A mixed potential sensor device and methods for measuring total ammonia (NH.sub.3) concentration in a gas is provided. The gas is first partitioned into two streams directed into two sensing chambers. Each gas stream is conditioned by a specific catalyst system. In one chamber, in some instances at a temperature of at least about 600.degree. C., the gas is treated such that almost all of the ammonia is converted to NO.sub.x, and a steady state equilibrium concentration of NO to NO.sub.2 is established. In the second chamber, the gas is treated with a catalyst at a lower temperature, preferably less than 450.degree. C. such that most of the ammonia is converted to nitrogen (N.sub.2) and steam (H.sub.2O). Each gas is passed over a sensing electrode in a mixed potential sensor system that is sensitive to NO.sub.x. The difference in the readings of the two gas sensors can provide a measurement of total NH.sub.3 concentration in the exhaust gas. The catalyst system also functions to oxidize any unburned hydrocarbons such as CH.sub.4, CO, etc., in the gas, and to remove partial contaminants such as SO.sub.2.