The redox reactions of methylene blue (MB+) often occur on a time scale of a few seconds to minutes. They may be followed visually for qualitative interpretations and spectrophotometrically for quantitative determinations. The experimental simplicity of MB+ reaction systems has also occasionally led to erroneous conclusions based upon oversimplified data treatment and assumptions. This paper compares spectrophotometric studies of MB+ reduction by ascorbic acid at low pH with previous conclusions based upon visual determinations of color loss. Spectrophotometric studies of the temporal decay of MB+ absorbance at 665 nm show that the reaction is first order in MB+, ascorbic acid, and HCl. A slower reaction occurs with only MB+ and ascorbic acid present. Regeneration of MB+ color by reaction with dissolved oxygen, the "blue bottle" reaction, is particularly significant when the reaction with ascorbic acid is slow. Methylene blue chemistry continues to provide a wealth of examples suitable for undergraduate kinetics studies.

The temperature-and pressure-dependent behavior of the cross section for optical absorption by the methyl radical is carefully considered, so we may define a criterion for selecting and correcting measurements of the rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 f C 2 H 6 . The low-temperature data of Slagle et al., Hippler et al., and Walter et al. and the high-temperature data of Glänzer et al., Hwang et al., and our latest results (previous paper in this issue) are used to define a data set which contains 217 points. Subsets of isothermal data show that the temperature dependence of the high-pressure rate coefficient may be described by the simple exponential function A ∞ exp{-T(K)/T ∞ }. Four different formulations for the pressuredependent behavior in the falloff region are used for the global fits: (1) the asymmetric Lorentzian broadening function of Gilbert et al.; (2) the Gaussian broadening function of Wang and Frenklach; (3) the empirical "a equation" introduced by Gardiner; (4) the extension of Lindemann's expression suggested by Oref. All formulations reproduce the data, but Oref's "J equation" produces the least correlation between the best-fit parameters, the least uncertainty in these parameters, and the smallest uncertainty in the predictions. These results are k ∞ (T) ) 8.78 × 10 -11 exp{-T(K)/723} cm 3 s -1 , k 0 (T) ) 9.04 × 10 -27 cm 6 s -1 , and J exp (T) ) {exp[T(K)/268] -1} 2 .

k2) in the second-order limit was found to be 1.3 ± 0.2 X 109 M™1 s™1.The rate constant for the combination of peroxy radicals, CH302 + CH302 -products (k3), was 2.3 ± 0.3 X 10® M™1 s™1, and for the combination of methyl radicals, CH3 + CH3 + M -*• C2H6 + M (ki), it was 3.1 ± 0.6 X 1010 M™1 s*1. The reactions were monitored by kinetic spectrophotometry of CH3 absorption at 2160 Á and CH302 absorption at 2480 Á.

HO. radicals were produced in the gas phase by flash photolysis of water vapor (3%) in an atmosphere of hydrogen, helium, or argon containing ~2% oxygen. Water is dissociated in the first continuum to Hand OH, and O. converts the H atoms to HO •. Hydrogen nearly doubles the amount of HO. produced by converting OH to H. The absorption spectrum of HOI is a broad band with a peak at 2050 A. The molar extinction coefficient, Em • ., based on measurement of the H~, formed in the hydrogen system, is 1770± 150M-l· cm-l. The rate constant for the bimolecular combination reaction, HO.+HO.-->H.O.+0 2 , was evaluated as 5.7±0.5X1()9M-l·seC I at 298°K and for the reaction HO.+OH-->H 2 0+O., k=1.2±0.2X lOuM-I·see-l. From auxiliary measurements of the rate of 0 3 formation it was also found that, in the flash photolysis of O. (2%) in H2, hot 0 atoms react with H2 to form OH and H which are then converted to H02•

The rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 f C 2 H 6 , was measured in incident shock-wave experiments on azomethane (0.12-1.3%) in argon. Methyl radicals were detected at 46 294 cm -1 (215.94 nm in air) by the tunable-laser flash-absorption technique. Postshock pressures between 114 and 230 kPa (1.13-2.27 atm) produced buffer gas concentrations between 5.4 × 10 18 and 1.