ASTM D6258 Determination of Solvent Red 164 Dye Concentration in Diesel Fuels
4. Summary of Test Method
4.1 The absorbance of each sample is recorded over a specified wavelength range, and the scan is analyzed using derivative analysis software to determine the dye concentration.
4.2 Derivative analysis methodology is employed to minimize interferences caused by variations in the color and composition of the fuel samples regularly tested using this test method.
4.2.1 Naturally occurring diesel test fuels range in color from water white to nearly black, and many of the samples tested using this test method have also had used oils and other products blended with them. These variations in color and composition have a significant effect upon absorbance characteristics of the samples in the region of the visible spectrum where azo dyes absorb. Standard operating procedures to correct for these background variations would involve running a neat (undyed) sample and subtracting out the background absorbance. In most situations involved with the application of this test method, however, neat material is not available, so no background corrections can be made.
4.2.2 The second derivative of the absorbance of these dyes in the visible region is a function of the fine structure of the dye's absorbance peak (versus its height or area) and is relatively unaffected by changes in background absorbance. Further, the specific sections (maxima and minima) of the second derivative spectra employed for this analysis are a function of the fine structure of the top of the absorbance curve which has been found to be unique to the azo dyes.
5. Significance and Use
5.1 This test method was developed to provide for the enforcement of 26 CFR 48.4082-1(b), which mandates that all tax-exempt diesel fuels be dyed with an amount of Solvent Red 164 at a concentration that is spectrally equivalent to 3.9 lb/103 bbl (11.1 mg/L) of Solvent Red 26. It is employed to verify that the correct amount of Solvent Red 164 is being added to tax-exempt product at terminals or refineries prior to sale, and to detect the presence of Solvent Red 164 in taxed product intended for on-road use.
5.1.1 Solvent Red 26 is the azo dye shown in Fig. 1. It is the standard against which the concentration of Solvent Red 164 is measured because it is available in a certified pure form. Solvent Red 164 is identical in structure to Solvent Red 26 except that it has hydrocarbon (alkyl) chains incorporated to increase its solubility in diesel and burner fuels. The exact composition of Solvent Red 164 will vary from manufacturer to manufacturer and lot to lot depending upon the extent of alkylation that occurs during production; however, its visible spectrum is virtually identical to the spectrum of Solvent Red 26. Solvent Red 164 is employed in the field (instead of Solvent Red 26) to dye tax-exempt diesel and burner fuels because of its higher solubility and relatively low cost.
