This part of IEC 62021 describes a procedure for 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 that may or may not be shown by 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 potentiometric methods, but they are generally of the same magnitude. The potentiometric method
uses an endpoint at pH 11,3 to ensure titration of all species, whereas the colourimetric methods uses an indicator
changing colour at approximately pH 9,5. This may lead to slightly higher results for oils with acidities above
0,3 mg KOH/g oil when using the potentiometric method.