Sulphur compounds, protection

Metal deactivator To form inactive protective films on metal surfaces which otherwise might catalyse oxidation and corrosion reactions Trialkyl and triaryl phosphites, organic dihydroxyphosphines, some active sulphur compounds, diamines in lubricating greases, mercaptobenzothiazole and phosphites... 

Tests by Clark for the Corrosion Sub-committee of the American Welding Society were carried out at severe marine and industrial sites. After four years, the greatest protection to steel was given by sprayed aluminium coatings combined with aluminium vinyl paint in the following environments id) sea air, ib) sea-water immersion, (c) alternate sea-water immersion and exposure to air (tidal conditions) and id) industrial atmospheres contaminated with sulphur compounds. 

Mincing, cooking and maturing expose meat products to oxidative stress for a long time so that antioxidants added for lipid protection are slowly destroyed on storage. Onion juice is a powerful antioxidant in meat products, more efficient than garlic juice. Lipid hydroperoxides are reduced to inactive hydroxyl derivatives by reaction with sulphur compounds present in those juices. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

The production of sulphur compounds is believed to be very important in the development of Cheddar cheese flavour. Residual sulphydryl oxidase activity may play a role in initially reoxidizing sulphydryl groups exposed upon heating cheesemilk the sulphydryl groups thus protected may be reformed during the ripening process. 

In refining the oil it is necessary to stabilize the crude by removing the liquefied petroleum gases (LPG), generally containing substantial amounts of H2S and other sulphurized compounds that must be eliminated to protect the LPG plant and produce propane and butane suitable for sale. A Reid vapour-pressure analyser monitors whether the crude has been properly stabilized. 

Selenium compounds are more active electron donors compared to analogous sulphur compounds and play a wide role in biochemical systems. They can decompose peroxides (Caldwell and Tappel 1964,1965) and can act as antioxidants (Dillard et al. 1978) and free radical scavengers (Ursini and Bindoli 1987). Masumoto and Siess (1996) and Masumoto et al. (1996) have shown that peroxy-nitrite rapidly oxidises ebselen and its main metabolite to the corresponding selenoxides. The results of Padmaja et al. (1997) suggest that CO2 partially protects methionine selenoxide from peroxynitrite-mediated oxidation and that 0=N-00-C02 or its derivatives do not mediate the oxidation of d,l-selenomethionine or methionine selenoxide. 

Alkaline chromate treatments for tin, e.g. the Protecta-Tin processes, have been developed by the Tin Research Institute. The solutions resemble the M.B.V. compositions for treating aluminium, but are more alkaline. Thin invisible films which resist staining by heat and sulphur-bearing compounds and give protection against humid atmospheres at pores are deposited. 

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 . 

Carbonyl compounds react with thiols, RSH, to form hemi-thioacetals and thioacetals, rather more readily than with ROH this reflects the greater nucleophilicity of sulphur compared with similarly situated oxygen. Thioacetals offer, with acetals, differential protection for the C=0 group as they are relatively stable to dilute acid they may, however, be decomposed readily by H20/HgCl2/CdC03. It is possible, using a thioacetal, to reverse the polarity of the carbonyl carbon atom in an aldehyde thereby converting this initially electrophilic centre into a nucleophilic one in the anion (31) ... 

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