Simultaneous Determination of Passivator and Antioxidant Additives in Insulating Mineral Oils by High-Performance Liquid Chromatography

Abstract

Insulating mineral oils frequently contain additives to improve their inherent characteristics, such as oxidation stability (antioxidants), electrostatic charging tendency, and compatibility with other materials (passivators). Despite standard test methods are available for the detection of individual additives, none of them covers the simultaneous detection of additives of different kind. For the first time the simultaneous determination of antioxidants and passivators most frequently added to mineral insulating oils is provided. The compounds investigated included three inhibitors (N-phenyl-1-naphthylamine, 2,6-di-tert-butylphenol, and 2,6-di-tert-butyl-p-cresol) and two passivators (benzotriazole and Irgamet 39). A solid-phase extraction step, aimed at the reduction of matrix oil components was optimized. Due to the hydrophobic characteristics of the additives, a reversed-phase chromatographic separation was used and optimized. The method here developed was applied to the analysis of inhibited and passivated transformer oils, compared with Standard IEC60666 method, which requires the use of Fourier transform infrared (FT-IR) spectrophotometry (measurement of the adsorption corresponding to the stretching ν(OH) of the phenol group at 3650 cm⁻¹), and finally applied inside a Proficiency Testing Program of the American Society for Testing and Materials International (ASTM). In a unique analysis, the method provides a tool to assess chemical composition and concentration of oil additivation, and it allows discrimination among phenolic inhibitors.

Keywords:

• antioxidants
• inhibitors
• mineral oil
• oil monitoring
• passivators
• SPE-HPLC

Acknowledgment

The authors would like to thank Dr. Giulia Brusotto and Dr. Marta Franz for their valuable help.

Notes

^a Concentration determined by standard addition method after SPE: 3290 mg · kg⁻¹.

^b Concentration declared by manufacturer: <3000 mg · kg⁻¹.

^c ; x _mj  = value measured with the SPE-HPLC procedure; x _M  = target value = 1090 mg · kg⁻¹; σ _p  = target standard deviation = 150 mg · kg⁻¹.

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