Piston dosing apparatus for fluid media, having a) a cylinder unit, which predetermines a cylinder chamber and is mounted in a connection arrangement; b) a piston arrangement (32; 32, 50) having a piston element (38; 50), which is movable in the cylinder chamber and, in certain regions, delimits an operating chamber in the cylinder chamber; wherein c) the cylinder unit has a filling opening, which communicates with the operating chamber and via which fluid medium can be fed to the operating chamber, and a discharging opening, which communicates with the operating chamber and via which fluid medium can be discharged from the operating chamber; d) the cylinder unit is movable relative to the connection arrangement and can assume a filling position and a discharging position; e) the connection arrangement has a filling connection, which can be connected to a source for fluid medium, and a discharging connection, which can be connected to an application device; f) in the filling position of the cylinder unit, the filling opening is connected fluidically to the filling connection and the discharging opening is separated in space, and thus electrically isolated, from the discharging connection; g) in the discharging position of the cylinder unit, the discharging opening is connected fluidically to the discharging connection and the filling opening is separated in space, and thus electrically isolated, from the filling connection, characterized in that h) the piston element (38; 50) of the piston arrangement (32; 32, 50) is formed by a piston of a piston unit which can be driven from the outside by a piston rod; or i) the piston element (38; 50) of the piston arrangement (32; 32, 50) is formed by a piston which is movable independently in the cylinder chamber, and the piston arrangement (32; 32, 50) also comprises a piston unit which can be driven from the outside by a piston rod, is arranged on that side of the independently movable piston remote from the operating chamber and can be brought into operative connection with the same; wherein j) in at least one position, the piston unit can be locked in a releasable manner with the cylinder unit, and therefore the cylinder unit is carried along by the piston unit as the latter moves. a) a cylinder unit, which predetermines a cylinder chamber and is mounted in a connection arrangement; b) a piston arrangement (32; 32, 50) having a piston element (38; 50), which is movable in the cylinder chamber and, in certain regions, delimits an operating chamber in the cylinder chamber; wherein c) the cylinder unit has a filling opening, which communicates with the operating chamber and via which fluid medium can be fed to the operating chamber, and a discharging opening, which communicates with the operating chamber and via which fluid medium can be discharged from the operating chamber; d) the cylinder unit is movable relative to the connection arrangement and can assume a filling position and a discharging position; e) the connection arrangement has a filling connection, which can be connected to a source for fluid medium, and a discharging connection, which can be connected to an application device; f) in the filling position of the cylinder unit, the filling opening is connected fluidically to the filling connection and the discharging opening is separated in space, and thus electrically isolated, from the discharging connection; g) in the discharging position of the cylinder unit, the discharging opening is connected fluidically to the discharging connection and the filling opening is separated in space, and thus electrically isolated, from the filling connection, characterized in that h) the piston element (38; 50) of the piston arrangement (32; 32, 50) is formed by a piston of a piston unit which can be driven from the outside by a piston rod; or i) the piston element (38; 50) of the piston arrangement (32; 32, 50) is formed by a piston which is movable independently in the cylinder chamber, and the piston arrangement (32; 32, 50) also comprises a piston unit which can be driven from the outside by a piston rod, is arranged on that side of the independently movable piston remote from the operating chamber and can be brought into operative connection with the same; wherein j) in at least one position, the piston unit can be locked in a releasable manner with the cylinder unit, and therefore the cylinder unit is carried along by the piston unit as the latter moves.