The quantitative determination of 2,6-ditertiary-butyl paracresol and 2,6-ditertiary-butyl phenol in a new electrical insulating oil measures the amount of this material that has been added to the oil as protection against oxidation. In a used oil it measures the amount remaining after oxidation has reduced its concentration. The test is also suitable for manufacturing control and specification acceptance.

When an infrared spectrum is obtained of an electrical insulating oil inhibited with either of these compounds there is an increase in absorbance of the spectrum at several wavelengths (or wavenumbers). 2,6 ditertiary-butyl paracresol produces pronounced increases in absorbance at 2.72 μm (3650 cm⁻¹), and 11.63 μm (860 cm⁻¹). 2,6 ditertiary-butyl phenol produces pronounced increases in absorbance at 2.72 μm (3650 cm⁻¹) and 13.42 μm (745 cm⁻¹).

When making this test on other than a highly oxidized oil or when using a double-beam spectrophotometer, it has been found convenient to obtain the spectrum between 2.5 μm (4000 cm⁻¹) and 2.9 μm (3450 cm⁻¹) because the instrument is compensated for the presence of moisture and the band is not influenced by intermolecular forces (associations). However, when testing a highly oxidized oil or when using a single-beam instrument better results may be obtained if the scan is made between 10.90 μm (918 cm⁻¹) and 14.00 μm (714 cm⁻¹).

Increased absorption at 11.63 μm (860 cm⁻¹) or 13.42 μm (745 cm⁻¹) or both, will identify the inhibitor as 2,6-ditertiary-butyl paracresol or 2,6-ditertiary-butyl phenol respectively (Note 1).

Note 1-The absorbance at 745 cm⁻¹ for 2,6-ditertiary-butyl phenol and at 860 cm⁻¹ for 2,6-ditertiary-butyl paracresol for equal concentrations will be in the approximate ratio of 2.6 to 1.