Phosphates, temperature factors

The four values correspond to the difference in temperature factors for atoms connected by a bond length, for atoms connected by a bond angle, for P-0 bond lengths, and for phosphate atoms connected by a bond angle or for atoms involved in hydrogen bonding. 

The formation of tricalcium phosphate in a cooling system is primarily a net result of system pH, calcium concentration, temperature, and phosphate concentration factors. An increase in any of these factors leads to an increased risk of phosphate deposition. 

Table 24.3. Mean crystallographic temperature factors B (A2) for base, sugar and phosphate atoms in the DNA dodecamer d(CGCGAATTCGCG) investigated under different conditions [867]. The mean vibrational amplitudes fi are obtained according to B = 8 n2fi2 ...

Temperature factors of atoms in DNA double helices increase from base to sugar to phosphate atoms (see Thble 24.3). This holds for all three types of double helices. It implies that water molecules associated with functional groups of bases are better defined than those bound to sugar atoms, and water molecules around phosphate groups are the least well defined in the electron density maps. 

Hydration numbers for phosphate and base atoms are comparable. This does not necessarily mean that they have the same affinity for hydration. Since, as shown in Thble 24.3, the temperature factors of phosphate groups are more than twice those of the bases, they and the associated water molecules are less well defined and more difficult to locate from electron density maps. We have to assume that phosphate groups are, in fact, more hydrated than shown in Thble 24.5. 

SR Laue data were recorded from a crystal of aluminium phosphate and analysed by Wood et al (1983). The R-factor on I in the Laue data refinement was 19% compared with a conventional monochromatic single crystal study (Thong and Schwarzenbach 1979) with an f -factor of 2.2% on F. For the Laue data 20% of the measurements were rejected on the basis of bad intensity agreements. Of the eight refined positional parameters six were within 2a and only one outside 3a. The temperature factors were somewhat less well determined. This was considered to be due to the significant extinction and multiple diffraction effects for a hard material such as aluminium phosphate. 

Haile JM (1997) Molecular E namics Simulation Elementary Methods. John Wiley and Sons, New York Hartzell CJ, Cygan RT, Nagy KL (1998) Molecular modeUng of the tributyl phosphate complex of europium nitrate in the clay hectorite. J Phys Chem A 102 6722-6729 Hass KC, Schneider WF, Curioni A, Andreoni W (1998) The chemistry of water on alumina surfaces Reaction dynamics from first principles. Science 282 265-268 Hazen RH (1976) Effects of temperature and pressiue on the cell dimension and X-ray temperature factors of periclase. Am Mineral 61 266-271... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Solid alkalis Solid alkalis may be used, in principle, for the corrosion control of drum boilers at all pressures but other factors, e.g. carryover or hideout a (reversible disappearance from solution on-load), may preclude them in some cases. However, they are used for feed-line treatment only in lower pressure plant where the boiler has increased tolerance to the higher solids burden which their use entails. Sodium hydroxide or, at very low pressures, sodium carbonate, (which is hydrolysed to the hydroxide at boiler temperatures) have been used, as have potassium and lithium hydroxides and various phosphate mixtures. (For a comparison of various alkalis for this purpose see References.)... 

Trost published a desulphonylation procedure for aryl alkyl sulphones using an excess of sodium amalgam in buffered ethanol126 (equation 52). Trost claimed that this is superior to earlier reactions using sodium amalgam in ethanol because of a couple of factors the use of the acid phosphate buffer to prevent formation of significant amounts of sodium methoxide is particularly important, since this can cause isomerizations in base-sensitive substrates, and the temperature should be kept low, but optimized for each substrate. 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

FIGURE 1.4 Dependencies of retention factors k on counterion (i.e., phosphate) concentration [X]. Experimental conditions Mobile phase, methanol-sodium dihydrogenphosphate buffer (50 50 v/v) (pHa 6.5 adjusted in the mixture with sodium hydroxide) flow rate, 1 mLmin temperature, 25°C CSP, 0-9-[3-(triethoxysilyl)propylcarbamoyl]-quinine bonded to silica [30] column dimension, 150 x 4 mm ID. 

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