After a review of most common sensor principles for the detection of CO2, with emphasis on solid state potentiometric sensors, the thermodynamic principles of recently developed potentiometric CO2 sensors with open reference electrodes are briefly discussed. These sensors are based on acid-base interactions and allow sealing-free cell arrangements with signals being independent of the oxygen partial pressure. Results concerning the CO2 and O-2 partial pressure as well as temperature dependence are presented. At least one of the reference electrodes could be identified to show a very fast response to varying CO2 partial pressures. Coulometric titrations were performed in order to check the phase stability of the reference electrodes. In this way the TiO2-rich part of the quasi-binary phase diagram TiO2-Na2O is elucidated.