USP<631>: Color and Achromicity

USP <631> Color and Achromicity is to assess the color or colorlessness of liquid samples is part of a harmonization effort across major pharmacopeias (USP, EP, JP), though adoption details vary by region. It provides two procedures: the traditional Method I (Organoleptic/Visual) and the modernized Method II (Instrumental/Quantitative). The instrumental method is the referee procedure in case of dispute, reflecting a modernization that emphasizes objective and reproducible measurement. This applies only when an individual USP monograph specifies testing per <631>.

In practice, USP <631> is applied infrequently and only when explicitly referenced in a monograph, with instrumental methods preferred for objectivity and reproducibility

When applied: Only when called for by an individual USP monograph. Intended for uniform liquid samples (or the liquid phase of dispersed systems after removing the dispersed phase).

Methods provided:

• Method I — Organoleptic (visual): Controlled visual comparison against specified reference/matching solutions under standardized conditions.
• Method II — Instrumental (quantitative): Acquire spectral data with a qualified spectrophotometer/colorimeter; compute CIE X,Y,Z values and report in CIELAB (L, a*, b*) using the defined illuminant/observer conditions (e.g., D65/10°) specified in <631>.
• Choice of method: The monograph dictates which procedure to use. Instrumental measurement is prioritized for objectivity; the visual method remains available when specified.