At its 165th session in November 2019, the European Pharmacopoeia Commission adopted a new version of one of its widely used general methods, chapter 2.2.2. Degree of coloration of liquids, which has been extensively revised to include the instrumental method. The revised chapter will be published in European Pharmacopoeia (Ph. Eur.) Supplement 10.3, available in July 2020 (implementation date: 1 January 2021).
This chapter now describes three methods:
- in method I (visual method), the colours are compared in diffused daylight, viewing horizontally against a white background;
- in method II (visual method), the colours are compared in diffused daylight, viewing vertically against a white background;
- method III is the instrumental method – this part of the text has been harmonised with the USP and the JP in the Pharmacopoeial Discussion Group (PDG).
Instrumental methods for measurement of colour provide more objective data than the subjective viewing of colours by a small number of individuals. With adequate maintenance and calibration, they can provide accurate, precise and consistent measurements of colour that do not drift over time. Through extensive colour-matching experiments involving human subjects with normal colour vision, distribution coefficients (weighting factors) have been established for each wavelength in the visible spectrum, giving the relative amount of stimulation of each receptor type caused by the light of that wavelength.
At present, however, the specifications indicated in the Ph. Eur. are all based on visual determination and an exact correlation between visual and instrumental results is not always possible, depending on the ability of the analyst to differentiate between colour grades (visual method) and on the equipment settings. Hence, when using chapter 2.2.2, the analyst is asked to report the results together with the method used: method I, II or III.
The instrumental method, when used instead of one of the visual methods currently prescribed in individual monographs, is nevertheless considered to be an alternative method and, as such, its use must meet the requirements of the Ph. Eur. General notices, which state: “the tests and assays described are the official methods upon which the standards of the Pharmacopoeia are based. With the agreement of the competent authority, alternative methods of analysis may be used for control purposes, provided that the methods used enable an unequivocal decision to be made as to whether compliance with the standards of the monographs would be achieved if the official methods were used. In the event of doubt or dispute, the methods of analysis of the Pharmacopoeia are alone authoritative.”