Preparation of low concentration nitrogen oxides gas from the pH controlled sodium nitrite solution

A new method was developed for preparation of nitrogen oxides gas of low constant concentration. This method is based on generation of nitrogen oxides from the sodium nitrite aqueous solution. The pH of the solution is controlled by adding a pH buffer. Nitrogen oxides is generated by passing air on the surface of the solution. The generated gas is diluted in the constant stream of air which has been cleaned by passing through a purification bed. The rate of gas generation depends on pH, concentration and temperature of the sodium nitrite solution. The gas generation from the solution began at pH 9.0, and a large amount of gas generation occurred at pH 5.0. The relation observed between concentrations of the solution (C_liq., μg/ml) and those of the generated gas (C^gas, ppm) is expressed as C^gas=k·Cⁿ_liq.……(1). If the pH of the solution is kept at a definite value, the gas of any desired concentration can be prepared according to the relation of Eq. (1). The temperature dependence of this method was much less than that of the permeation tube method. Variation of the concentration of the generated gas with temperature is 0.01 ppm/°C {at (20~35) °C, 0.36 ppm of NO_x}. The concentration of the generated gas reached to a constant level within about 60 min after starting air flow, and it was kept constant for several hours. The coefficient of variation of the concentration of the generated gas was 2% at 0.36 ppm of NO_x.