Rapid determination of nitrogen oxides in exhaust gases by ultraviolet spectrophotometry

A rapid and accurate method for determining nitrogen oxides in exhaust gases has been developed. The nitrogen oxides (NO_x) in a sample were oxidized by ozone and absorbed into a weak acidic solution to be converted to nitrate. The determination of NO_x was performed by measuring the absorbance of the solution at one wavelength ranging from 215 nm to 230 nm where nitrate absorbed strongly and the absorption was little affected by coexistent gases in the sample. Only 5 min was required for one determination. The method can be used for the determination of (2×10^-6~2×10^-4) M nitrate converted from NO_x with an error of ±2.4%. The lower determination limit was apporoximately 1 ppm by using a 11 sample gas container with the absorption solution of 20 ml. Though the molar absorptivity, 6400 at 215 nm, was smaller than those by other chemical methods for NO_x determination, the useful determination range was little defferent compared with the other methods because the present method required neither pretreatment nor preparation for chromophores. High concentration of hydrocarbons and organic gases caused positive errors for the analytical result. However, unless the sample contains more than 500 ppm of those gases, NO_x can be measured without correction. The analytical data of NO_x in flue and automobile exhaust gases were in fair agreement with those obtained by Zn-reduction NEDA method.