Inhibition action

However, such a level can still be considered too high for vehicles having 3-way catalytic converters. In fact, results observed in the United States (Benson et al., 1991) and given in Figure 5.20 show that exhaust pollutant emissions, carbon monoxide, hydrocarbons and nitrogen oxides, increase from 10 to 15% when the sulfur level passes from 50 ppm to about 450 ppm. This is explained by an inhibiting action of sulfur on the catalyst though... 

According to Nyman the pharmacological action of hyoscine is considerably modified in the quaternary compounds of the alkaloid, e.g., the inhibiting action on salivary secretion is greatly increased in the methonitrate, as is also the spasmolytic activity, but the mydriatic action is unchanged and the central sedative activity disappears. 

Inhibiting action on the sympathetic functions of the vegetative system. 

Borates are often the base for light-duty cleaners associated with the cleaning of light metals, due to their inhibiting action and mild pH. They can also be used, to a certain extent, as a substitute for phosphates when a phosphate-free product is required. 

Variable results have been reported with this pigment and an examination of its inhibitive action has led to the conclusion that under rural and marine conditions, where the pH of the rain-water is above 5, it behaves as an inert pigment owing to its limited solubility. However, in industrial and urban areas, where the pH of the rain-water may be in the region of 4 or lower, it is converted into the more soluble monohydrogen phosphate. This reacts in the presence of oxygen, with the steel surface to form a mixture of tribasic zinc and ferric phosphates, which being insoluble protects the steel from further attack. 

Inhibitive Pigment a pigment which retards or prevents the corrosion of metals by chemical and/or electrochemical means, as opposed to a purely barrier action. Red lead and zinc chromate are examples of inhibitive pigments as opposed to red iron oxide which has little or no inhibitive action. 

Organic or inorganic inhibitors This distinction is based on the chemical nature of the inhibitor. However, in their inhibitive action many compounds that are organic in nature as, for example, the sodium salts of carboxylic acids, often have more similarities with inorganic inhibitors. 

Although halide ions are aggressive in near-neutral solutions they can be used to improve the action of inhibitors in acid corrosion (see Practice Acid Solutions). Variations exist among the halides, e.g. chloride ions favour the stress-corrosion cracking of Ti in methanol whereas iodide ions have an inhibitive action ... 

N— compounds used as acid inhibitors include heterocyclic bases, such as pyridine, quinoline and various amines. Carassiti describes the inhibitive action of decylamine and quinoline, as well as phenylthiourea and dibenzyl-sulphoxides for the protection of stainless steels in hydrochloric acid pickling. Hudson e/a/. refer to coal tar base fractions for inhibition in sulphuric and hydrochloric acid solutions. Good results are reported with 0-25 vol. Vo of distilled quinoline bases with addition of 0 05m sodium chloride in 4n sulphuric acid at 93°C. The sodium chloride is acting synergistically, e.g. 0-05m NaCl raises the percentage inhibition given by 0-1% quinoline in 2n H2SO4 from 43 to 79%. Similarly, potassium iodide improves the action of phenylthiourea . 

An exception to the lone pair or donor electron requirement of organic inhibitors is provided by the quaternary ammonium compounds. Meakins reports the effectiveness of tetra-alkyl ammonium bromides with the alkyl group having C 10. Comparative laboratory tests of commercial inhibitors of this type have been described . The inhibiting action of tetra-butyl ammonium sulphate for iron in H S-saturated sulphuric acid has been described, better results being achieved than with mono-, di- or tri-butylamines . 

Figure 2. Structural formulas of some lactones isolated from plants which have a growth-inhibiting action...

PA exerts an inhibiting action on free radical polymerizations. Bartlett et al (Ref 28) ascribe this to the following reactions ... 

The sulfenic acids have been found to be extremely active radical scavengers showing rate constants of at least 107 m"1 s 1 for the reactions with peroxyl radicals at 333 K17. It has also been suggested that the main inhibiting action of dialkyl sulfoxides or related compounds in the autoxidation of hydrocarbon derives from their ability to form the transient sulfenic acids on thermal decomposition, i.e.17... 

Another important characteristic of inhibitors is the time of their inhibition action. If an inhibitor is consumed only in chain termination reactions, this time is determined by the initial concentration [InH]0, stoichiometric coefficient of inhibition / and Vj. In this case, the rate of inhibitor consumption is vInH = v //. Side reactions of InH with dioxygen and hydroperoxide shorten the inhibitory period and increase the rate of inhibitor consumption. Therefore, an inhibitor is efficient when it provides a minimal chain length v and its own loss in side reactions w is low. Assuming that an efficient inhibitor has w < 0.25, we get the inequality 4k 2[InH][02] < v which can be transformed, by substituting the correlation equation from Table 14.7, into the following equation... 

It should be taken into account that the reaction of chain propagation occurs in polymer more slowly than in the liquid phase also. The ratios of rate constants kjlkq, which are so important for inhibition (see Chapter 14), are close for polymers and model hydrocarbon compounds (see Table 19.7). The effectiveness of the inhibiting action of phenols depends not only on their reactivity, but also on the reactivity of the formed phenoxyls (see Chapter 15). Reaction 8 (In + R02 ) leads to chain termination and occurs rapidly in hydrocarbons (see Chapter 15). Since this reaction is limited by the diffusion of reactants it occurs in polymers much more slowly (see earlier). Quinolide peroxides produced in this reaction in the case of sterically hindered phenoxyls are unstable at elevated temperatures. The rate constants of their decay are described in Chapter 15. The reaction of sterically hindered phenoxyls with hydroperoxide groups occurs more slowly in the polymer matrix in comparison with hydrocarbon (see Table 19.8). 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

Kobayashi, N. and K. Fujinaga. 1976. Synergism of inhibiting actions of heavy metals upon the fertilization and development of sea urchin eggs. Sci. Engin. Rev. Doshisha Univ. 17 54-69. 

Emorfazone (3) exhibits activities equal to or even more potent than those of basic analgesic anti-inflammatory drugs [25, 32, 35, 36], with no inhibiting action on prostaglandin biosynthesis [37,38]. Inhibition of vascular permeability [39] and of bradykinin release [37] was observed with (3). It has been found that the anti-inflammatory action of (3) may be due in part to pituitary-adrenocortical stimulation [40]. 

Hydroquinone, nitrobenzene, dinitrobezene and benzothiazine are generally used as inhibitors in polymer industry. The inhibiting action of these can be shown as follows ... 

Atmospheric oxygen also acts as an inhibitor. Its inhibiting action is due to its biradical nature... 

Results. Both the dimethyl and di-isopropyl ester were found to inhibit the cholinesterase activity of human plasma, and their action was stronger than that of eserine. Of the two esters, the di-isopropyl had a more powerful cholinesterase-inhibiting action than the dimethyl ester. An accurate quantitative comparison was made of the action of the di-isopropyl ester with that of eserine sulphate. Under the conditions of Adrian s experiment, the ester at 1/80 million had about the same cholinesterase-inhibiting action as eserine sulphate at 1/14 million, i.e. the di-isopropyl ester was about 5 times as active as eserine sulphate when compared weight for weight, and about 3 times as active when compared in molar solution. 

A confluent monolayer of Madin-Darby canine kidney (MDCK) cells was grown in 96-well plates. Serial tenfold dilutions in minimal essential medium were prepared from the aliquots of allantoic fluid taken from the irradiated specimen. These dilutions were applied to MDCK cells and incubated for 48 h at 36 °C in 5% C02. The cells were then washed two times for 5 min with phosphate buffered saline (PBS) and incubated for 1 h with 100 pi of 0.5 mg/ml solution of 3-(4,5-dimethyl-thiazolyl-2) 2,5-diphenyltetrazolium bromide (MTT, ICN Biochemicals Inc., Aurora, Ohio). After lh, the colored deposit was dissolved in 100 pi DMSO, and optical density in the wells was measured on plate reader Victor 1420 (Perkin Elmer, Finland). Based on the data obtained, the infectious titer of the vims was determined as a decimal logarithm of reciprocal to the dilution of the specimen causing destruction of 50% of cells. The inhibiting action of irradiation was evaluated by decreasing the vims titer. 

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