Abstract: Cartridge unit for an electrochemical sensor includes a cartridge (2) prefilled with electrolyte and having a first end closed by a selectively permeable membrane (7), and a second end (6) having a fastening device (11) arranged for fastening the cartridge to the electrochemical sensor (24). The cartridge unit further has a supporting member (3) to which the cartridge (2) is detachably fastened, and closing devices (4, 20) for closing the second end (6) of the cartridge (2). These closing devices (4, 20) are able to be opened by a user to allow the cartridge (2) to be fastened to the electrochemical sensor (24) and then to be detached from the supporting element (3).