Comparative colour solutions for methods I and II evaluating colouring of liquids according to PhB MMII are of identical composition. For method I, they can be stored without any time limit, whereas for method II are to be prepared just immediately prior to their use. The present paper deals with the examination of stability of comparative colour solutions PhB MMII kept in test-tubes made of colourless borosilicate glass with ground-glass stoppers, protected from light. The colour differences ΔE* in the uniform colour space CIELAB calculated from transmittance spectra measured in the course of preservation of solutions revealed that comparative colour solutions PhB 2002 show higher stability when there is a higher concentration of colour components in the solution. In solutions H9, HŽ7, and Ž7, the time period from their preparation to the time not exceeding the difference of ΔE* 1.5 units ranged within 3 days, in ZŽ7 it was at least 14 days, and in Č7 at least 20 days. In solutions H5, HŽ4, Ž4, ZŽ4, and Č4, it was at least 25 days, in solutions H2, HŽ2, Ž2, ZŽ2, and Č2 at least 41 days. These findings make questionable the justification of preservation of colour solutions according to PhB MMII for method I for an unlimited time period. At the same time they show that the solutions are not unstable to such an extent that they should be prepared for method II just prior to use in all cases.

        Key words: Czech Pharmacopoeia – comparative colour solutions – stability of colouring – colority – CIELAB