An adapter for a potentiometric sensor having a sensor shaft, which has a reference liquid opening in its exterior surface. The adapter comprises an annular chamber member having a sensor opening for receiving the sensor shaft. Arranged in the sensor opening are first and second sealing rings for the sensor shaft. The axial position of the reference liquid opening lies between the first and second sealing rings. Formed between the sealing rings and the sensor shaft is an annular chamber, which is in communication with the reference liquid opening. The annular chamber member further includes a duct extending between the annular chamber and a reference feed opening. The adapter further includes a process connection member having a process connection opening, which surrounds the sensor shaft, and whose axis is aligned with the axis of the sensor opening. The axial position of the process connection member is fixed relative to the annular chamber member, and the process connection member is freely rotatable relative to the annular chamber member.