Induction period, and

Thermal Decomposition of GIO2. Chloiine dioxide decomposition in the gas phase is chaiacteiized by a slow induction period followed by a rapid autocatalytic phase that may be explosive if the initial concentration is above a partial pressure of 10.1 kPa (76 mm Hg) (27). Mechanistic investigations indicate that the intermediates formed include the unstable chlorine oxide, CI2O2. The presence of water vapor tends to extend the duration of the induction period, presumably by reaction with this intermediate. When water vapor concentration and temperature are both high, the decomposition of chlorine dioxide can proceed smoothly rather than explosively. Apparently under these conditions, all decomposition takes place in the induction period, and water vapor inhibits the autocatalytic phase altogether. The products of chlorine dioxide decomposition in the gas phase include chlorine, oxygen, HCl, HCIO, and HCIO. The ratios of products formed during decomposition depend on the concentration of water vapor and temperature (27). 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

The yield of product is increased to 81% (analyzed by evolution of M-butane) if 0.67 g. (0.0033 mole) of aluminum isopropoxide is added to the suspension of magnesium before addition of the halide solution. Alternatively, an equivalent amount of 2-propanol and iodine (giving 0.01 mole of CsII OMgl) may be added. These modified procedures (particularly the second) also shorten the induction periods and render unnecessary any special drying of the reagents and apparatus and the use of fresh magnesium. 

Inhibitors or retarders that give inert products are called ideal .173 The term ideal inhibitor has also been used to describe a species that stops all polymerization until such time as it is completely consumed (i.e. the induction period) and then allows polymerization to proceed at the normal rate. However, in many cases the products formed during inhibition or retardation are not inert. Four... 

Magnitudes of n have been empirically established for those kinetic expressions which have found most extensive application e.g. values of n for diffusion-limited equations are usually between 0.53 and 0.58, for the contracting area and volume relations are 1.08 and 1.04, respectively and for the Avrami—Erofe ev equation [eqn. (6)] are 2.00, 3.00 etc. The most significant problem in the use of this approach is in making an accurate allowance for any error in the measured induction period since variations in t [i.e. (f + f0)] can introduce large influences upon the initial shape of the plot. Care is needed in estimating the time required for the sample to reach reaction temperature, particularly in deceleratory reactions, and in considering the influences of an induction period and/or an initial preliminary reaction. 

Hofer et al. [671] observed that the decompositions of Ni3C and Co2C (the iron compounds melt) obeyed the zero-order equation for 0.3 < a < 0.9 (596-628 K and E = 255 kJ mole-1) and 0.2 < a < 0.75 (573-623 K and E = 227 kJ mole-1), respectively. The magnitudes of the rate coefficients for the two reactions were closely similar but the nickel compound exhibited a long induction period and an acceleratory process which was not characteristic of the reaction of the cobalt compound. Decomposition mechanisms were not discussed. 

The investigators studied various blends of the three polymers in order to control the rate of chain scission and thus influence the induction period and onset of drug release. None of the blends provided the desired 1-week zero-order kinetics. However, blends of different microsphere types did show promise in vitro (88). 

The permanganate oxidation of oxalic acid has been studied exhaustively and has been reviewed by Ladbury and Cullis . It is characterised by an induction period and a sigmoid dependence of rate upon time. Addition of manganous ions eliminates the induction period and produces first-order decay kinetics . Addition of fluoride ions, however, practically eliminates reaction . ... 

The same samples, after a pretreatment in flowing oxygen (10%) at 625 K, were used as catalysts for the oxidative dehydrogenation of ethanol and methanol in the same reactor. The reaction mixture consisted of O2 (3 or 5%), methanol vapor (3%) or ethanol vapor (5%) and He (balance), all delivered by Tylan mass flow controllers or vaporizer flow controllers. Products were analyzed by gas chromatography. The catalysts exhibited no induction period and their activities were stable over many days and over repeated temperature cycles. 

For the amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as starting catalyst, an induction period and a sigmoid-shaped kinetic curve (plot of substrate conversion versus time) were also observed, consistent with metal-particle formation. But, for Ph2PH BH3... 

Search for More Active Catalyst. An extensive screening effort was undertaken to find a catalyst more active than Et-DuPhos-Rh. As a result of this effort, Et-FerroTane-Rh and some other competitive catalysts were found. The reactivity of Et-FerroTane-Rh and Et-DuPhos-Rh, is presented in Figure 3.9. The reaction rate with Et-FerroTane-Rh catalyst is very high with a small induction period, and the total time for reaction completion is drastically less than with Et-DuPhos-Rh. 

Interaction in hexane to produce the 2,4-di-fert-butyl derivative often starts only after an induction period and may then proceed very violently. Careful temperature control is imperative. 

A mixture of the reactants being heated at 60°C to effect a Diels-Alder addition exploded. Onset of this vigorously exothermic reaction was probably delayed by an induction period, and presence of a solvent and/or cooling would have moderated it. 

The oxidation of aqueous ethanedial to glyoxylic acid is inclined to induction periods and then runaway, cessation and renewed runaway, as the nitric acid is progessively added. Probably this is the same problem as with formic acid, oxalic acid (above). 

Oxidation of the trioxane ( paraldehyde ) to glyoxal by action of nitric acid is subject to an induction period, and the reaction may become violent if addition of the trioxane is too fast. Presence of nitrous acid eliminates the induction period. 

Reduction of the double bond in the anilide by magnesium in methanol is vigorously exothermic after an induction period, and efficient cooling is then necessary. 

With pure monomer and diluents a polymerization with a half-life of 8 hr has been recorded with these catalysts. There, stability would appear to be indefinite provided water or oxygen are not admitted to the system. In one experiment described in Table XII the monomer feed is switched off after 60 min and the dissolved monomer exhausted from the polymer slurry. If monomer is reintroduced some days later, polymerization begins again without an induction period and the rate was little changed from that previously observed. 

The primary nucleation process is divided into two periods in CNT one is the so called induction period and the other is the steady (or stationary) nucleation period (Fig. 2) [16,17]. It has been proposed by CNT that small (nanometer scale) nuclei will be formed spontaneously by thermal fluctuation after quenching into the supercooled melt, some of the nuclei could grow into a critical nucleus , and some of the critical nuclei will finally survive into macroscopic crystals. The induction period is defined as the period where the nucleation rate (I) increases with time f, whereas the steady period is that where I nearly saturates to a constant rate (fst). It should be noted that I is a function of N and t,I = I(N, t). In Fig. 2, N and N mean the size of a nucleus and that of the critical nucleus, respectively. The size N is defined... 

The purpose of this section is to present direct evidence of nucleation during the induction period by means of synchrotron small angle X-ray scattering (SAXS). In the classical nucleation theory (CNT), the number density distribution function of nuclei of size N at time t, f(N, t), is expected to increase with an increase of t during the induction period and saturates to a steady f(N, t),fst(N) in the steady period. The change off(N, t) should correspond to that of the scattering intensity of SAXS. 

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

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