Vitreous water

The formation of trapped electrons during radiolysis of vitreous water-alkaline matrices is known to be accompanied by the formation of O anion radicals which serve as acceptors for etr [27]. The trap for the O anion radicals in these matrices, suggested in ref. 36, is represented schematically in Fig. 10. When water alkaline matrices irradiated at 77 K are heated to T 150 K, i.e. to a temperature at which glass softening starts, the concentrations of et" and O decrease simultaneously [37]. This appears to be related to the occurrence of a reaction between them... 

Fig. 27. Kinetic curves for the decay of et, in a vitreous water-alkaline solution (6M NaOH) containing 0.025 M of CrO2- at 178 K. The points represent the experimentally observed values [43] the lines are the curves drawn from calculations using eqn. (35) of Chap. 4 (curve 1) and eqn. (27) (curve 2) i D = 3lA and D = 2.2 x 10 12cm2s-1.

In order to check the proposed model of et photobleaching, in refs. 40 and 62 the kinetics of et photobleaching in the presence of acceptor additives in vitreous water-alkaline and water-ethylene glycol matrices at 77 K was studied. Typical curves for photobleaching are presented in Fig. 31. The addition of acceptors is shown to result in an essential increase in the rate of e,r. photobleaching, the kinetics of e,r decay in the presence of additives being described by an exponential law in accordance with eqn. (28). 

Rate constants of the reactions of eaq with various acceptors in liquid water at 298 K and the characteristic times, t, for the reactions of et with the same acceptors in vitreous water-alkaline solutions at 77 K... 

Therefore, experiments are performed on immobilized liquids , or in other words on amorphous water (also called vitreous water or glassy water). Currently, three structurally distinct amorphous states of water are known low- (LDA) , high- (HDA) and very high- (VHDA) density amorphous ice We emphasize that HDA is not a well defined state but rather comprises a number of substates. It has been suggested to use the nomenclature uHDA ( unrelaxed HDA ) ", eHDA ( expanded HDA ) " and/or rHDA ( relaxed HDA ) to account for this. Even though no signs of micro-crystallinity have been found in neutron or X-ray diffraction studies, it is unclear whether... 

Figure 17. Temperature dependence of heat capacity of potato starch-water mixture sample containing 58.5 mass% of H2O [4,73, 125] AB - vitreous saturated solution of water in starch + partially crystalline and vitreous water surplus phase BC - highly elastic saturated solution of water in starch + partially crystalline and vitreous water surplus phase AD - vitreous saturated solution of water in starch + entirely crystalline water surplus phase DE - highly elastic saturated solution of water in starch + crystalline water surplus phase EF - melting of water surplus phase GH - highly elastic saturated solution of water in starch liquid water surplus phase.

Fluoridation of potable water suppHes for the prevention of dental caries is one of the principal uses for sodium fluoride (see Water, municipal WATER treatment). Use rate for this appHcation is on the order of 0.7 to 1.0 mg/L of water as fluoride or 1.5 to 2.2 mg/L as NaF (2). NaF is also appHed topically to teeth as a 2% solution (see Dentifrices). Other uses are as a flux for deoxidiziag (degassiag) rimmed steel (qv), and ia the resmelting of aluminum. NaF is also used ia the manufacture of vitreous enamels, ia pickling stainless steel, ia wood preservation compounds, caseia glues, ia the manufacture of coated papers, ia heat-treating salts, and as a component of laundry sours. 

Enameling meets decorative as weU as protective requirements. Ceramic enamels are mainly based on alkaH borosiHcate glasses. The part to be enameled is dipped into or sprayed with a sHp, ie, a water suspension of glass fragments called frit. The sHp coating is dried and fused in an enameling furnace under careful heat control (see Enamels, porcelain or vitreous). 

Chemical Properties. Stoichiometric vitreous sihca contains two atoms of oxygen for every one of sihcon, but it is extremely doubtful if such a material really exists. In general, small amounts of impurities derived from the starting materials are present and various stmctural defects can be introduced, depending on the forming conditions. Water is incorporated into the glass stmcture as hydroxyls. 

Vitreous sihca does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubihty is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 sihca dissolves mainly as soluble sihcates. Solubihty also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10 300 atm), for example, the solubihty, S, of Si02 in g/kg H2O can be expressed as foUows, where d ls the density of the vapor phase and T is the absolute temperature in Kelvin. 

Cristobahte can also form on vitreous siUca at temperatures as low as 400°C when the pressure is equal to 35 MPa (<350 atm) and the glass is immersed in weak NaOH solutions (108). In stronger NaOH solutions, quart2 is formed. The formation of the crystalline phases is a result of the hydrolysis of the anions present. No crystallisation occurs with HF, H2SO4, and H PO in KHSO solutions or in pure water. 

The surface conductivity of vitreous siUca is also low compared to other siUcate glasses. Because vitreous siUca is not hydroscopic, water films containing exuded alkaUes do not readily form on its surfaces. The surface conductivity, however, can increase significantly with increasing relative humidity. A change in the relative humidity from 20 to 80% produces a millionfold increase in the surface conductivity (174). 

Vitreous silica is used for gas-heated or electrically heated devices ia various shapes, eg, as a tube or muffle because of its electrical resistivity, impermeabihty, and low expansion. In its simplest form, an electric-resistance furnace consists of a vitreous siUca tube or pipe on which the resistance element is wound (see Furnaces, ELECTRIC). Because of its iadifference to temperature gradients, a tubular furnace of vitreous siUca maybe made to operate at different temperatures at various portions of the tube, either by arrangement of the heating elements or by cooling sections of the tube with water. Vitreous siUca pipes may be employed ia vacuum-iaduction and gas-fired furnaces (see Vacuum technology) (221). 

The Topex oceanographic satelHte used a laser-based retroreflector array for positioning. The retroreflectors were manufactured from Corning Code 7958 fused siHca, a sol—gel-derived low water vitreous siHca material (248). 

The relationships between condensed phases ia the B2O3—H2O system are shown ia Figure 1 (42). There is no evidence for stable phases other than those shown. B2O3 melts and glasses containing less than 50 mol % water have mechanical and spectroscopic properties consistent with mixtures of HBO2 and vitreous B2O3. 

Sedimentary rocks (like sandstone) have a microstructure rather like that of a vitreous ceramic. Sandstone is made of particles of silica, bonded together either by more silica or by calcium carbonate (CaCOj). Like pottery, it is porous. The difference lies in the way the bonding phase formed it is precipitated from solution in ground water, rather than formed by melting. 

Vitreous ceramics are different. Clay, when wet, is hydroplastie the water is drawn between the clay particles, lubricating their sliding, and allowing the clay to be formed by hand or with simple machinery. When the shaped clay is dried and fired, one component in it melts and spreads round the other components, bonding them together. 

The uses in the glass and ceramics industries reflect the diagonal relation between boron and silicon and the similarity of vitreous borate and silicate networks (pp. 203, 206 and 347). In the UK and continental Europe (but not in the USA or Japan) sodium perborate (p. 206) is a major constituent of washing powders since it hydrolyses to H2O2 and acts as a bleaching agent in very hot water ( 90°C) in the USA domestic washing machines rarely operate above 70°, at which temperature perborates are ineffective as bleaches. 

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