An assembly (1) for preparing mineral water in situ, comprising a container (2) filled with a mineral concentrate (4) and a dosing device (6) connected to the holder for dispensing the concentrate from the container in a dosed manner, the dosing device being provided with a housing (15) comprising an inlet (16), an outlet (18), a liquid flow path (20), a pump (22), and a magnetisable or magnetic operating element (24). The operating element is being included in the housing for carrying out a predetermined movement relative to the housing with the aid of a changing magnetic field and the operating element is mechanically connected to the pump for driving the pump when the operating element carries out the predetermined movement. The operating element is manufactured, from corrosion resistant steel, or the operating element is provided with a corrosion resistant coating such as gold, silver or Teflon or the operating element is manufactured, from a magnetisable and/or magnetic ceramic material.