# Regulation (EC) No 648/2004 of the European Parliament and of the Council of 31 March 2004 on detergents (Text with EEA relevance), Division 2.1: Principle

Extent: E+W+S+N.I.

The method is based on the fact that the cationic dye methylene blue forms blue salts with anionic surfactants (MBAS), which can be extracted with chloroform. To eliminate interference, the extraction is first effected from alkaline solution and the extract is then shaken with acidic methylene blue solution. The absorbency of the separated organic phase is measured photometrically at the wavelength of maximum absorption of 650 nm.

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Source: https://www.legislation.gov.uk/eur/2004/648/annex/VIII/division/2/division/2.1 (the official version)
Page: https://openlegislation.uk/eu-regulation-2004-648/annex-VIII/division-2/division-2.1
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.