The method currently in use to determine the content of nitrogen dioxide in the air (atmospheric and industrial premises) is based on absorption of nitrogen dioxide by a solution of potassium iodide, and the subsequent determination of the nitrite ion formed by the Griss-Illosvaya reagent. The basis of the method is the conversion of sulfanilic acid into a diazo compound by nitrous acid. The compound, reacting with alpha-naphthyl-amine, forms a red dye. The developers of the method took it that nitrogen dioxide is completely converted into the NO2(-) ion when it reacts with potassium iodide. Of practical interest is an explanation of the degree to which nitrogen dioxide is converted into the nitrite ion when gas-air mixtures are sampled in a potassium iodide solution. Establishment of the conversion factor of NO2 to the NO2(-) ion through experiment is greatly complicated by the lack of adequately reliable methods for obtaining the specified, very small, concentrations of nitrogen dioxide in the air. The use of known liquid vapor metering devices for this purpose are ruled out as are gas metering devices. This study used an already developed diffusion type metering device in which the quantity of air introduced into the vapor flow can be measured directly from the decrease that takes place in the volume of liquid in a calibrated glass capillary. The measuring microscope used to evaluate liquid loss was able to determine with a high degree of accuracy extremely small changes in the quantity of liquid.