Chromium trioxide (CrOg) was tested as a replacement for the potassium permanganate-sulfuric acid solutions designed to oxidize NO for conversion and detection as NO2. The CrOg oxidizer was prepared by soaking firebrick in a 17% solution of pure CrOg, draining, and drying at 105°C. The final loading was about 10% by weight. Since the optimal relative humidity (R.H.) working range of the CrOg oxidizer without sulfuric acid is higher (35-90% R.H.) than the material mixed with sulfuric acid, a humidifier with a shortened inlet tube was used so that the airstream was blown over water rather than bubbled through it. This gave a relative humidity of about 70-80%. Three to 5 grams of the hydrated oxidizer was placed in a midget impinger. An interesting and desirable feature of the CrOg oxidizer is its color change when hydrated (dull yellow) or dehydrated (dull pink) and when its oxidizing ability is spent (greenish brown). The midget impinger should be changed before more than half has depleted. At ambient levels of NO the oxidant should last at least 3 months. The oxidation efficiency is close to 100%.The development of suitable analytical procedures for determining NO paralleled the historical sequence of improvements in NO2 analysis. Chronologically, the need for trace analysis of NO started with the manufacture of city gas from coal. Methods for oxidizing NO with a mixture of KMn04 and H2SO4 were described by Guyer and Weber (1933), Rollings (1937), andShindman andYeaw (1942).Other systems for oxidizing NO to NO2 were described by Johnston (1954), Thomas et al. (1956), Remy (1956), and Ripley et al. (1964). Such systems used such compounds as ozone, heated I2O5, acidified MnC>2, periodate, persulfate, N2O5, CIO2, and oxygen. Each method, however, has certain disadvantages for continuous sampling and analysis. An excess of ozone converts nitrogen oxides all the way to nitric acid in the presence of atmospheric moisture. Therefore, ozone must be precisely metered into the gas stream with no more than a 100% excess over the amount of NO. A disadvantage of the use of heated iodine pentoxide is the fact that many other reducing pollutants also liberate iodine from this reagent. Gaseous oxygen (O2) oxidizes nitric oxide slowly and can be used only for concentrations above 100 ppm. Periodate, persulfate, and acidified permanganate are not quantitative oxidizers, and acidified manganese dioxide is not stable. Chlorine dioxide has been demonstrated to give good conversions. However, it is not readily available, and any large excess bleaches the azo dye reagent and must, therefore, be precisely metered into the air stream, similarly to ozone (Thomas et al. 1956). Remy (1956) mentioned the use of CrOg as an oxidizing
