Initial induction period

Properties. Silver difluoride melts at 690°C, bods at 700°C, and has a specific gravity of 4.57. It decomposes in contact with water. Silver difluoride may react violently with organic compounds, quite often after an initial induction period. Provisions must be made to dissipate the heat of the reaction. Small-scale experiments must be mn prior to attempting large-scale reactions. 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

M aqueous solutions of iodopentaminecobalt(lll) decompose with first-order kinetics at 45 °C with = 6.0x 10" sec 10" M solutions decompose faster after an initial induction period at the normal rate. Product analysis shows the fast decomposition to be a mixture of a redox process leading to iodine and substitution leading to aquopentaminecobalt(iri) and iodide. Addition of sodium iodide (to 10 M) accelerates the decomposition and... 

Quite different types of rhodium compound can give very similar reaction rates in a system which shows a kinetic dependence on the rhodium catalyst concentration. In particular, rhodium(III) halides and rho-dium(I) phosphine complexes give almost identical reaction rates after an initial induction period. Thus, in the case of these two systems, it appears that a common species is being formed. 

Unless the reaction mixture is heated before the addition process is started, there may be an initial induction period that may cause the reaction to become extremely vigorous once the mixture heats to reflux temperature. 

Mechanisms involving glycol bond fission have been proposed for the oxidation of vicinal diols, and hydride transfer for other diols in the oxidation of diols by bromine in acid solution.The kinetics of oxidation of some five-ring heterocyclic aldehydes by acidic bromate have been studied. The reaction of phenothiazin-5-ium 3-amino-7-dimethylamino-2-methyl chloride (toluidine blue) with acidic bromate has been studied. Kinetic studies revealed an initial induction period before the rapid consumption of substrate and this is accounted for by a mechanism in which bromide ion is converted into the active bromate and hyperbromous acid during induction and the substrate is converted into the demethylated sulfoxide. 

O Brien and Saeed, nsing ethylenediamine as compexant, higher deposition temperatnres, and glass as a snbstrate, fonnd that the thickness of the CdS film increased linearly with time (after an initial induction period) and also that there was no increase in the size of the nncleii (both in contrast to the previous study) [40], In spite of the different experimental conditions, the mechanism of the depositions in both stndies appears to be essentially the same, i.e., hydroxide-mediated catalysis of thionrea decomposition. 

Farmer s experiments were repeated and extended by Garner and Hailes [41]. They examined the behaviour of mercury fulminate at about 100°C and came to the conclusion that during the initial induction period, decomposition is accompanied by a slow evolution of gas at a constant velocity (linear decomposition). At the end of this phase the main decomposition period begins with an increased rate of gas evolution. The authors noticed that if the fulminate is finely ground, rapid evolution of gas begins at once, without any initial period. 

In concomitance with the displacement observed by i.r., an evolution of the catalytic activity has been observed while studying the liquid-phase epoxidation of cyclohexene in the presence of (EGDA)- Mo(VI), freshly prepared or after four months of conditioning at room temperature under inert atmosphere. As usual, the appearance of epoxide was followed by gas chromatographic analyses or by direct titration of oxirane oxygen and the disappearance of hydroperoxide was monitored by iodometric titration. In figure we report concentration-time for typical runs in ethylbenzene at 80°C obtained with the experimental procedure already described (ref. 9). It may be seen that with a freshly prepared catalyst an induction period is observed which lowers the initial catalytic activity. Our modified Michaelis-Menten type model equation (ref. 9) cannot adequately fit the kinetic curves obtained due to the absence of kinetic parameters which account for the apparent initial induction period (see Figure). 

In our earlier work [13] we studied the ethanolysis of TMOS. It was noticed that in slow reactions that were catalyzed by carboxylic acids, a change in rate occurred. After an initial induction period, the rate would increase. It was postulated that an initial equilibrium between the alcohol and the catalyst to form some sort of complex was required to occur before the rate of the alcoholysis reaction would become rate determining ... 

The essence of the steady-state assumption is that the concentration of a reactive intermediate (X) is assumed to build up during a brief initial induction period, but then, during most of the rest of the reaction, its formation and decomposition are balanced (i.e. d [ X ] /d f = 0) so that its concentration remains essentially constant. Obviously, during the final stages of the reaction, the concentration of the intermediate decreases to zero. 

With many Nd catalyst systems multimodal or at least bimodal MMDs are obtained. From this observation it is concluded that there are several active species. A particularly highly active species which causes high molar masses at low monomer conversions is present at the start of the polymerization. Once the initial induction period is over and consistent kinetics are observed, often this species is no longer active. The fate of the species is not clear. Does this species die off after the initial high activity or is it transformed to a species which exhibits a significantly lower activity Is the early species heterogeneous and the normal species homogeneous ... 

Successive experiments, Fig. 1 (c), on the same sample give slightly higher rates for H2 evolution, that now occur without initial induction period. This may be related to the reduction of the metallic particles in the previous experiments, since it vanishes when samples are pre-treated with H2 at 298K before irradiation. The new rates of H2 evolution remains almost unchanged during several set of experiments (each 9h irradiation) on both samples, while 02 was not detected. 

The kinetics of the reduction of [Rh(bipy)2Cl2]+ in alkaline aqueous ethanol (under H2) revealed a two stage process an initial induction period, during which time Rh1 species formed, and a faster, autocatalytic region. The kinetics could be fit to the expression d[Rh ]/df = k[Rhm]2[Rh1]. The reaction rate was retarded by the addition of excess bipy, suggesting the suppression of a dissociation equilibrium, and the rate constant varies with [OH- ], but npt with [Cl- ]. A five-step mechanism for the autocatalytic process was proposed, involving the formation (via an unspecified mechanism) of [Rh(bipy)2]+, followed by the oxidative-addition of H2 to [Rh(bipy)2]+, and chloro-bridged dimeric and trimeric intermediates.823... 

The crystallization curves can be broken down into three parts. There is an initial induction period during which the primary nuclei are formed. These primary nuclei are the smallest crystalline entities that are stable enough to allow further growth at that temperature (i.e., do not re-melt). This induction period is followed by a period of fast spheru-lite growth called primary crystallization (Figure 10-23). (If you haven t observed... 

During the initial induction period the net branching factor 0 is n ative, giving a non-branched chain system (cf. Sect. 8.2). However, if induction period, because of the changes in concentration of NO2 and NO, then a sudden increase in chain centre concentration would occur as 0 passes through zero and becomes positive. This is taken to occur at Pe. 

During the initial induction period, when 0 is n ative, the major reactions are those of the H2—NO2 system. The branching reaction (ii) is outweighed by the fast reaction (xxxix) of O atoms, and by reaction (iv) and its successors. Reaction (xxxix) becomes less important as [NO2 ] decreases, and at the same time the increasing concentration of NO favours reaction (xxxvii) of HO2 at the expense of (v) or (x). Since reactions (xxxiii) and (xxxiv) have similar rates (see Sect. 8.2) the effect of replacement of NO2 by NO on these termination steps is negligible. 0 therefore increases during the induction period. 

If the development rate by negatively charged developing agent ions is monitored until all the silver halide is reduced, it follows a typical S-shaped curve that has been interpreted as a consequence of autocatalysis caused by the increasing surface area of the developed silver. An alternative interpretation by Gavrik [23] holds that this autocatalysis is a consequence of simpler kinetics and that development is better represented by simple relations where developed silver mass is proportional to development time with an initial induction period and a final exhaustion period when all the silver halide has been reduced. Levenson [24], however, has reaffirmed the autocatalytic view. 

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