IEC 62021-1 Insulating liquids - Determination of acidity - Part 1: Automatic potentiometric titration
1 Scope
This part of IEC 62021 describes the procedure for the determination of the acidity of unused and used electrical mineral insulating oils.
NOTE 1 In unused and used mineral insulating oils, the constituents that may be considered to have acidic characteristics include organic acids, phenolic compounds, some oxidation products, resins, organometallic salts and additives.
The method may be used to indicate relative changes that occur in a mineral insulating oil during use under oxidizing conditions regardless of the colour or other properties of the resulting mineral oil.
The acidity can be used in the quality control of unused mineral oil.
As a variety of oxidation products present in used mineral oil contribute to acidity and these products vary widely in their corrosion properties, the test cannot be used to predict corrosiveness of a mineral oil under service conditions.
NOTE 2 The acidity results obtained by this test method may or may not be numerically the same as those obtained by colorimetric methods, but they are generally of the same magnitude.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60475, Method of sampling liquid dielectrics
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 acidity
quantity of base, expressed in milligrams of potassium hydroxide per gram of sample, required to titrate potentiometrically a test portion in a specified solvent to obtain a pH of 11,5
3.2 unused oil
mineral oil which has not been used in, or been in contact with, electrical equipment
4 Principle
The test portion is dissolved in solvent and titrated potentiometrically with alcoholic potassium hydroxide using a glass-indicating electrode and a reference electrode. The meter readings are plotted automatically against the respective volumes of titrant and the end-point is taken when the volume corresponds to a pH of 11,5. This was found to include all inflection points, with very little effect on the result from the rapid change in pH after the last inflection point. While use of inflection points is more accurate, it was found that measurement of weak inflection points was more instrument-dependent and gave poorer reproducibility.
