The invention relates to a dispensing unit for dispensing fluid into an external container. The dispensing unit comprises: a connector, a reagent cylinder for storing the fluid, and a locking ring. The connector comprises an upper fastener, and a bottom wall with at least one passage therethrough. The reagent cylinder comprises at least one compartment for storing the fluid, which compartment comprises at least one opening through a cylinder bottom. The at least one opening is located so that for at least one mutual orientation of the reagent cylinder and the connector, fluid can flow from the compartment to the external container via the at least one opening and the at least one passage. The locking ring comprises a second fastener which fastens to the upper fastener of the connector, so that when the locking ring is tightened, the reagent cylinder is pressed towards the interior bottom of the connector.