ASTM D3231 Standard Test Method for Phosphorus in Gasoline
9. Procedure
9.1 Clean all glassware before use with cleaning acid or by some procedure that does not involve use of commercial detergents. These compounds often contain alkali phosphates that are strongly adsorbed by glass surfaces and are not removed by ordinary rinsing. It is desirable to segregate a special stock of glassware for use only in the determination of phosphorus.

9.2 Observe the usual precautions of cleanliness, careful manipulation, and avoidance of contamination in order to obtain satisfactory accuracy with the small amounts of phosphorus involved.

9.3 Transfer 2 more or less 0.2 g of zinc oxide into a conical pile in a clean, dry, unetched ignition dish.

9.4 Make a deep depression in the center of the zinc oxide pile with a stirring rod.

9.5 Pipet the gasoline sample (Note 5) (see 7.2 for suggested sample volume) into the depression in the zinc oxide. (Warning - In addition to other precautions, cool the ignition dish before adding the additional aliquots of gasoline to avoid a flash fire.) Record the temperature of the fuel if the phosphorus content is required at 15.6°C (60°F) and make correction as directed in 10.2.

NOTE 5 - For the 10-mL sample use multiple additions and a syringe. Hold the tip of the needle at approximately 2/3 of the depth of the zinc oxide layer and slowly deliver 2 mL of the sample; fast sample delivery may give low results. Give sufficient time for the gasoline to be absorbed by the zinc oxide. Follow step 9.6. Cool the dish to room temperature. Repeat steps 9.5 and 9.8 until all the sample has been burned.

9.6 Cover the sample with a small amount of fresh zinc oxide from the reagent bottle (use the tip of a small spatula to deliver approximately 0.2 g). Tap the sides of the ignition dish to pack the zinc oxide.

9.7 Prepare the blank, using the same amount of zinc oxide in an ignition dish.

9.8 Ignite the gasoline, using the flame from a Bunsen burner. Allow the gasoline to burn to extinction (Note 5).

9.9 Place the ignition dishes containing the sample and blank in a hot muffle furnace set at a temperature of 621 to 704°C (1150 to 1300°F) for 10 min. Remove and cool the ignition dishes. When cool gently tap the sides of the dish to loosen the zinc oxide. Again place the dishes in the muffle furnace for 5 min. Remove and cool the ignition dishes to room temperature. The above treatment is usually sufficient to burn the carbon. If the carbon is not completely burned off, place the dish into the furnace for further 5-min periods.

NOTE 6 - Step 9.9 can also be accomplished by heating the ignition dish with a Meker burner, gradually increasing the intensity of heat until the carbon from the sides of the dish has been burned; cool to room temperature.

9.10 Pipet 25 mL of H2SO4 (1 + 10) into each ignition dish. While pipeting, carefully wash all traces of zinc oxide from the sides of the ignition dish.

9.11 Cover the ignition dish with a borosilicate watch glass and warm the ignition dish on a hot plate until the zinc oxide is completely dissolved.

9.12 Transfer the solution through filter paper to a 100-mL volumetric flask. Rinse the watch glass and the dish several times with distilled water (do not exceed 25 mL) and transfer the washings through the filter paper to the volumetric flask.

9.13 Prepare the molybdate-hydrazine reagent.

9.14 Add 50 mL of the molybdate-hydrazine reagent by pipet to each 100-mL volumetric flask. Mix immediately by swirling.

9.15 Dilute to 100 mL with water and mix well. Remove stoppers from flasks after mixing.

9.16 Place the 100-mL flasks in the constant-temperature bath for 25 min so that the contents of the flasks are below the liquid level of the bath. The temperature of the bath should be 82.2 to 87.8°C (180 to 190°F) (Note 1).

9.17 Transfer the 100-mL flasks to the cooling bath and cool the contents rapidly to room temperature (Note 4).

9.18 Allow the samples to stand at room temperature before measuring the absorbance.

NOTE 7 - The color developed is stable for at least 4 h.

9.19 Set the spectrophotometer to the wavelength of maximum absorbance as determined in 8.9. Adjust the spectrophotometer to zero absorbance, using distilled water. If using a dual-beam spectrophotometer, place distilled water in both cells.

9.20 Measure the absorbance of the samples at the wavelength of maximum absorbance. If using a dual-beam spectrophotometer, measure the absorbance of the samples at the wavelength of maximum absorbance with distilled water in the reference cell.

9.21 Subtract the absorbance of the blank from the absorbance of each sample (8.10).

9.22 Determine the micrograms of phosphorus in the sample, using the calibration curve from 8.12 and the corrected absorbance.

10. Calculations
10.1 Calculate the milligrams of phosphorus per litre of sample as follows:
P, mg/L = P/V
where:
P = phosphorus read from calibration curve, µg and
V = gasoline sample, mL.

To convert to grams of phosphorus per U.S. gallon of sample, multiply mg P/L by 0.0038.

10.2 If the gasoline sample was taken at a temperature other than 15.6°C (60°F), make the following temperature correction:
mg P/L at 15.6°C = [mg P/L at t][1 + 0.001 (t - 15.6)]
where:
t = observed temperature of the gasoline, °C.

10.3 Concentrations below 2.5 mg/L or 0.01 g/gal should be reported to the nearest 0.01 mg/L or 0.0001 g/U.S. gal.
10.3.1 For higher concentrations, report results to the nearest 1 mg P/L or 0.005 g P/U.S. gal.