An ammonia and isocyanic acid measuring method for soot containing exhaust gases

The set-up and operating principle of an analyzer, which can be used without any filter for the continuous measurement of ammonia (NH3) and isocyanic acid (HNCO) in soot containing exhaust gases is described. Ammonia and isocyanic acid are dissolved in a buffer solution, in which ammonia is measured directly with an ammonia-sensitive electrode. The sum of ammonia and isocyanic acid is measured with the same method after hydrolysis of HNCO to NH3 and CO2. Ammonia and isocyanic acid concentrations of up to ∼500 ppm can be measured with a mean relative error of 3.6% at a standard deviation of 4.0%. The investigation of the urea thermohydrolysis, which is an important step in the selective catalytic reduction (SCR) of nitrogen oxides in Diesel exhaust gases, revealed the emission of toxic isocyanic acid besides ammonia from the SCR catalyst.