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2,6-Di-tert-butyl-p-cresol (DBPC), dibenzyl disulfide (DBDS), and 1,2,3-benzotriazole (BTA) are additives that may be found concomitantly in the oil matrix of power transformer. DBPC and DBDS act as antioxidants while, BTA is a corrosion inhibitor that protects copper conductors inside the transformer unit from corrosion. A powerful analytical method is, therefore, required to determine these additives at trace levels in the transformer oil. This work describes a unique single liquid-liquid extraction pretreatment step prior to the determination of the components by gas chromatography (GC) and high-performance liquid chromatography (HPLC) techniques. The optimum volume ratio used in the pretreatment step was determined as 5:2:5 for mineral oil/n-hexane/acetonitrile, respectively. Relatively, the method is simple and quick with a minimal use of solvents. Analytical results indicate that the method is relatively sensitive, accurate, and precise for each of the three components in fresh and used mineral oil. The calibration curves for the three components demonstrate a significant increase in sensitivities. Detection limits found were, 100 mg L(-1) (0.01% w/v), 0.80 mg L(-1) , and 2.04 mg L(-1) for DBPC, DBDS, and BTA, respectively. The Student's t values determined at 95% confidence level indicate that there is no significant difference between the experimental means obtained by this method and the standard method for each component.

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This study examines the kinetics of the net depletion of dibenzyl disulfide (DBDS) from its solution in mineral insulating oil due to copper corrosion at elevated temperatures in power transformers. The concentration of DBDS is determined using liquid-liquid extraction followed by GC detection. Measurements, at 24 h intervals, up to 168 h, show that the depletion of DBDS from its solutions in mineral insulating oil at 150 C and initial concentrations of 3.00, 6.00, 9.00, 12.00 and 15.00 mg L −1 is very slight compared to identical measurements in the presence of copper strips in the mineral oil. The analysis of the kinetic data reflects the complexity of the corrosion process and its occurrence at a concentration as low as 1 mg L −1 which is much lower than what has been previously believed. Measurements on solutions at initial DBDS concentrations of 150 mg L −1 indicate that even when the insulation oil matrix is in contact with copper strips the corrosion does not occur to any measurable extent below 100 C and starts to speed up appreciably only beyond 125 C. From data at 100, 125 and 150 C an energy of activation equal to 123 kJ mol −1 is estimated.

The incubation criterion of the 62535 standard procedure of the International Electrochemical Commission (IEC) was modified to obtain the depletion profiles for different concentrations of each of 1,2,3-benzotriazol (BTA) and dibenzyl disulfide (DBDS) when both are present in a mineral oil matrix. Measurements on BTA concentrations ranging from 0 to of 70 mg L −1 show that its depletion profile after incubation for 72 h, at room temperature, and at 150 o C in the presence and absence of a copper strip, is the same irrespective of the DBDS concentration. Similar measurements on DBDS at concentrations ranging from 10 to 300 mg L −1 show that identical depletion profiles are obtained as long as the BTA concentration is maintained in excess of 5 mg L −1 . The results show that a minimum BTA concentration of 5 mg L −1 is needed to make the copper windings in contact with the mineral oil passive and in turn suppress their sulfur corrosion by DBDS.