ASTM D2269 Test Method for Evaluation of White Mineral Oils by Ultraviolet Absorption
9. Correction for Inhibitor Content
9.1 A correction for the absorbance due to inhibitor may be made as described below if a sufficient amount of the same inhibitor contained in the sample and the inhibited sample are available to prepare a blend. The concentration of additional inhibitor to be added to the inhibited sample should be equal to the concentration contained in the inhibited sample.
NOTE 5 - The inhibitor content is usually expressed in pounds per thousand barrels. Pound per thousand barrels x 0.00285 = grams per litre. For example, 3 lb/1000 barrels x 0.00285 = 0.00855 g/litre.
9.2 Weigh a minimum of 50 mg of inhibitor in a volumetric flask, fill to volume with inhibited sample and mix thoroughly. Make further dilutions with inhibited sample, as necessary, to obtain the desired concentration of added inhibitor.
9.3 Run the original inhibited sample in accordance with Test Method D2269 and label it Run A.
9.4 Run the blend containing known amount of added inhibitor in accordance with Test Method D2269 and label it Run B.
9.5 Calculate the difference in absorbance between Run B and Run A at corresponding wavelengths as follows:
ΔA = Ab - Aa
where:
ΔA = difference in absorbance at a given wavelength,
Ab = absorbance of Run B at the same wavelength, and
Aa = absorbance of Run A at the same wavelength.
9.5.1 Correct the absorbance at each wavelength as follows:
Ac = Aa - DA
where:
Ac = corrected absorbance at each wavelength.
9.6 Mark the points of inflection on the spectrum from Run A at wavelengths lower and higher than those which were increased.
9.6.1 Draw a baseline tangent to the curve connecting the two points.
9.6.2 Read the absorbance, Ad, along the baseline at each wavelength.
9.7 Compare Ad and Ac at each position. If Ac is less than Ad, it is indicated that the inhibitor content of the original sample is less than the amount added.
9.8 Use the absorbance Ac or Ad, whichever is greater, to compare to the solvent control through the range 260 to 350 nm, inclusive.
