Conductance stopped flow

Dynamic processes of complex formation of metal ions with poly-4-vinylpyridine (PVP) (Eqs. (4) and (5)) have been studied by means of the conductance stopped flow (CSF) and conductance pressure-jump (CPJ) technique 30). 

Kitano, H. Hasegawa, J. Iwai, S. Okubo, T. Kinetic study of the compiexation of cryptand 222 with alkaline earth ions by the conductance stopped-flow method. J. Phys. Chem. 1986, 90 (23). 6281-6284. 

The procedure which had originally been used by Lehn et al. involved slow addition (over a period of ca. 8 h) of ca. 0.1 M solutions of diamine and diacyl halide in benzene. Dye et al. found that the reactions could be conducted more rapidly as long as stirring was kept efficient. This observation suggested the use of a mixing chamber of the type normally used for stopped-flow kinetic studies. Utilizing this type of set-up, the latter authors were able to obtain a 70% yield for 1, slightly inferior to the yield reported by Lehn, but a similar yield of 3 which is better than that previously ob-tained. Note that the chemical features of this synthetic method are essentially identical to the approach shown in Eq. (8.1) and differ primarily in the mechanics. 

The reaction between peroxysulphuric acid and thiocyanate ions was investigated by Smith and Wilson by a stopped-flow conductance method. Their results can be summarized as follows. If / and m are the numbers of moles of cyanate and sulphur dicyanide produced per mole of thiocyanate consumed, the overall stoichiometric equation is... 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Monoprotonation of the [2.1.1]-cryptand occurs rapidly but protonation of the monoprotonated species by hydronium ion and other acids can be followed kinetically in various solvents (Cox et al., 1982, 1983). In methanol, protonation of ii+ species by substituted acetic and benzoic acids to give i+i+ has been studied using the stopped flow technique with conductance detection. The values of the rate coefficients (kHA) for protonation (81) vary with the acidity of the donor acid from kHA = 563 dm3mol-1s-1 (for 4-hydroxy-benzoic acid) to kHA = 2.3 x 105 dm3mol 1s 1 (for dichloroacetic acid). 

K. Ishihara, H. Miura, S. Funahashi and M. Tanaka, Inorg. Chem. 27, 1706 (1988), describe a high-pressure, stopped-flow arrangement with conductivity monitoring. 

This analyser is a computer-controlled automated batch analyser, using a stop-flow principle to analyse for pH, conductivity, turbidity and colour. TTie principle of analysis for each module is based on the recommended methods as detailed in the Examination of Waters and Associated Materials issued by the Standing Committee of Analysts of the Department of the Environment. The temperature of the sample hquid flow is measured in order that temperature-compensated results of pH and conductivity can be quoted. 

The present performance of high field NMR instruments allows the recording of a spectrum in quantities of micrograms. Under these conditions, it is possible to install a flow cell of only a few microlitres into the magnet of the instrument which allows the spectra of the analytes to be recorded. The experiment is conducted with a very small flow rate of the mobile phase (D20 or CD3CN) or in the stop-flow mode. In this mode, the mobile phase is momentarily stopped in order to record the spectrum. This technique, which requires very expensive materials, is of limited use. It is used mainly to isolate and identify very unstable compounds that cannot be isolated through classical means. 

The stopped-flow method uses syringe-type pumps, (a), to feed the components, A and B, through a mixing cell, (c), into the reaction cell, (d), which can be an optical cell (Fig. 3.3-5). The pumps, mixing cell, and reactor are well thermostatted. The flow is stopped when the syringe, (e), is loaded and operates a switch, (f), to start the monitoring device. The change in concentration is detected either by spectroscopy or conductivity measurement. 

What is the precise mechanistic process by which bromine (and by extension chlorine) adds to alkenes Studies designed to probe this question are generally difficult to conduct due to the very fast product-forming steps. By applying stopped-flow spectrokinetics to sterically crowded alkenes such as adamantylideneadamantane (Ad2>, where steric hindrance retards the final addition step by hampering rearside nucleophilic attack of the counterion at the bromonium carbons of the intermediate ion pairs, it... 

Conductivity and Optical Detection Using p-Jump Relaxation 75 Evaluation of p-Jump Measurements 76 Commercially Available p-Jump Units 78 Application of Pressure-Jump Relaxation Techniques to Soil Constituents 81 Stopped-Flow Techniques 91 Introduction 91... 

Hi-Tech Scientific Limited (Salisbury, England) recently introduced a stopped-flow (SF-51) instrument with conductivity detection that uses a five-mixer aging block that gives preparative quench aging times in the range of 1.0 ms to >10 s (Fig. 4.16). Preparative quench and stopped-flow experiments can be performed under total thermostatted, anaerobic, and chemically inert conditions. The entire stopped-flow package consists of the sample handling unit, a spectrophotometer, and a data processor based on the Apple lie. 

Figure 4.18. Typical reaction curves observed by using the stopped-flow method with electrical conductivity detection at Cp = 0.7 g dm-3 and 298 K (a) Li+, (b) K+, Rb+, and Cs+. [From Ikeda et al. (1984a), with permission.]...

Electrophoresis can also be conducted on-line, as an element of industrial process monitoring and/or control. In this case a slip-stream sample is usually withdrawn from a process vessel, diluted in a mixing tank to reduce the sample turbidity, and then pumped through an electrophoresis cell that is fitted with stop-flow solenoid valves. The flow is stopped for long enough to make an electrophoresis measurement and then resumed. The sampling can be either intermittent or continuous. An example is described in reference [265]). 

The direct coupling of liquid chromatography with proton NMR has been attempted numerous times. Early experiments of coupled HPLC- H-NMR were conducted in a stop-flow mode or with very low flow rates [193-195]. This was necessary to accumulate a sufficient number of spectra per sample volume in order to improve the signal-to-noise ratio. Other problems associated with the implementation of on-line HPLC-NMR have included the need for deuterated solvents. However, with the exception of deuterium oxide, the use of deuterated eluents is too expensive for routine analysis. Therefore, proton-containing solvents, such as acetonitrile or methanol, must be used. To get rid of the solvent signals in the spectra, the proton NMR signals of the solvents have to be suppressed. 

Honaker and Preiser reported the first fundamental kinetic mechanism of chelate extraction in 1962 [1]. They elucidated that the rate-determining step for the extraction of divalent metal ions with dithizone was the formation of their 1 1 complexes in the aqueous phase. They proposed that a simple batch extraction method could be used as an alternative method of the complicated stopped-flow method, which was the only method available at the time, to measure such a fast reaction rate. Since the 1970s, hydrometallurgy has been developed in many countries, and extensive kinetic studies on the metal extraction have been conducted in an effort to improve the extraction rate as well as to develop effective and reusable extractants. The extractants used in hydrometallurgy are required to be highly hydrophobic and readily coordinative with various metal ions. On the basis of the interfacial adsorptivity of the extractant, Flett et al. [2] expected an interfacial reaction mechanism in the chelate extraction process. There was, however, no experimental evidence to prove the interfacial mechanism directly [3]. 

The use of stop-flow techniques to observe the formation of carbenium ions in actual polymerising systems was introduced by Pepper et al. about ten years ago and is presently exploited by various research groups with increasingly fast equipment. These experiments consist essentially in mixing monomer and catalyst solutions in an appropriate flowing system coupled vrith a rapid detection apparatus which takes absorption spectra and can measure other physical parameters, such as the electrical conductivity of the reaction mixture. This technique is certainly the most appropriate for studying the rise and fate of ionic active species in cationic polymerisation and the few, but remarkable, results obtained so far will be reviewed in the various sections dealing vrith specific systems. 

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