ASTM D2668 Standard Test Method for 2,6-di-tert-Butyl- p-Cresol and 2,6-di-tert-Butyl Phenol in Electrical Insulating Oil by Infrared Absorption
7. Procedure
7.1 Using the quantitative scan mode, make at least three scans (see Note 2) of the test specimen on which the determination of the 2,6-ditertiary-butyl paracresol or 2,6-ditertiary-butyl phenol content is to be made, at the desired band. Do this in accordance with Practices D2144, except that cells having path lengths as specified in 6.2 are preferred. Use the same cell that was used in obtaining the spectra for the calibration curve. For the Differential Method, fill the reference cell with an appropriate base oil free of oxidation inhibitors. Use a spectrophotometer which meets the requirements specified in Section 6 of Practices D2144 and instrumental conditions identical to those used in obtaining the spectra for the calibration curve. In particular, it should be noted that Fourier-transform rapid scan infrared spectrophotometers may also be used and in some cases are preferred for the measurement.
7.2 For routine tests, a single scan of the test specimen is adequate as long as the instrument is capable of meeting the criteria of 8.2 using single scans to replace average values. For referee purposes, use the average of three scans of the test specimen performed manually or the average of the same number of scans used to standardize the instrument for the test specimen, if performed automatically.
7.3 Manually Determined Results - Read the values of absorbance at points A o and A on each of the three spectra to the nearest 0.001 absorbance unit; obtain the differences and average them. Using this average value, determine the concentration from the calibration curve.
7.4 Automatically Determined Results - Using the average value of the absorbance difference between A and Ao that is determined automatically by the instrument for the same number of scans performed on the test specimen as for the standards, determine the concentration of the test specimen from the calibration curve.
