Ammonia and Sulfur Dioxide Electrodes

The ammonia electrode operates by the diffusion of dissolved ammonia from the sample through a highly gas-permeable membrane into an internal proprietary filling solution. Diffusion proceeds until a reversible equilibrium is established between the ammonia level of the sample and the level in the filling solution. Hydroxide ions are formed in the filling solution by the ammonia-water reaction 

The OH level in the filling solution which is detected by the sensing element, is directly proportional to the sample ammonia level. The electrode potential follows the approximate Nernst relation 

The sulfur dioxide electrode works on exactly the same general principle as the ammonia electrode. Equilibrium is established between the sulfite level in the sample and the sulfite level in the proprietary filling solution. Hydrogen ions formed in the internal filling solution according to the reaction of SO2 and HjO 

---