ASTM D3348 Rapid Field Test for Trace Lead in Unleaded Gasoline
ASTM D3348 Standard Test Method for Rapid Field Test for Trace Lead in Unleaded Gasoline (Colorimetric Method)
8. Calibration
8.1 Prepare a calibration curve as follows, using at least four gasoline standards of known lead content that cover the range from 0.01 to 0.10 g Pb/gal (2.64 to 26.4 mg Pb/L).
8.1.1 Rinse the 2-mL graduated pipet three times with the gasoline sample. Add 2.0 mL of the sample to a 1-oz glass vial. Add 2.0 mL of iodine/TEAC/CHCl3 solution (Reagent A) from another pipet, to the vial containing the gasoline. Tightly cap the vial.
8.1.2 Place the vial on the ultraviolet light and set the timer to give the sample a 3-min exposure.
8.1.2.1 (Warning - Ultraviolet light can be harmful to the eyes. A protective shield has been provided in the prototype kit. DO NOT remove it or otherwise defeat its purpose. DO NOT stare at the light.)
8.1.3 After exposure, remove and uncap the vial. Measure 10.0 mL of ammonium nitrate solution (Reagent B) into the 10-mL graduated cylinder. Add this to the vial containing the sample. Recap and shake the vial vigorously for 1 min. (The timer in the kit may be used.)
8.1.4 Place a clean 18-mm test tube in the aluminum measuring block. Add 5.0 mL of PAR solution (Reagent C) to the test tube using the mark on the block such that the upper level of liquid in the tube is equal to the mark on the block. Place the plastic funnel in the test tube. Fold a piece of filter paper and place in the funnel.
8.1.5 When the two layers of liquid in the vial have separated (8.1.3), pour the entire contents of the vial inside the filter paper. The aqueous layer will filter into the test tube; the gasoline/CHCl3 layer will remain in the filter paper. Tap the funnel to add any remaining drops of aqueous solution to the test tube. Remove the funnel and discard its contents. Swirl the test tube gently using a wrist action to obtain a uniform color.
NOTE 6 - The lead-PAR complex formed in 8.1.5 must be measured within 10 min after starting 8.1.5.
NOTE 7 - A few drops of the organic layer may come through the filter paper. This will not alter the results and can therefore be tolerated. However, if more than 10 drops do come through, refilter the aqueous layer through a fresh filter paper into a clean empty test tube.
NOTE 8 - Swirling may cause air bubbles to be trapped in the liquid. Wait for these to settle before continuing. Wipe the test tube off with a clean towel to remove any fingerprints that may be present on the surface of the tube.
8.2 Zero and standardize the colorimeter as follows:
8.2.1 Set the colorimeter at 490 nm. Set zero absorbance (100 % transmittance) with water in an 18-mm test tube. Read and record the absorbance (or percent transmittance) obtained for the standards.
8.2.2 Plot the absorbance values versus concentration on rectangular coordinate paper. (If percent transmittance values are used, plot them versus concentration using semilog paper, with the percent transmittance values on the log scale.) Draw a best fit line by eye. (See Fig. 2 and Fig. 3 for examples.) When plotting absorbance versus concentration note that the curve does not pass through the origin.
