Anodic half-reaction in the $Na^+$ ionic sensors using $Na_2CO_3$ and $MCO_3$ ($M=Cs_2,K_2,Li_2,Ca$) as a sensitive membrane is derived in continuous flow system to explain $CO_2$ sensing characteristics. For various gas-sensitive membranes, a well known overall reaction, $MCO_3;=;MO;+;CO_2$, cannot be applied for the EMF behaviors of these kinds of sensors. So, the anodic reaction is found to involve $Na_2CO_3$ and $M^{++}$-containing oxide phases by employing the ion exchange reaction at the interface of solid electrolyte and the sensitive membrane to maintain ionic balance in the whole cell. Based on the electrode reaction derived in flow system, differences of cell potentials between continuous and discontinuous flow systems were also discussed. These EMF differences were considerably caused by the partial pressures of oxygen and $CO_2$ as well as irreversible chemical reactions between electrode materials and $CO_2$ atmosphere.