Measured induction periods Mechanical

Various investigators have tried to obtain information concerning the reaction mechanism from kinetic studies. However, as is often the case in catalytic studies, the reproducibility of the kinetic measurements proved to be poor. A poor reproducibility can be caused by many factors, including sensitivity of the catalyst to traces of poisons in the reactants and dependence of the catalytic activity on storage conditions, activation procedures, and previous experimental use. Moreover, the activity of the catalyst may not be constant in time because of an induction period or of catalyst decay. Hence, it is often impossible to obtain a catalyst with a constant, reproducible activity and, therefore, kinetic data must be evaluated carefully. 

Antibiotics may be classified by chemical structure. Erythromycin, chloramphenicol, ampicillin, cefpodoxime proxetil, and doxycycline hydrochloride are antibiotics whose primary structures differ from each other (Fig. 19). Figure 20 shows potential oscillation across the octanol membrane in the presence of erythromycin at various concentrations [23]. Due to the low solubility of antibiotics in water, 1% ethanol was added to phase wl in all cases. Antibiotics were noted to shift iiB,sDS lo more positive values. Other potentials were virtually unaffected by the antibiotics. On oscillatory and induction periods, there were antibiotic effects but reproducibility was poor. Detailed study was then made of iiB,sDS- Figure 21 (a)-(d) shows potential oscillation in the presence of chloramphenicol, ampicillin, cefpodoxime proxetil, and doxycycline hydrochloride [21,23]. Fb.sds differed according to the antibiotic in phase wl and shifted to more positive values with concentration. No clear relationship between activity and oscillation mode due to complexity of the antibacterium mechanism could be discovered but at least it was shown possible to recognize or determine antibiotics based on potential oscillation measurement. 

The induction period is measured experimentally at the constant sum of concentrations of two antioxidants, namely, Co = [S]o + [InH]0 = const. Theoretically this problem was analyzed in [9] for different mechanisms of chain termination by the peroxyl radical acceptor InH (see Chapter 14). It was supposed that antioxidant S breaks ROOH catalytically and, hence, is not consumed. The induction period was defined as t = (/[InH /v, where vV2 is the rate of InH consumption at its concentration equal to 0.5[InH]o. The results of calculations are presented in Table 18.1. 

The thermal decomposition and photolysis of this alkyl have been studied by Buchanan and Creutzberg112. The pyrolysis mechanism is not fully understood. The overall process is first-order and is unaffected by an 8.5-fold increase in surface-to-volume ratio. Based on measurements of pressure increase, the reaction exhibits an induction period ranging from 2-3 minutes at 513 °C to 40 minutes at 466 °C. Short chains are apparently involved. A polymer initially of empirical formula (BCH2) but slowly losing hydrogen to form (BCH) is deposited on the surface. The mechanism probably involves the reactions... 

Clear indications of the induction period and of an increase in the reaction rate after copolymerization has started were found for isothermal runs by DSC measurements by Peyser and Bascom 941 even for melt copolymerization. According to the copolymerization mechanism, the induction period is interpreted as a gradual increase in the concentration of active centres45,52 and is identical with the time for reaching the maximum on the conductivity curves57). An induction period has also been established by other measurements 39,40>73.90.95), where it is often considered as an imprecision in the determination of the monomer concentration, mixing effect, temperature establishement, or it is not considered at all. 

Hofer et al. [29] used a magnetic method to measure the isothermal kinetics of the decompositions of cobalt and nickel carbides. The reaction of CojC was zero-order (0.20 < a< 0.75) with Z , = 226 kJ mol in the range 573 to 623 K, and became deceleratory thereafter. The behaviour of NijC differed in that there was a short induction period, but there was again a period of constant rate (0.3 < ar < 0.9) with , = 295 kJ mol between 593 and 628 K and the final period was deceleratory. There was no evidence for the intervention of a lower carbide. The mechanisms of these reactions were not discussed. 

It is obvious that conclusions about the amount of enzyme synthesis could not be based on the amount of cellulolytic activity found in the culture medium since the amount of activity found depended on the time at which measurements were made. On the basis of the hypothesis that the cellulolytic enzymes are bound to the surface of the hyphae and are released to the culture medium only slowly after the growth period, we undertook to investigate the release of the enzymes for, in order to determine if an induction-repression mechanism operates, it is necessary to measure total enzyme activity. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

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