Stopped-flow apparatus

Figure C3.1.2. Stopped-flow apparatus with motor-driven syringes. Syringe plungers force tire reactants A and B tlirough a mixing chamber into a spectral cell. Kinetic data collection begins when tire effluent syringe plunger is pushed out to contact an activation switch, about a millisecond after tire initiation of mixing. (Adapted from Pilling M J and Seakins P W 1995 Reaction Kinetics (Oxford Oxford University Press)...

The example above of tire stopped-flow apparatus demonstrates some of tire requirements important for all fonns of transient spectroscopy. These are tire ability to provide a perturbation (pump) to tire physicochemical system under study on a time scale tliat is as fast or faster tlian tire time evolution of tire process to be studied, the ability to synclironize application of tire pump and tire probe on tliis time scale and tire ability of tire detection system to time resolve tire changes of interest. 

Recently, Hurwitz and Kustin have reinvestigated this exchange reaction using the same isotopic procedure and the 2-butanone separation method, in conjunction with a stopped flow apparatus. A rate coefficient of 2.3 x 10 l.mole . sec was obtained for the conditions, temperature 25 °C and ionic strength 0.1 M. Application of the Marcus theory to results obtained for the reaction... 

Espenson has shown that the reaction of c/j-Co(en)2(N3)2 with takes place by an inner-sphere mechanism. This Co(III) complex was selected for investigation because it is particularly reactive towards and also the dissociation of monoazido vanadium(lll) is relatively slow. At low concentrations (2-20 X 10 M) the second-order rate coefficient is 32.9 l.mole . sec at 25 °C, [H ] = 0.10 M and [i = 1.0 M. At higher concentrations ( 0.1 M), using a stopped-flow apparatus, the kinetics are apparently first order at 520 mfi, a wavelength where shows negligible absorbance. The rate coefficient under... 

Beattie and Basolo have investigated the reactions of the substitution-inert octahedral complexes of Pt(IV) with tris(bipyridine)chromium(II). A rapidmixing, stopped-flow apparatus was made use of in the majority of experiments. 

Examination of the permanganate oxidation of furfural and 5-substituted furfurals, at pH 11.5-13.3 in a stopped-flow apparatus, viz. 

Oscilloscope traces obtained on mixing Ce(IV) perchlorate and H2O2 in a stopped-flow apparatus reveal an initial build-up of absorption at 350 nm complete within a few msec, suggesting formation of a complex, followed by a first-order decay almost complete within 20 msec and independent of initial [Ce(IV)], [H2O2] and [HjO-"] and of added Ce(III) at 25 °C k, = (2.8+0.2) x 10 sec and a similar value is found over a temperature range of 18-43 °C implying E = 0. Breakdown of a Ce(IV)-peroxide complex to H02 followed by oxidation of H02 to O2 is proposed . 

The oxidation of Fe(II) by CI2 is fast ( 2 = 80 + 5 l.mole . sec at 25 X, fi = 3 M) and yields mostly FeCP on the several millisecond time-scale of a stopped-flow apparatus. This does not allow differentiation between one- and two-equivalent mechanisms. The analogous oxidation of hypochlorous acid, ki =... 

It has long been known that substitution at the anion of Zeise s salt, [Pt(CH=CH2)Cl3], is, thanks to the high trans effect of the coordinated ethene, very fast. Recent developments in low-temperature stopped-flow apparatus have now permitted the study of the kinetics of substitution at Zeise s and other [Pt(alkene)Cl3] anions in methanol solution. These substitutions obey the customary two-term rate law (i.e. with kohs = ki+ /s3[nucleophile]), with large negative AS values for the k2 term as expected for Sn2 processes (196). 

Therefore, in order to obtain information about the nature of the brominating species present in the reaction mixture, and on its stability, spectroscopic measurements were carried out in the absence of olefin on methanolic Br2 solutions containing increasing amount of NaN3. (14) When bromine (4.3 x 10 3 M) and methanolic solution of NaN3 (between 4.7 x 10 2 to 2.37 xlO 1 M) were rapidly mixed in a stopped-flow apparatus, at 25 °C, no kinetic of disappearance of Br2 could be observed, but only the presence of a new absorption band (> ax 316 nm) and its subsequent decrease could be measured. The disappearance of the absorption band followed a first order rate law. The observed kinetic constants are reported in Table I. 

Fig. 1.15 Second-order superoxide disproportionation constant vs pH at 25 °C. Potassium superoxide ( 1 mM) in pH a 12 was mixed in a stopped-flow apparatus with buffers at various pH s and the change in absorbance at 250 nm monitored. The decays were second-order and data were treated in a similar manner to that described in Fig. 1.3. The full line fits Eqn. (1.231) using the parameters given in the text. Reprinted with permission from Z. Bradid and R. G. Wilkins, J. Am. Chem. Soc. 106, 2236 (1984). (1984) American Chemical Society.

If a reaction is accompanied by a change in A//, then the temperature of that reaction sensed with time is a measure of the rate. Although the method has found little use generally, it can be linked to a stopped-flow apparatus and this allows the determination of the thermal properties of a transient. The heat of formation of an intermediate, which decomposes with k = 0.27 s , in the complicated luciferase-FMNH2 reaction with Oj can be measured by stopped-flow calorimetry. 

Transients such as OJ or HO can be generated in solution by pulse radiolysis of O2. If such solutions are contained in one syringe of a stopped-flow apparatus they may be mixed with substrate and the final mixture examined spectrally. For flow experiments these transients must, of course, have lifetimes longer than a few millisecond. For the examination of more labile transients, production may be by laser photolyses or pulse radiolysis, and the substrate under examination must be then incorporated in the pulsed solution. Care has now to be taken that substantial amounts of the substrate are not lost (by reaction) as a result of the pulse. 

C. Paul, K. Kirschner and G. Haenisch, Anal. Biochem. 101, 442 (1980) determine the deadtime for a stopped-flow apparatus using a disulfide exchange reaction. 

J. D. Ellis, K. L. Scott, R. K. Wharton, and A. G. Sykes, Inorg. Chem. 11, 2565 (1972), who observe optical density changes when 1 M acid solutions are mixed with water in a Durrum-Gibson stopped-flow apparatus. Such traces could be incorrectly assigned to chemical reactions. 

D. S. Auld, K. Geoghegan, A. Galdes and B. L. Vallee, Biochemistry 25, 5156 (1986) C. Balny, T.-L. Saldana and N. Dahan, Anal. Biochem. 163, 309 (1987) describe a high-pressure, low temperature, stopped-flow apparatus. 

The interaction of Co(II) complexes (CoL +) with Oj can be studied by mixing in a stopped-flow apparatus Co (syringe 1) with a ligand L/O2 mixture (syringe 2). What are the advantages of this procedure and what are likely to be the problems ... 

MCPBA]o=0.0005 M and all others O.OIOOM unless otherwise stated. All data have been measured in our labs using traditional and stopped flow apparatus unless otherwise stated. Initial compounds are Cobalt(II) and manganese (II) acetate tetrahydrates. 

Later studies on acetyl chloride hydrolysis were those of Hudson and Moss90, using a stopped-flow apparatus at 27°C and working with dioxan-water mixtures (19.6-75.6 v/v), and Cairns and Prousnitz102 with acetone/water (15.35% v/v) at low temperatures. The value of the Grunwald-Winstein103... 

Figure 3.3-5. Stopped flow apparatus [19]. a, Syringe type pump b, thermostat c, mixing cell d, reaction cell e, stop syringe f, switch g, photo multiplier h, monochromatic filter i, lamp j, controller k, transducer 1, computer.

The stopped-flow apparatus for measuring enzyme-catalyzed reactions very soon after mixing enzyme and substrate. 

Figure 4.16. Stopped-flow apparatus (SF-51) from Hi-Tech Scientific Limited with datapro...

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