Experimental Studies solution equilibria

Two series of experiments have been conducted which elucidate the stability of pure magnesian sepiolite in aqueous solution, each using a different approach in procedure and each giving concordant and complimentary results. Wollast, et al., (1968) have used sea water as a basis for their chemical system where they have controlled the pH and SiOj content by addition of NaOH and sodium meta-silicate. This procedure incidentally changes the Na concentration of the system from that of normal sea water but this does not seem to alter the phase relations. 

SiO concentrations of the supernatant liquid. The result of these experiments is to establish that the precipitation of sepiolite from solution 2+ 

Significant in this series of experiments is the fact that Mg concentration was high enough to precipitate the silica added without significantly 

Thus the solution always contains Mg in approximately constant abundance, which makes it effectively a perfectly mobile component. The same is true for H+ since pH changes little after precipitation of the sepiolite even though the reaction consumes (OH). The experimental system is then open with respect to these two components. A determination of the solubility product constant of a natural iron-calcium-aluminous sepiolite confirms generally the above results (Christ, et al , 1973). 

The study by Siffert (1962) was conducted using another method resulting In fixed chemical ratios of the components in solution which significantly affects the character of the product precipitated. The molecular ratio of Si/Mg In the initial solution is fixed (values of 1.43 and 0.70 were 

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In all of these experimental studies, the equilibrium was established at an elevated temperature (>100°C) and the subsequent analysis was carried out at room temperature. This can only be done if the kinetics of all condensation equihbria are very slow at room temperature and fast enough to establish a thermodynamic equilibrium at the experimental temperature. It was found that the reaction kinetics of the hydrolysis and condensation reactions were very slow at room temperature and a high pH. Using the rate constants for the hydrolysis of diphosphate and triphosphate from other literature references, a half-Hfe of about 30-100 days can be estimated for this species in a dilute solution at room temperature and pH 9-11 [47, 48]. 

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

Lee et al. (1974) conducted an experimental study on the equilibrium for the assemblage of stannoidite-chalcopyrite-bornite-mawsonite-S2 (gas) in a temperature range from 430 to 300°C. Curves A and B in Fig. 1.183 correspond to /s2-temperature relationships for this equilibrium assemblage for ape = 1 and ape = 0.1, where ape is the activity of the CusFe2FeSn2Si2 component in stannoidite solid solution. 

In experimental studies, it is common practice to attempt to bracket a measured solubility by reacting a sample with undersaturated as well as supersaturated solutions. As is shown in Figure 26.2, however, this technique might equally well identify a steady-state condition as an equilibrium state. 

Most thermodynamic data for solid solutions derived from relatively low-temperature solubility (equilibration) studies have depended on the assumption that equilibrium was experimentally established. Thorstenson and Plummer (10) pointed out that if the experimental data are at equilibrium they are also at stoichiometric saturation. Therefore, through an application of the Gibbs-Duhem equation to the compositional dependence of the equilibrium constant, it is possible to determine independently if equilibrium has been established. No other compositional property of experimental solid solution-aqueous solution equilibria provides an independent test for equilibrium. If equilibrium is demonstrated, the thermodynamic properties of the solid solution are also... 

More than half a century ago, Bawden and Pirie [77] found that aqueous solutions of tobacco mosaic virus (TMV), a charged rodlike virus, formed a liquid crystal phase at as very low a concentration as 2%. To explain such remarkable liquid crystallinity was one of the central themes in the famous 1949 paper of Onsager [2], However, systematic experimental studies on the phase behavior in stiff polyelectrolyte solutions have begun only recently. At present, phase equilibrium data on aqueous solutions qualified for quantitative discussion are available for four stiff polyelectrolytes, TMV, DNA, xanthan (a double helical polysaccharide), and fd-virus. 

Wollast, et al., 1968) at 1 atmosphere, 25°C, demonstrates the inherent stability of this mineral at surface conditions. These experimental studies establish the necessity of an alkaline solution (pH 8) and sili v on-centration in aqueous solution controlled by the presence of amorphous silica (20-150 ppm). Calculations based upon laboratory synthesis data suggest that sepiolite could form in equilibrium with quartz (i.e., 10 ppm... 

In the previous sections concerning reference and standard states we have developed expressions for the thermodynamic functions in terms of the components of the solution. The equations derived and the definitions of the reference and standard states for components are the same in terms of species when reactions take place in the system so that other species, in addition to the components, are present. Experimental studies of such systems and the thermodynamic treatment of the data in terms of the components yield the values of the excess thermodynamic quantities as functions of the temperature, pressure, and composition variables. However, no information is obtained concerning the equilibrium constants for the chemical reactions, and no correlations of the observed quantities with theoretical concepts are possible. Such information can be obtained and correlations made when the thermodynamic functions are expressed in terms of the species actually present or assumed to be present. The methods that are used are discussed in Chapter 11. Here, general relations concerning the expressions for the thermodynamic functions in terms of species and certain problems concerning the reference states are discussed. 

In the foregoing we have discussed the determination of the chemical potentials as functions of the temperature, pressure, and composition by means of experimental studies of phase equilibria. The converse problem of determining the phase equilibria from a knowledge of the chemical potentials is of some importance. For any given phase equilibrium the required equations are the same as those developed for the integral method. The solution of the equation or equations requires that a sufficient number of... 

Rupture of emulsion bilayers. Experimental verification of the theory [399,402,403] of hole nucleation rupture of bilayer has also been conducted with emulsion bilayers [421]. A comparative investigation of the rupture of microscopic foam and emulsion bilayers obtained from solutions of the same Do(EO)22 nonionic surfactant has been carried out. The experiments were done with a measuring cell, variant B, Fig. 2.3, a large enough reservoir situated in the studied film proximity was necessary to ensure the establishment of the film/solution equilibrium. The emulsion bilayer was formed between two oil phases of nonane at electrolyte concentration higher than Cei,cr-... 

Since the type of solutions encountered in extractive distillation involve mixtures of polar compounds or polar with nonpolar ones, the solutions are usually nonideal, and predicting the phase equilibrium from pure component data only is practically impossible. Theoretical and experimental studies through the years, however, have established certain trends which are used to search for and screen potential solvents. 

The problem of crystal growth from supercooled liquids has been formulated in terms of a similar model based on the interfacial tension of microcrystals in the solution. A number of experimental studies which have been made have given further support to the qualitative concepts of the model. The time lag in nucleation required for the distribution of nuclei to change from the equilibrium value at saturation to the stationary concentration at supersaturation has been discussed in some detail by Kantrowitz. ... 

Inorganic phosphate ligands are important with respect to the behavior of actinides in the environment and as potential waste forms. There have been a number of experimental studies to determine the equilibrium constants in the actinide-phosphoric acid system, but they have been complicated by the formation of relatively insoluble solid phases and the formation of ternary actinide complexes in solution. 

Silicate solutions of equivalent composition may exhibit different physical properties and chemical reactivities because of differences in the distributions of polymer silicate species. This effect is keenly observed in commercial alkali silicate solutions with compositions that lie in the metastable region near the solubility limit of amorphous silica. Experimental studies have shown that the precipitation boundaries of sodium silicate solutions expand as a function of time, depending on the concentration of metal salts (29,58). Apparendy, the high viscosity of concentrated alkali silicate solutions contributes to the slow approach to equilibrium. 

Quantitative information on the equilibrium flexibility of actually existing chain molecules permitting the determination of their A (or a) values can be obtained from the experimental study of their conformational, hydrodynamic, optical and other properties in dilute solutions ... 

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