Incongruent solution

The isothermal evaporation of a sat. soln. of two salts with a common ion leads to the separation of both salts, but with sat. soln. of three salts, this may or may not occur. The isothermal evaporation of solution II furnishes as solid phases the same three salts as are present as solutes. These soln. were called by W. Meyer-hofer congruent solutions, because, on isothermal evaporation, they furnish as solid phases the same salts as are present as solutes in soln. On the other hand, with solution I the salts which separate as solid phases are different from those present in soln., for sodium chloride appears in place of ammonium hydrocarbonate. In contrast with congruent soln., W. Meyerhofer styled them incongrucnt solutions here the sat. soln. contains a different salt from those which are present as solid phases. The gradual addition of sodium chloride to solution I changes the composition of the soln. until the incongruent solution I passes into II ammonium hydrocarbonate does not change when added in a similar manner to solution II. 

Some elements in aquatic systems exist only at low concentrations (pg/L range) in spite of readily soluble minerals. This phenomenon is not always caused by a generally small distribution of the concerned element in the earth crust mineral as for instance with uranium. Possible limiting factors are the formation of new minerals, co-precipitation, incongruent solutions, and the formation of solid-solution minerals (i.e. mixed minerals). 

Fig. 50 Calcite solubility and incongruent solution calcite-dolomite (calcite -precipitation and dolomite dissolution)...

With the decrease in as carbon dioxide was consumed in the formation of carbonates, incongruent solution of dolomite was possible, with the formation of saponite (or talc) as a result of reaction with silica ... 

Although aqueous hydrolytic equilibria such as equation (1) provide an overall framework for the discussion of isopolyanions, several species have been synthesized in nonaqueous media, and are unstable in water. Other isopolyanions that are crystallized from aqueous solution, may be incongruently soluble and have polymeric structures. It will, however, prove to be convenient to treat all isopolyanions as though they were derived by equation (1), and characterize each by its acidity , Z (=p/q). Note that Z for a specific anion is not necessarily equal to the value of p/q for the solution from which the anion was crystallized.10... 

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

The rhombohedral 7-Cr2. S3 solid solution series has a stability field ranging in composition from 46.84 to 45.12 wt. % S, but its precise incongruent melting tern-... 

Solution processes, in which one mineral is dissolving, while another mineral is inevitably precipitating, are called incongruent. Thus, if dolomite is added to water in equilibrium with calcite (SI = 0) then dolomite dissolves until equilibrium for dolomite is established. That leads consequently to an increase for the concentrations of Ca, Mg, and C in water, which in turn inevitably causes supersaturation with respect to calcite and thus precipitation of calcite. 

Chemical weathering of minerals results not only in the introduction of solutes to the aqueous phase but often in the formation of new solid phases. Dissolution is described as congruent, where aqueous phase solutes are the only products, or incongruent, where new solid phase(s) in addition to aqueous phase solutes are the products. These reactions... 

Initially in this study, it was planned to critically evaluate AG data for complex clays, including chlorite, illite, and the smectites. However, there is much evidence that these clays dissolve Incongruently so that the apparent equilibria in solution are determined by secondary phases, such as gibbsite, boehmite, amorphous silica, and ferric oxyhydroxldes. The smectites are frequently the dominant clays in the colloidal size fraction in natural sediments. They have very large exchange capacities, and exhibit wide chemical variations. Usually, one or more of these factors have not been considered in the experimental solubility work. Even if appropriate corrections could be made, it is uncertain whether a AG value so obtained would have applicability to natural systems. 

Incongruent mass transfer in the case of parabolic kinetics based on diffusion control can be readily explained by differences in the diffusion coefficients of chemical species. However, the proposed mechanisms invoked to explain linear kinetics involve the dissolution of the bulk silicate phase, which, over extended periods, should result in the chemical components being transferred to solution from the silicate in their stoichiometric ratios (25). However, many field studies have shown that silicates weather incongruently in nature (27). This is supported by our studies of ground waters associated with vitric rhyolite tuffs. 

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