This International Standard specifies a semi-micro colour-indicator method for the determination of acidic constituents in new or used petroleum products and lubricants soluble in mixtures of toluene and propan-2-ol, or capable of existing as suspensions in such mixtures, and where the suspended material is sufficiently dissociated that its acidic components can be titrated. The method is especially intended for applications in which the amount of sample available to be analysed is too small to allow accurate analysis by ISO 6619 or ISO 6618. It is applicable to the determination of acids having dissociation constants in water larger than 10-9. Extremely weak acids having dissociation constants smaller than 10-9 do not interfere. Salts titrate if their hydrolysis constants are larger than 10-9. NOTE 1 This method may be used to indicate relative changes in acid number that occur in an oil during use under oxidizing conditions. Although the titration is made under definite equilibrium conditions, the method does not measure an absolute acidic property that can be used to predict performance of an oil under service conditions. No general relationship between bearing corrosion and acid number is known. NOTE 2 Since this test method requires substantially less sample than ISO 6618 or ISO 6619, it provides an advantageous means of monitoring an oxidation test by changes in acid number by: a) minimizing test sample depletion for acid number analyses and thus minimizing the disturbance of the test, and b) allowing additional acid number analyses to be made while maintaining the same test sample depletion, and thus providing additional data. NOTE 3 Some oils, such as many cutting oils, rustproofing oils, and similar compounded oils, or excessively dark-coloured oils, may be more difficult to analyse by this method due to obscurity of the colour-indicator end-point. These oils can be analyzed by ISO 6619 provided sufficient sample is available. However, this situation is much less likely using this International Standard than using ISO 6618, since the sample is more dilute during the titration and the end-point colour change is more stable. The acid numbers obtained by this method may or may not be numerically the same as those obtained by ISO 6619, but they should be of the same order of magnitude. NOTE 4 The results obtained using this method have been found to be numerically the same as those obtained using ISO 6618, within the precision of the two methods, for new or oxidized lubricants of the type primarily intended for hydraulic or steam turbine type service. The oxidized lubricants were obtained using ISO 4263. This correlation is shown by the correlation coefficient r = 0,989 with slope s = 1,017 and intercept y = +0,029, calculated using the acid numbers obtained using both titration methods for the samples used for the precision statement (see clause 11).