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Specifies a method for determination of acid number by potentiometric titration, of lubricating oils and additives soluble in mixtures of propan-2-ol, dimethylsulfoxide and toluene. It is applicable in the range 0,1 mg KOH/g to 250 mg KOH/g and to unused and used lubrication oils and additives. In the case of engine oils and oils with a high polymer content, e.g. viscosity index improvers precision will be poor as the electrode performance will be impaired. Precision data are only valid for industrial oils and related products.