Impurities inhibitive effect

Addition of about 0 04% arsenic will inhibit dezincification of a brasses in most circumstances and arsenical a brasses can be considered immune to dezincification for most practical purposes . There are conditions of exposure in which dezincification of these materials has been observed, e.g. when exposed outdoors well away from the sea , or when immersed in pure water at high temperature and pressure, but trouble of this type rarely arises in practice. In other conditions, e.g. in polluted sea-water, corrosion can occur with copper redeposition away from the site of initial attack, but this is not truly dezincification, which, by definition, requires the metallic copper to be produced in situ. The work of Lucey goes far in explaining the mechanism by which arsenic prevents dezincification in a brasses, but not in a-/3 brasses (see also Section 1.6). An interesting observation is that the presence of a small impurity content of magnesium will prevent arsenic in a brass from having its usual inhibiting effect . 

A remarkable property of the sulphides of the alkaline earth metals and of beryllium and zinc is their power, when certain impurities are present, to exhibit phosphorescence after exposure to bright light. The phenomenon is not due to slow oxidation and is still observable in samples which have been kept hermetically sealed for years it is obvious, therefore, that the effect is a physical one and not analogous to the phosphorescence observable with sulphur (p. 37). The nature and amount of impurity present considerably affect the phosphorescence, chlorides for example causing an increase some impurities inhibit the action.2... 

Cullis and Hinshelwood (13) noted that their spectroscopically pure hexane contained an impurity. With pure a-hexane the concentration of formaldehyde remained constant during the induction period, but the inhibiting effect of formaldehyde was still observed. 

A process designed to generate product B from a raw-material stream containing the reactant A consists of a reactor in which the reaction A —> B takes place, followed by a condenser, where product B is separated in liquid form and the unreacted A is recycled to the reactor in vapor form, as in Figure 4.1. The feed stream contains a small quantity of noncondensing impurity I, which is eliminated by purging a small portion of the recycle stream. The impurity has an inhibitive effect on the reaction, which is reflected in the rate expression ... 

At a first glance, this controller is sufficient for maintaining the product purity. However, simulation results indicate that, in order to maintain x-q at the desired level when the system is subjected to a small (5%) increase in the mole fraction yi o, the recycle flow rate R would need to rise to 501.3mol/ min (a fivefold increase from the nominal value). Thus, due to its inhibitive effect on the reaction rate, the accumulation of the impurity I is highly detrimental to the operation of the process. Consequently, the control of the impurity levels in the reactor is of critical importance and directly linked to the main objective of product-purity control. 

It has been reported (II, 21) that the presence of mono- and dichloro-ethylene carbonate decreases the yield and molecular weight of PVCA. This can be confirmed for the radiation-induced polymerization when these compounds are present at levels above 1 %. However, as inhibition experiments demonstrated, the inhibition effectiveness of mono- and dichloroethylene carbonate was too small to account for the induction period observed. Furthermore, even after careful separation of these two substances, an unpolymerizable monomer with a chlorine content of about 0.1% was obtained. It should be noted that radiation-initiated polymerization of VCA is especially sensitive to inhibiting impurities, and that the induction period can be easily overrun by using conventional initiators. 

The properties of tin is shown in Table 10.1 [1]. The valence number is 2 or 4. Valence 2 is always positive however, valence 4 has amphoteric properties showing +4 or —4 according to its reaction partner. The metal has at least two allotropic modifications, i.e., the a- and /5-forms. White tin, which is usually seen, is the /5-tin. The transformation temperature from white to gray a-tin is 13.2°C. The transformation rate increases as the temperature decreases, and reaches a maximum at — 48°C. The small amounts of Bi, Sb, Pb, Ag and Au in the tin retard the transformation. Thus, commercial grade tin resists the transformation because of the inhibiting effect of these elements which are present as impurities. Tin is inert and does not react with air and water at ordinary temperatures. But at high temperatures it forms a very thin oxide layer on the surface. In oxygen, tin hardly shows any reactivity at ordinary temperatures. However, when tin is heated in... 

Li et al. (2010) tested four different Australian raw coals as fuels in direct carbon MCFCs. They found that the cell performances are highly dependent on a coal s intrinsic properties, in particular its chemical composition and concentration of oxygen-containing surface groups. Impurities such as AI2O3 and Si02 lead to an inhibitive effect, whereas CaO, MgO, and Fc203 exhibit a catalytic effect on the electrochemical carbon oxidation reaction. 

Fears of the failure of fabricated products made of block tin or high-tin alloys at low temperatures, or of the disintegration of tin in storage, are largely unfounded, because of the inhibiting effect of the common impurities and the difficulty in initiating the transition. Authenticated instances of the transformation taking place under natural conditions are rare [51]. 

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