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This lnternational Standard specifies two methods for the measurement of the isocyanate content of aromatic isocyanates used as polyurethane raw materials. Method A is primarily applicable to refined toluene diisocyanate (TDI), methylene-bis-(4-phenylisocyanate) (MDI) and their prepolymers. Method B is applicable to refined, crude or modified isocyanates derived from toluene diisocyanate, methylene-bis-(4-phenylisocyanate) and polymethylene polyphenylisocyanate. It can also be used for isomer 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. It is not applicable to blocked isocyanates.