ASTM D3231 Standard Test Method for Phosphorus in Gasoline
8. Calibration
8.1 Transfer by buret, or a volumetric transfer pipet, 0.0, 0.5, 1.0, 1.5, 2.0, 3.0, 3.5, and 4.0 mL of phosphorus standard solution into 100-mL volumetric flasks.
8.2 Pipet 10 mL of H2SO4(1 + 10) into each flask. Mix immediately by swirling.
8.3 Prepare the molybdate-hydrazine reagent. Prepare sufficient volume of reagent based on the number of samples being analyzed.
8.4 Pipet 50 mL of the molybdate-hydrazine reagent to each volumetric flask. Mix immediately by swirling.
8.5 Dilute to 100 mL with water.
8.6 Mix well and place in the constant-temperature bath so that the contents of the flask are submerged below the level of the bath. Maintain bath temperature at 82.2 to 87.8°C (180 to 190°F) for 25 min (Note 1).
8.7 Transfer the flask to the cooling bath and cool the contents rapidly to room temperature. Do not allow the samples to cool more than 2.8°C (5°F) below room temperature.
NOTE 4 - Place a chemically clean thermometer in one of the flasks to check the temperature.
8.8 After cooling the flasks to room temperature, remove them from the cooling water bath and allow them to stand for 10 min at room temperature.
8.9 Using the 2.0-mL phosphorus standard in a 5-cm cell, determine the wavelength near 820 nm that gives maximum absorbance. The wavelength giving maximum absorbance should not exceed 830 nm.
8.9.1 Using the red-sensitive phototube and 5-cm cells, adjust the spectrophotometer to zero absorbance at the wavelength of maximum absorbance using distilled water. If using a dual-beam spectrophotometer, place distilled water in both cells. Use the wavelength of maximum absorbance in the determination of calibration readings and future sample readings.
8.9.2 The use of 1-cm cells for the higher concentrations is permissible.
8.10 Measure the absorbance of each calibration sample including the blank (0.0 mL phosphorus standard) at the wavelength of maximum absorbance with distilled water in the reference cell. Great care shall be taken to avoid possible contamination. If the absorbance of the blank exceeds 0.04 (for 5-cm cell), check for source of contamination. It is suggested that the results be disregarded and the test be rerun with fresh reagents and clean glassware.
8.11 Correct the absorbance of each standard solution by subtracting the absorbance of the blank (0.0 mL phosphorus standard).
8.12 Prepare a calibration curve by plotting the corrected absorbance of each standard solution against micrograms of phosphorus. One millilitre of phosphorus solution standard provides 10 µg of phosphorus.
