供试品中的羟基与邻苯二甲酸酐酯化。多余的邻苯二甲酸酐水解，形成的邻苯二甲酸用标准体积氢氧化钠溶液中和。如果样品含有游离酸度或碱度，则进行相应的校正。根据校正的羟基数，可通过给出的公式计算相对分子质量。

The hydroxy groups present in a test portion are esterified with phthalic anhydride. The excess phthalic anhydrids is hydrolyzed and the phthalic acid formed is neutralized with standard volumetric sodium hydroxide solution. If the sample contains free acidity or alkalinity a corresponding correction is made. From the corrected hydroxyl number, the apparat relative molecular mass may be calculated by a formula given.