Congruent solution

At pseudo steady state, the thickness of the cation depleted layer is a function of pH. Thickness will decrease with increasing H+ ion concentration in bulk solution. Congruent dissolution will result immediately from low pH solutions. In practical situations, the depleted layer thickness has been estimated to be 2-17 A (Schott et al., 1979). For A1 or Si dissolving from a feldspar grain at pH 4, typical parameters for Eq. 4 are... 

Addition of the appropriate amount of water to anhydrous H3PO4, or crystallization from a concentrated aqueous solution of syrupy phosphoric acid, yields the hemihydrale 2H3PO4.H2O as a congruently melting compound (mp 29.3 "). The crystal structure shows the presence of 2 similar H3P()4 molecules which, together with the H2O molecule, are linked into... 

The dichlorodibenzo-p-dioxin component was isolated by passing a dioxane solution of the mixture through acetate ion exchange resin to remove phenolics. The eluted product was recrystallized from benzene. The x-ray powder diffraction pattern of the precipitate was identical with that of 2,7-dichlorodibenzo-p-dioxin. Analysis of the mother liquor by GLC showed a singular peak consistent with 2,7-dichlorodibenzo-p-dioxin. The mother liquor was cooled to 5°C and yielded transparent crystals. This material had an x-ray diffraction pattern congruent to a sample of 2,8-dichlorodibenzo-p-dioxin obtained from A. E. Pohland (FDA). The two patterns were quite distinct from each other 14, 15). 

We start with compounds "A" and "B", one on each end of the pheise diagram, i.e.- 100% A or 100% B. There are two regions in between these limits where solid solutions of A B occur. More Importantly, a region exists where the composition freezes and melts congruently (that is- the melt does not decompose. 

In very dilute HCl solutions, specifically those with a pH above 5-48, the 4 1 5 phase was found to be insoluble. By contrast, addition of concentrated HCl to the 4 1 5 phase was shown to lead to formation of the 1 1 2 phase (Sorrell, 1977). Below 35wt% HCl, the 4 1 5 phase was found to dissolve congruently. Since the 1 1 2 phase was also found to dissolve congruently in hydrochloric acid solutions with concentrations above 23 wt %, it follows that there is a range of concentrations over which both phases are soluble in aqueous HCl. This behaviour explains why the zinc oxychlorides have proved to be unsatisfactory in attempts to use them as dental cements. The preparation of such cements from concentrated aqueous solutions of ZnClj results in the formation either of the 1 1 2 phase alone or of mixtures of the 4 1 5 and 1 1 2 phases, neither of which is stable in the presence of water. Preparing dental cements from less concentrated solutions also results in the formation of mixed phases, unless the bulk composition has excessive amounts of ZnO present. In these latter cases the cement stability is acceptable but it lacks both a workable consistency and a reasonable working time. 

Calcareous minerals and evaporite minerals (halides, gypsum) are very soluble and dissolve rapidly and, in general, congruently (i.e., yielding upon dissolution the same stoichiometric proportions in the solution as the proportions in the dissolving mineral and without forming new solid phases). Their contribution to the total dissolved load in rivers can be estimated by considering the mean composition of river... 

The thickness, y, of the cation depleted layer at steady state with decrease with increasing [H+] in the solution. Therefore congruent dissolution will result immediately from low pH solution. 

The term congruent dissolution often refers to a process by which a mineral is dissolved in stoichiometric proportions into solution without formation of a new solid phase, and this usage is convenient for isotopic studies because it constrains the phases or components that may... 

Figure 5. Possible pathways by which Fe isotopes may be fractionated during dissimilatory Fe(III) reduction (DIR). Dissolution, if it occurs congruently, is unlikely to produce isotopic fractionation (Afi. If Fe(II) is well complexed in solution and conditions are anaerobic, precipitation of new ferric oxides (A3) is unlikely to occur. Significant isotopic fractionation is expected during the reduction step (A2), possibly reflecting isotopic fractionation between soluble pools of Fe(III) and Fe(II). The soluble Fe(III) component is expected to interact with the cell through an electron shuttle compound and/or an outer membrane protein, and is not part of the ambient pool of aqueous Fe. Sorption of aqueous or soluble Fe(II) to the ferric oxide/hydroxide substrate (A4) is another step in which isotopic fractionation may occur. Modified from Beard et al. (2003a).

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe " solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

Considerable effort has been expended in growing single crystals of compositions in the solid solution, with particular emphasis on the superconducting composition. A variety of methods have been employed, all flux type, as the solid solution members are not congruently melting. There have been a few detailed reports on growth of single crystals from melts in the... 

Equations such as equation (1) above imply that the oxidative dissolution of pyrite is congruent, directly liberating Fe2+, SO4, and H+ to solution. However, in the common circumstance that water is insufficiently abundant to immediately transport the oxidation products away from the mineral surfaces, pyrite oxidation more commonly results initially in the accumulation of various hydroxysulphate evaporite minerals. These minerals form efflorescent crusts, typically white and yellow in colour, on the surfaces of pyrite-rich coals and mudstones (Fig. 1), and they effectively store the oxidation products in a readily soluble form until some hydro-logical event delivers sufficient water to dissolve and transport them away. Because pyrite often occurs in mudstones, where Al-bearing clay minerals are in contact with acidic pyrite oxidation waters, A1 is frequently released from the clays and is also stored in these hydroxysulphate phases. When these minerals finally dissolve, they result in abrupt and extreme increases in dissolved acidity. For this reason, they have been termed acid generating salts (AGS) (Bayless... 

After one day, a direct correlation between leachant pH and release of Si in the solution can be seen (Fig. 9a). Later on (3- and 10-day corrosion), the increase in pH becomes much slower, suggesting that the leaching of alkaline elements is not a congruent process, and pH stabilizes rapidly during matrix dissolution. In fact, the proportion of CaO in the matrix of HT materials (11-38 wt%) directly controls the amount of Ca that will be readily leached out (see Fig. 9b). It can thus be expected that Ca-poor/Si-rich HT materials possess a higher durability than Ca-rich/Si-poor ones. The relationship between CaO in the matrix of HT materials, Ca2+ extracted into solution by the leachant, and pH is given by ... 

Using the data in Example Problem 2.1 and assuming that a = 668 J, determine the composition of a mixture of Cs and Rb that will melt congruently at 282.7 K—that is, will go from a solid solution to a liquid solution without changing composition (going through phase separation). 

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