Pulsed accelerated flow method

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

Fig. 3.2 The operation of flow methods. The distance x and the combined flow rate govern the time that elapses between mixing and when the combined solutions reach the observation, or quenching, point. In the stopped flow method, observation is made as near to the mixer as is feasible, and monitoring occurs after the solutions are stopped. In the pulsed accelerated flow method, observation is within the mixer.

The pulsed-flow method evolved further into the pulsed-accelerated-flow method [7], Here, the solutions have a range of flow velocities owing to the constant flow... 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

Pulsed continuous flow is a method in which continuous flow is established for a short time. This method can reduce reagent consumption to 5 ml, and fast jet mixers have lowered the accessible reaction half-time to the 10 ps range. Pulsed accelerated flow may be viewed as an adaptation of pulsed continuous flow in which the flow rate through the mixer and observation chamber is varied during the course of one kinetic run. This method can be used for reactions with half-times down to 10 ps. This method is limited to first-order reaction conditions. 

A novel method, a periodic pulse technique in which a catalyst is exposed to forced redox cycles, was applied to the oxidation of C2II4 on Pt/Al Os catalysts. It was found that the catalysis are sintered at relatively low temperature, i.e., at 588 K, in reaction atmosphere, and that the periodic pulse technique actually accelerates the sintering. The sintering in the continuous flow reaction for ca. 1000 h could be reproduced within ca, 100 h, indicating the possibility of rapid estimation of catalyst life. It is suggested that the sintering in reaction atmosphere is governed by the chemical factors rather than the thermal ones. 

Especially for the synthesis of nanogels (e.g. by intramolecular cross-linking of polymer molecules with high molecular weights), the method of pulse irradiation is useful, see Fig. 10. The polymer is dissolved and the solution, flowing through a quartz irradiation cell (effective volume about 1 ml), is pulse-irradiated with electrons of several MeV (in our experiments 6 MeV) generated by a linear accelerator (ELU-6, Eksma, Russia). Pulse frequency of 5.0 Hz and pulse duration of 3 ps were applied at a flow rate of 5.0 ml/s. 

A multitude of methods have been developed for the measurement of the mobility of charge carriers in liquids. Many methods employ pulsed sources of charge carriers, as radioactive isotopes, lasers, or accelerators. In other methods a continuous charge carrier injection is modulated by an electric field. Sometimes electric field impulses are applied. In a few cases liquid flow was superimposed on the action of the electric field. Liquids with low vapor pressure allow mobility measurements by monitoring the decay of a surface charge layer. 

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