The invention relates to a metering device (D) for metering a reagent by dissolution in a liquid flow, characterized in that it comprises a plurality of parts: —a fixed base (200) grouping together said enclosure and a liquid inlet, means for bringing the liquid into contact with the reagent roller (G) and liquid outlet means, —one or more cartridges (300) forming a separate detachable volume grouping together said reagent magazine, means for forming at least one liquid jet directed substantially radially towards the roller (G) to be dissolved, and means for supporting the roller (G), said cartridge (300) coming to be associated with said base (200) which is preformed with at least one area for receiving said cartridge (300) for the purpose of dissolving the reagent.