KINETICS OF THE REACTION OF NITRIC-OXIDE WITH OXYGEN IN AQUEOUS-SOLUTIONS

An understanding of the rate of reaction of nitric oxide (NO) with oxygen in aqueous solutions is needed in assessing the various actions of NO in the body. A novel approach was developed for studying the kinetics of this reaction, which permitted simultaneous and continuous measurements of the concentrations of NO and the principal product, nitrite (NO2-). Nitric oxide was measured using a chemiluminescence detector, with continuous sampling achieved by diffusion of NO through a membrane fitted into the base of a small, stirred reactor. The results with various initial NO and O-2 concentrations confirmed that the rate of reaction is second-order in NO and first-order in O-2. The rate of reaction of NO was described by the expression 4K(1) [NO](2)[O-2], where K-1 was (2.1 +/- 0.4) x 10(6) M(-2) s(-1) at 23 degrees C and (2.4 +/- 0.3) x 10(6) M(-2) s(-1) at 37 degrees C. The value of k(1) was the same at pH 4.9 and 7.4. The rate of formation of NO2- equaled the rate of reaction of NO (within experimental uncertainty of a few percent), and there was no detectable formation of nitrate (NO3-). This confirmed that NO and NO2- were the only NOx species present in significant amounts and supported the validity of pseudo-steady-state assumptions for NO2 and N2O3, which are intermediates in the conversion of NO to NO2-. A kinetic model was developed to predict temporal variations in the concentrations of NO, O-2, NO2-, NO2, and N2O3 in the system studied, which were due in part to transport across the gas-liquid interface and across the membrane at the base of the reactor. The analysis emphasizes the importance of quantifying interphase mass transfer when studying reaction rates in open systems, a point which has not always been recognized in previous studies with nitric oxide.