.0, and 10 mL of alkyl nitrate standard solution (7.5) from a buret into each of five separate 100-mL volumetric flasks and dilute to the mark with diesel fuel.
8.2 Measure 1.0-mL portions of each standard into each of five 10-mL volumetric flasks. To another 10-mL flask add 1.0 mL of blank diesel fuel. Dilute all flasks to volume with isopropyl alcohol.
8.3 Pipet 1.0 mL of each standard solution (8.2) into each of five separate 125-mL separatory funnels. To another funnel add 1.0 mL of the blank solution. This covers concentrations from 0.03 to 0.30 volume % alkyl nitrate.
8.4 Add 1.0 mL of m-xylenol solution (7.9) to each separatory funnel and swirl to mix thoroughly.
8.5 Add 40 mL of H2SO4 (7.11). The m-xylenol must be added before the sulfuric acid. Otherwise low or negative results will be obtained.
8.6 Secure the stopper with springs or rubber bands and shake the separatory funnel and its content for 30 min on the automatic shaking machine set at fast speed.
NOTE 2 - The amount of contact the sample has with the H2SO4 determines the extent of hydrolysis. Therefore, the speed and time of shaking should be the same for both the samples and the calibration standards.
8.7 To each separatory funnel add 25 mL of isooctane.
8.8 Shake the separatory funnel for 1 min by hand. Allow the layers to separate and discard the acid (bottom) layer. Add 25 mL of distilled water and again shake for 1 min to wash out residual acid. Discard the water (bottom) layer.
8.9 Pipet 10.0 mL of NaOH solution into the separatory funnel, shake for 1 min and allow the phases to separate for at least 10 min or when phases separate cleanly (Note 3). Drain a few drops of the lower phase to rinse the stem and discard. Fill a 1-cm absorption cell and read the absorbance at 452 +/- 5 nm with respect to water.
NOTE 3 - If no color forms, check the pH. When the solution is not alkaline, add an additional 10.0 mL of NaOH solution and correct calculations for the increased solution volume.
8.10 Subtract the absorbance of the blank from that of each standard to obtain corrected absorbance values. Calculate the slope of the curve as follows:
Slope, C = A/B
where:
A = sum of the volume percent alkyl nitrate standards and
B = sum of corrected absorbances.
