Acid hydrates, crystalline

Biochemistry and chemistry takes place mostly in solution or in the presence of large quantities of solvent, as in enzymes. As the necessary super-computing becomes available, molecular dynamics must surely be the method of choice for modeling structure and for interpreting biological interactions. Several attempts have been made to test the capability of molecular dynamics to predict the known water structure in crystalline hydrates. In one of these, three amino acid hydrates were used serine monohydrate, arginine dihydrate and homoproline monohydrate. The first two analyses were by neutron diffraction, and in the latter X-ray analysis was chosen because there were four molecules and four waters in the asymmetric unit. The results were partially successful, but the final comments of the authors were "this may imply that methods used currently to extract potential function parameters are insufficient to allow us to handle the molecular-level subtleties that are found in aqueous solutions" (39). 

There is IR and Raman evidence for a phase transition for nitric acid hydrate, near 200 K.536 RAIR and ab initio calculations gave information on the structure and vibrational wavenumbers for crystalline nitric acid.537 A DRIFTS study has been performed on the interaction of gas-phase IIN03 with ice and acid (HC1, HN03) hydrate surfaces.538 An ab initio calculation of the vibrational wavenumbers of nitric acid hydrates, HN03.(Fl20)n, where n = 1, 2 or 3, has been reported.539... 

There is considerable evidence for the association of Bi3+ ion with nitrate ions in aqueous solution. The nitrate ions appear to be mainly bidentate, and all members of the set Bi(N03) (H20)3 +. .. Bi(N03)4 appear to occur.70 From acid solution various hydrated crystalline salts such as Bi(N03)3 5H20, Bi2(S04)3 and double nitrates of the type M3[Bi(N03)6]2,24H20 can be obtained. Treatment of Bi203 with nitric acid gives bismuthyl salts such as BiO(N03) and Bi202(0H)(N03). Similar bismuthyl salts are precipitated on dilution of strongly acid solutions of various bismuth compounds. Bismuthyl salts are generally insoluble in water. 

The hexaquo ion exists in very strongly acid solutions of ferric salts, in the several ferric alums, MIFe(S04)2- 12H20, and presumably also in the highly hydrated crystalline salts. 

To examine the effects of coordination and hydrogen bonding, Nakamoto el al. made extensive IR measurements of the COO stretching frequencies of various metal complexes of amino acids in D2O solution, in the hydrated crystalline state, and in the anhydrous crystalline state. The results showed that, in any one physical state, the same frequency order is found for a series of metals, regardless of the nature of the ligand. The antisymmetric frequencies increase, the symmetric frequencies decrease, and Ihe separation between the two frequencies increases in the following order of metals ... 

Water content is adjusted to the total surfactant concentration of 30-42 % wt. The residual sulphite in the product may be oxidised to sulphate. The sulphonation proceeds also well when using partially hydrated crystalline sodium sulphite in a jacketed shear-stress reactor. This process modification is especially appropriate for manufacturing concentrated sulphosuccinate monoesters as flakes or vermicelli (often with plasticisers and fillers added in situ) suitable in mild synthetic soap bars [78]. The Cn-ig alcohols (I), ethoxylated (x2-4 mole EO) alcohols (II), and fatty monoethanolamides (III) esters of sulphosuccinic acid, mainly as sodium and alkanolamine salts, are of most practical importance as very mild high-foaming surfactants useful for personal care products and in wool, fur, and leather treatment. Very mild disodium PEG-5 laurylcitrate sulphosuccinate (in combination with sodium lauryl ethersulphate) serve for cosmetics produced by Witco as "Rewopol SB CS 50". 

Few if any of these various disilicic acids, which are highly polymerized in two dimensions, are identical. The relation of these to the previously discussed hydrated crystalline silicas obtained from hydrated sodium polysilicates is not known. It is evident that a large number of crystalline hydrated silicas may exist, some more stable than others, but all obtained from crystalline silicates by ion exchange. No crystalline hydrate silica is likely to be formed directly in the silica-water system in the absence of cations to bring about the organization of a regular polysilicate structure. 

The various methods of preparing monosilicic acid may be summarized as follows. A saturated solution of monosilicic acid, SifOH), containing about 0.01% SiOj, is obtained when pure amorphous silica is equilibrated with water at room temperature. A more concentrated (supersaturated) solution can be obtained only indirectly by liberating monosilicic acid from its compounds under carefully controlled conditions at low temperature and low pH, dilute solutions remain supersaturated with respect to amorphous silica for appreciable periods. For example, at pH 3 and 0°C, solutions of monosilicic acid up to O.l (0.6% SiOj) can be prepared by spontaneous hydrolysis of monomeric silicon compounds, sich as silicon tetrachloride or methyl orthosilicate, and also by reacting monomeric silicates, such as sodium or magnesium orthosilicates or hydrated crystalline sodium metasilicate, with dilute acid. 

Powders of this type have invariably been made from hydrated crystalline salts of metals, mainly alkali metals or metal complexes, by removing the cations with acid. These crystalline polysilicates have been discussed in Chapter 2 and the crystalline... 

At room temperature, salts are solid crystalline substances that contain the cation of a base and the anion of an acid. Hydrated salts contain specific numbers of water molecules as a part of their crystalline structures. Salts can be prepared by reacting an appropriate acid with one of a number of other materials. 

Mootz, D. and Wunderlich, H., Crystalline structure of acid hydrates and oxonium salts. IV Dioxonium-1,2-ethane disulfonate, Acta Crystallogr. B26,1820-1825 (1970). 

However, as we discussed in Chapter 7, the hydrated protmi or oxonium ion, [H30] +, is an important species in aqueous solution Ahyd// (H, g) = —1091 kJmol (seeSectimiT.O). The [H30] ion (10.1) is a well-defined species which has been crystallographically characterized in various salts. The irais [H502] (Fig. 10.1) and [H904] have also been isolated in crystalline acid hydrates. The [H502] and [11904] irais are members of the general family of hydrated protons [H(0H2) ] (m = 1 to 20) and we return to these ions in Section 10.6. 

Silica as SiO is generally used in water-related discussions, when, in fact, more than 22 phases of silica, as silicic acid and silicates, have been identified. For example, H SiO is recognized as ortho (or mono) silicic acid and H SiO is metasilicic acid, with the difference being the degree of hydration. Crystalline silica, as SiO, dissolves in water to form silicic acid, as shown in Equation 7.10 ... 

This occurs naturally as a white solid in various crystalline forms, in all of which six oxygen atoms surround each titanium atom. Titanium dioxide is important as a white pigment, because it is nontoxic. chemically inert and highly opaque, and can be finely ground for paint purposes it is often prepared pure by dissolving the natural form in sulphuric acid, hydrolysing to the hydrated dioxide and heating the latter to make the anhydrous form. 

Mercurous Nitrate. Mercurous nitrate [10415-75-5] Hg2N20 or Hg2(N02)2, is a white monoclinic crystalline compound that is not very soluble in water but hydrolyzes to form a basic, yellow hydrate. This material is, however, soluble in cold, dilute nitric acid, and a solution is used as starting material for other water-insoluble mercurous salts. Mercurous nitrate is difficult to obtain in the pure state directly because some mercuric nitrate formation is almost unavoidable. When mercury is dissolved in hot dilute nitric acid, technical mercurous nitrate crystallizes on cooling. The use of excess mercury is helpful in reducing mercuric content, but an additional separation step is necessary. More concentrated nitric acid solutions should be avoided because these oxidize the mercurous to mercuric salt. Reagent-grade material is obtained by recrystaUization from dilute nitric acid in the presence of excess mercury. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Tricalcium phosphate, Ca2(P0 2> is formed under high temperatures and is unstable toward reaction with moisture below 100°C. The high temperature mineral whidockite [64418-26-4] although often described as P-tricalcium phosphate, is not pure. Whidockite contains small amounts of iron and magnesium. Commercial tricalcium phosphate prepared by the reaction of phosphoric acid and a hydrated lime slurry consists of amorphous or poody crystalline basic calcium phosphates close to the hydroxyapatite composition and has a Ca/P ratio of approximately 3 2. Because this mole ratio can vary widely (1.3—2.0), free lime, calcium hydroxide, and dicalcium phosphate may be present in variable proportion. The highly insoluble basic calcium phosphates precipitate as fine particles, mosdy less than a few micrometers in diameter. The surface area of precipitated hydroxyapatite is approximately... 

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