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This International Standard specifies methods for the measurement of the acidity of aromatic isocyanates and prepolymers used as polyurethane raw materials. Method A is a straightforward titration without a blank and is primarily applicable to products derived from crude or modified methylene-bis-(4-phenylisocyanate), polymethylene polyphenylisocyanates and toluene diisocyanate having moderate to high ( > 100 μg/g ) levels of acidity. Method B is applicable to refined, crude or modified isocyanates derived from methylene-bis-(4-phenylisocyanate), polymethylene polyphenylisocyanate and toluene diisocyanate having low ( < 100 μg/g ) levels of acidity, and a blank is employed. These methods can also be used for isomeric mixtures of toluene diisocyanate, methylene-bis-(4-phenylisocyanate) and polymethylene polyphenylisocyanate. Other aromatic isocyanates may be analysed by this method if precautions are taken to verify suitability.