Bicarbonate ion is an effective activator for hydrogen peroxide in the oxidation of sulfides. Kinetic and spectroscopic results support the formation of peroxymonocarbonate ion (HCO 4 -) as the oxidant in the catalytic reactions. The reaction of hydrogen peroxide and bicarbonate to form HCO 4occurs rapidly at 25 °C (t 1/2 ≈ 300 s) near neutral pH in aqueous solution and alcohol/water mixtures, and an equilibrium analysis of the reaction by 13 C NMR leads to an estimate of the electrode potential for the HCO 4 -/HCO 3couple (1.8 V vs NHE). Solubility of the bicarbonate catalyst is enhanced by the use of NH 4 HCO 3 rather than by the use of group 1 salts, which tend to have lower solubility in the mixed solvents and can lead to phase separation. Rate laws and mechanistic analyses are presented for the oxidation of ethylphenylsulfide and related sulfides. The second-order rate constants for sulfide oxidations by HCO 4are ∼300-fold greater than those for H 2 O 2 , and this increase is consistent with expectations based on a Brønsted analysis of the kinetics for other heterolytic peroxide oxidations. At high concentrations of H 2 O 2 , a pathway that is second order in H 2 O 2 is significant, and this path is interpreted as a general acid catalysis by H 2 O 2 of carbonate displacement accompanying substrate attack at the electrophilic oxygen of HCO 4 -. Increasing water content up to 80% in the solvent increases the rate of oxidation. The BAP (bicarbonate-activated peroxide) oxidation system is a simple, inexpensive, and relatively nontoxic alternative to other oxidants and peroxyacids, and it can be used in a variety of oxidations where a mild, neutral pH oxidant is required. Variation of bicarbonate source and the cosolvent can allow optimization of substrate solubility and oxidation rates for applications such as organic synthesis and chemical warfare agent decontamination.

We assessed the reproducibility of the microdilution checkerboard method for measuring antibiotic synergy. Five strains of Pseudomonas aeruginosa were tested with four antibiotic combinations by using 10 replicates each. Twenty-five percent of replicate sets gave discordant classification results (i.e., a 7:3 or worse split in categorization). Determination of the individual MICs of each antibiotic alone was excellent; all 10 replicates were within 1 twofold dilution for 95% of the 80 sets of 10 replicates. The microdilution checkerboard method either should not be used or should be used with at least five replicates per determination, with .80% agreement among the replicates required for classification.The microdilution checkerboard has been one of the traditional methods for the measurement of antibiotic synergy. Synergy has generally been defined as requiring a fourfold reduction in the MIC of both antibiotics in combination, compared with each used alone, i.e., a fractional inhibitory concentration (FIC) index of c0.5 (5). Since there is an inherent 1-dilution variability in the determination of the MIC of each antibiotic used alone (8), this variability is additive and could become significant for the combination wells. We therefore decided to determine how much inherent variability occurs with microdilution checkerboard synergy testing under ideal conditions, namely, replicate microdilution plates inoculated on the same day with the same inoculum by as meticulous a technique as possible. We tested microdilution checkerboard plates prepared by two different methods, using five strains of Pseudomonas aeruginosa and four antibiotic combinations.(This work was presented at the 92nd

A detailed mechanism for the oxidation of aryl sulfides by peroxymonocarbonate ion in cosolvent/water media is described. Kinetic studies were performed to characterize the transition state, including a Hammett correlation and variation of solvent composition. The results are consistent with a charge-separated transition state relative to the reactants, with an increase of positive charge on the sulfur following nucleophilic attack of the sulfide at the electrophilic oxygen of peroxymonocarbonate. In addition, an average solvent isotope effect of 1.5 +/- 0.2 for most aryl sulfide oxidations is consistent with proton transfer in the transition state of the rate-determining step. Activation parameters for oxidation of ethyl phenyl sulfide in tert-butyl alcohol/water are reported. From the pH dependence of oxidation rates and (13)C NMR equilibrium experiments, the estimated pK(a) of peroxymonocarbonate was found to be approximately 10.6.

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