A mixing unit for ready-to-use flushing solutions with a mixing chamber, with which conduits for ultrapure water and for concentrate communicate, in order to form a ready-to use flushing solution therein is characterized in that the concentrate is filled into a concentrate bag, which is provided with a concentrate bag connector, which is connectable to a connector on the mixing unit, that the connector on the mixing unit is covered by a pivotal concentrate valve in its closed state so that a sealed flushing space is defined around the connector and that a flushing liquid conduit communicates with the connector on the mixing unit so that flushing liquid is conveyable into the flushing space, which can completely clean the connector internally and externally.