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.