A number of investigators have studied the kinetics of the reaction between nitric oxide and oxygen. It was the subject of a long controversy between Raschig and Lunge. They used similar experimental methods, absorbing the reacting gaseous mixture in sodium hydroxide or sulfuric acid and titrating the amount of nitrogen dioxide formed with potassium permanganate. Raschig claimed experimental evidence pointing to the formation of nitrogen trioxide as an intermediate product; while Lunge's results indicated that nitrogen dioxide was formed directly, strictly in accordance with the law of a third-order reaction. Raschig's view regarding the intermediate, nitrogen trioxide, is also held by Briner and co-workers,2 and by Jolibois and Sanfourche.3 That the reaction possesses a small, but marked, negative temperature coefficient was first noted by Foerster and Blich.4 Recently, Briner, Pfeiffer and Malet5 have studied the reaction at low temperatures, down to -193°; Bodenstein,6 in 1918, worked at the temperatures 0°, 30°, 60°and 90°; later, Bodenstein and Linder7 con-1 The material in this article is taken from a dissertation submitted by R. Leonard Hasche in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Johns Hopkins University.

In a previous investigation,2 the effect of increased glass surface on the velocity of the reaction between nitric oxide and oxygen was studied. It appeared that this rapid reaction is not greatly affected by surface; that it takes place chiefly in the gas phase. When we consider that the rate of the reaction which we measure is the sum of the rates by different paths, altering the kind or amount of surface may be supplying or removing a catalyst. Accordingly, the net speed may be altered by introducing or eliminating paths through which the reaction may proceed. As possibilities of catalysts in the present case we may consider nitrogen tetroxide or moisture. Cohn and Jung3 have recently shown in their study of the photochemical union of chlorine and hydrogen that with a pressure of water vapor under 10-7 mm. of mercury, no combination occurs; at a pressure of 10-6 mm., 88% is converted and at 10-3 mm. maximum catalysis results. This means that in case a film of moisture on a surface acts as a catalyst, only a very low moisture content, perhaps a monomolecular layer, is necessary to produce the maximum effect. More recently Norrish4 has calculated from the description of the apparatus used in the experiments of Cohn and Jung that at a pressure of 10 ~3 mm. of mercury the monomolecular layer on the surface of the reaction vessel was just complete. Bodenstein and Dux5 also found that evacuation to 10 ~3 mm.

Was used and, being a father weak acid, this exaggerated the effect. The Same titration using a 0.2cc. offset gave the curve shown in Fig. 5. It is quite possible to decrease the offset further if great care be observed to ascertain that the burets are quite accurate. If such care be not observed small and irregular potential readings may be obtained or else the cusps may turn downward indicating that the wrong buret is in the lead. For ordinary use with 0.1 N or 0.05 N solutions an offset of 0.2 cc. has been found to be most satisfactory. SummaryIt has been found that in many cases the method described above and referred to as "differential titration" is simpler and more accurate than electro-titration as usually practised. The reasons for this are summarized as follows.1. The necessity of plotting curves is eliminated.2. The use of the troublesome calomel half-cell is avoided.3. Slight errors due to "drifting" (very slow change of potential at an electrode upon standing) are compensated. 4. By the use of solutions almost identical in composition all trouble from diffusion is avoided.