A new type of detector involving heat effects in a catalyst bed has been developed and applied to the measurement of nitrogen oxides. This detector is called the reaction-heat detector (RHD) and consists of a catalyst bed of vanadium pentoxide, a bed of aluminum powder as a preheater and a differential type thermocouple with two junctions. One junction of the thermocouple was placed in the center of the catalyst bed and the other in the bed of aluminum powder as a reference. The sample containing NOx was introduced with a known amounts of ammonia into a constant air stream as a carrier gas. The determination of NOx was perfomed by measuring the heat of reaction produced by the catalytic reaction between the NOx and the added ammonia. The optimum conditions for the measurement were as follows: flow rate, 150 ml/min (s.v. =1200-1); added concentration of NH3, 1 % catalyst temperature, 325°C. Under such conditions, detector response was proportional to the concentration of NOx ranging (20~1200) ppm. The effects of interference gases were examined to apply the RHD to the selective measurement of NOx in exhaust gas. The RHD seems to be applicable for the measurement of NOx in exhaust gas unless the gas contains sulfur dioxide of more than 300 ppm or high concentration of hydrocarbons. Especially, the detector was well suited for the actual measurement of NOx in automobile exhaust gas compared with other methods.
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