SIMULTANEOUS LASER-ABSORPTION MEASUREMENTS OF CN AND OH IN A SHOCK-TUBE STUDY OF HCN-]PRODUCTS(OH)

Simultaneous, quantitative, narrow-line laser absorption measurements of CN time-histories at 388.444 nn and OH time-histories at 306.687 nm have been made in incident and reflected shock wave experiments using dilute mixtures of nitric acid (HNO3) and HCN in argon. The thermal d ecomposition of HNO3 serves as a rapid source of OH upon shock-heating , and the OH subsequently reacts predominantly with the KCN in the tes t gas mixture. The rate coefficient for the reaction (1a) HCN + OH --> CN + H2O was determined in the temperature range 1120-1960 K via deta iled kinetics modeling of the simultaneously acquired CN and OK measur ements. These data are in good agreement with lower temperature measur ements of the rate of the reverse reaction (-1a) when recent values of the heats of formation of CN and HCN are used. The expression k(1a) = 3.90 x 10(6)T(1.83) exp(-5179/T) cm(3)mol(-1)s(-1), valid for tempera tures 500 to 2000 K, effectively represents the experimental measureme nts. The estimated uncertainty of the expression for k(1a) is +/- 30%, based on the experimental uncertainties of the individual rate coeffi cient studies. Analysis of the decay region of the experimental OH tim e-histories yielded the total rate coefficient k(1) (all product chann els) for the reaction of HCN with OH for temperatures ranging from 149 0 to 1950 K. These measurements are consistent with a previous theoret ical analysis of the three primary addition-isomerization-dissociation processes for the HCN + OH reaction at combustion temperatures when t he contribution to k(1) from reaction (1a) is included. (C) 1995 John Wiley & Sons; Inc.