The apparatus makes it possible to change a desired sample volume—even during operation—in a simple manner without mechanical intervention, with any accumulation solids of a sample contained in a sample collecting vessel being avoided, so that a sample representative of the medium is obtained. The apparatus comprises a control unit which controls a pneumatic control gear and switches a pump by means of which the medium is drawn into a sample collecting vessel via a hose or tube used as a suction line. A detection unit incorporated in the suction line signals the presence of the medium to the control unit, which then controls the time period for taking the desired volume of the sample. The sample volume obtained is drained off into a sample container via a drain device.