THE INFLUENCE OF PHOSPHORUS OXIDES AND ACIDS ON THE RATE OF H+OH RECOMBINATION

The addition of small amounts of phosphine combustion products (phosphorus oxides and acids) to water vapor resulted in a large increase in the relaxation rate of OH to equilibrium concentrations following H2O photolysis. The thermochemical properties of 17 simple phosphorus species were assembled and the equilibrium densities of the phosphorus species were calculated under typical test conditions. The thermochemical properties of many of the phosphorus species have not been determined but for the purposes of this analysis they were estimated from the results of ab initio quantum chemical calculations available in the literature. Of the phosphorus species examined, eight were selected for construction of a reaction set. Thirteen species (7 H(x)O(y) species and 6 H(x)O(y)P(z) species) were ultimately used to assemble a set of 33 reactions which comprise all of the bimolecular reactions (including recombination reactions) likely to participate in the overall reaction chemistry. Rate constant values were estimated using standard methods for the metathesis reactions and using simplified RRKM calculations to estimate the rates of recombination reactions. The increase in the observed rate of OH recovery was well described by this reaction set at the experimental conditions of 1970 K and 532 Torr. The two reaction mechanisms OH + PH2 --> HOPO2, H + HOPO2 --> H2O + PO2, and H + PO2 --> HOPO, OH + H-2 --> H2O + H, H + HOPO --> H-2 + PO2 appear to be the predominant pathways for recombination of H and OH radicals to form H2O and for reestablishment of chemical equilibrium when phosphine combustion products are present in the reaction mixture.