A pulsed-accelerated-now spectrophotometer with UV-visible capability
is described that permits measurement of pseudo-first-order rate const
ants as large as 500 000 s(-1) (t(1/2) = 1.4 mu s). Chemical rate proc
esses are resolved from physical mixing rate processes by variation of
now velocities under conditions of turbulent now, Two mixing processe
s are found in the mixing/observation tube, One mixing rate constant,
valid for the full length of the tube, is directly proportional to the
now velocity, The other mixing behavior, proportional to the square o
f the now velocity, is found only in the immediate vicinity of the 10
inlet reactant streams that collide with one another in the middle of
the observation tube, Contributions from the latter mixing become more
important for very fast reactions that take place close to the inlet
jets, These miring models and improved signal/noise permit the measure
ment of rate constants for very fast reactions, Applications of the PA
F method to electron-transfer, proton-transfer, hydrolysis, and non-me
tal redox reactions are reported for pseudo-first-order and second-ord
er reactions.