Water, acid attack

Ditungsten trisiUcide [12138-30-6], W2Si2, gray in color and having an sp gr of 10.9, is insoluble in water, acid, or alkaline solutions. It is readily attacked by HNO —HE and fused alkah-metal carbonates and hydroxides. 

High Pressure Boiler Water Treatment. High pressure boilers usually have feed water composed of demineralized makeup water and a high percentage of condensate returns. Because of these conditions, high pressure boilers are prone to caustic and acid attack. Low pressure boilers that use dernineralized water and condensate as feed water are also susceptible to caustic and acid attack. 

MetaUic arsenic is not readily attacked by water, alkaline solutions, or nonoxidizing acids. It reacts with concentrated nitric acid to form orthoarsenic acid [7778-39-4] H AsO. Hydrochloric acid attacks arsenic only in the presence of an oxidant. 

Aluminum has high resistance to atmospheric conditions as well as to industrial fumes and vapors and fresh, brackish, or salt waters. Many mineral acids attack aluminum, although the metal can be used with concentrated nitric acid (above 82 percent) and glacial acetic acid. Aluminum cannot be used with strong caustic solutions. 

Dissolved oxygen, water, acid, and metal-ion concentrations can have a pronounced effect on acid corrosion. For example, copper is vigorously attacked by acetic acid at low temperatures at temperatures above boiling, no attack occurs because no dissolved oxygen is present. 

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

The sulfate process is based on the attack of activated beryl by sulfuric acid to form water-soluble Be and A1 sulfates and insoluble silica. A flow diagram summarizing this process is given in Scheme 1. Activated beryl must be used since the natural ore is resistant to acid attack except by HF, which is prohibitively expensive. High-grade beryl ore (> 10% BeO) is normally activated by a heat-treatment process... 

Figure 7-6. Mechanism for catalysis by an aspartic protease such as HIV protease. Curved arrows Indicate directions of electron movement. Aspartate X acts as a base to activate a water molecule by abstracting a proton. The activated water molecule attacks the peptide bond, forming a transient tetrahedral Intermediate. Aspartate Y acts as an acid to facilitate breakdown of the tetrahedral intermediate and release of the split products by donating a proton to the newly formed amino group. Subsequent shuttling of the proton on Asp X to Asp Y restores the protease to its initial state.

It is not affected by halogens or acids, except for phosphoric and hydrofluoric acids. Phosphoric acid attacks fused silica at temperatures of 300-400°C, and hydrofluoric acid attacks it at room temperature, forming silicon tetrafluoride and water. At high temperatures silica reacts with caustic alkalis, certain metallic oxides, and some basic salts, and cannot be used for incinerating these materials. Over 1600°C, fused silica is reduced to silicon by carbon. It can also be reduced at high temperature by hydrogen. It is unaffected by water under normal conditions but is attacked by strong solutions of alkalis. 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

Methanol is not miscible with hydrocarbons and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. On the other hand, anhydrous ethanol is completely miscible in all proportions with gasoline, although separation may be effected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol. Also benzene or acetone can be used. The wear problem is believed to be caused by formic acid attack, when methanol is used or acetic acid attack when ethanol is used. 

Serine hydroxymethyl transferase catalyzes the decarboxylation reaction of a-amino-a-methylmalonic acid to give (J )-a-aminopropionic acid with retention of configuration [1]. The reaction of methylmalonyl-CoA catalyzed by malonyl-coenzyme A decarboxylase also proceeds with perfect retention of configuration, but the notation of the absolute configuration is reversed in accordance with the CIP-priority rule [2]. Of course, water is a good proton source and, if it comes in contact with these reactants, the product of decarboxylation should be a one-to-one mixture of the two enantiomers. Thus, the stereoselectivity of the reaction indicates that the reaction environment is highly hydro-phobic, so that no free water molecule attacks the intermediate. Even if some water molecules are present in the active site of the enzyme, they are entirely under the control of the enzyme. If this type of reaction can be realized using synthetic substrates, a new method will be developed for the preparation of optically active carboxylic acids that have a chiral center at the a-position. 

White powder, hexagonal graphite-like form or cubic crystal cubic form similar to diamond in its crystal structure, and reverts to graphite form when heated above 1,700°C density 2.18 g/cm melts at 2,975°C (under nitrogen pressure) sublimes at 2,500°C at atmospheric pressure insoluble in water and acid attacked by hot alkalies and fused alkali carbonates not wetted by most molten metals or glasses. 

Holmium chloride is obtained from rare-earth minerals. Recovery steps are discussed above (see Holmium). The rare-earth mineral is cracked by acid attack by heating with hydrochloric acid. The water-soluble chloride salt is filtered and separated from insoluble residues. The hydrated chloride salt is heated at 350°C in a current of hydrogen chloride to yield anhydrous H0CI3. Heating in air in the absence of hydrogen chloride yields holmium oxychloride, HoOCl. Hohnium chloride may be purified by distdlation or vacuum sublimation. 

White metal with brdhant metaUic luster face-centered cubic crystals density 10.43 g/cm at 20°C, and 9.18 g/cm at 1,100°C melts at 961.8°C vaporizes at 2,162°C vapor pressure 5 torr at 1,500° C pure metal has the highest electrical and thermal conductive of aU metals, electrical resistivity of pure metal at 25°C 1.617x10 ohm-cm elastic modulus 71GPa (10.3x10 psi) Poisson s ratio 0.39 (hard drawn), 0.37 (annealed) viscosity of hquid silver 3.97 centipoise at 1,043°C thermal neutron absorption cross section 63 1 barns insoluble in water inert to most acids attacked by dilute HNO3 and concentrated H2SO4 soluble in fused caustic soda or caustic potash in the presence of air. 

Gray metallic soM cubic structure very hard, hardness > 8.0 Mohs density 6.73 g/cm3 melts at 3,532°C insoluble in water slightly soluble in concentrated sulfuric acid soluble in hydrofluoric acid and oxidizing acids, such as nitric and perchloric acids attacked by oxidizers... 

Sulfuric acid and hydrofluoric acid are used as catalysts in the production of gasoline alkylate. After processing, this acid must be removed from the finished alkylate. This is typically accomplished by water washing or caustic washing the alkylate. However, if residual sulfuric acid or hydrofluoric acid remains in the fuel or alkylate, the acid can initiate corrosion. The acid is very aggressive toward initiating corrosion of ferrous metal. It is difficult for filming-type corrosion inhibitors to overcome acid attack of metal. 

First the aqueous C02 dissociates in water, producing carbonic acid, then the acid attacks the anorthite, leaching Ca2+ and neutralizing the acid, and finally, calcium carbonate precipitates. 

Acid-catalysed hydrolysis. The carbonyl group of an ester is not sufficiently electrophilic to be attacked by water. The acid catalyst protonates the carbonyl oxygen, and activates it towards nucleophilic attack. The water molecule attacks the protonated carbonyl carbon, and forms a tetrahedral intermediate. Proton transfer from the hydronium ion to a second molecule of water yields an ester hydrate. The intramolecular proton transfer produces a good leaving group as alcohol. A simultaneous deprotonation by the water and loss of alcohol gives a carboxylic acid. 

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