规定方法的原理是将酸酐硬化剂或促进剂试验部分中的少量游离酸与罗丹明6G反应，使试验溶液呈粉红色，使用光谱仪在约510 nm的波长下测量有色溶液的吸光度，并根据校准曲线确定游离酸含量。该方法适用于大多数酸酐，但聚酯和低聚物酸酐除外，这可能会导致结果过于分散。

The principle of the method specified is reacting a small amount of free acid present in a test portion of acid anhydride hardener or accelerator with rhodamine 6G to colour the test solution pink, measuring the absorbance of the coloured solution at a wavelength of about 510 nm using a spectrometer, determining the free-acid content from a calibration curve. The method is suitable for most anhydrides except for polyester and oligomeric anhydrides which may give too wide a scatter of results.