Effect of impurities on solubility

So-called pure solutions are rarely encountered outside the analytical laboratory, and even then the impurity levels are usually well within detectable limits. Industrial solutions, on the other hand, are almost invariably impure, by any definition of the term, and the impurities present can often have a considerable effect on the solubility characteristics of the main solute. 

If to a saturated binary solution of A (a solid solute) and B (a liquid solvent) a small amount of the third component C (also soluble in B) is added, one of four conditions can result. First, nothing may happen, although this is comparatively rare, in which case the system remains in its original saturated state. Second, component C may react or otherwise combine or react chemically with A by forming a complex or compound, thus altering the whole nature of the system. In the third case, the presence of component C may make the solution super-saturated with respect to solute A, which would then be precipitated. In the fourth case, the solution may become unsaturated with respect to A. The terms salting-out and salting-in are commonly used to describe these last two cases, particularly when electrolytes are involved. 

The salting-out effect of an electrolyte added to an aqueous solution of a non-electrolyte, can often be represented by the empirical equation 

When considerable quantities of a soluble impurity are present in, or deliberately added to, a binary solution, the system may be assessed better in terms of three components, expressing the data on a triangular ternary equilibrium diagram (see section 4.6). 

Certain impurities such as aluminum in minute amounts not only reduce the rate of dissolution of silica, but by chemisorption on the surface of silica, even in amounts less than a monomolecular layer, reduce the solubility of silica at equilibrium. Jeph-cott and Johnston (179a) have shown that the apparent solubility of amorphous finely divided silica in water, which they find to be 0.017% at 37 C, is reduced to 0.003-0.0097% when aluminum oxide is added to the system and to less than 0.0001% when powdered aluminum is present. Since it was also shown that the addition of silica to a suspension of alumina depressed the solubility of alumina, it seems likely that a surface containing both SiOj and AljOa is formed on both the silica and alumina phases and has a lower solubility than either oxide. Earlier Denny. Robson, and Irwin (179b) had shown similar effects of iron and aluminum. 

This observation emphasizes the point that in attempting to measure the solubility of silica in the form of coarse particles, where the surface area of the silica in the system is rather low, it may be very difficult to obtain an accurate measure of solubility unless traces of aluminum and other metals forming insoluble silicates are ridigly excluded from the system. 

The fact that the amorphous silica on the ocean floor, found in enormous deposits of skeletons of diatoms, does not dissolve in seawater, has been a mystery. Lewin (180) found that as soon as these organisms died, silica began to dissolve, but only slowly, and the concentration in solution reached only 30 ppm. Removal of organic matter had no effect, but treatment with hot concentrated nitric acid greatly increased the rate of dissolution, as did treatment with mixed oxalate and EDTA solutions at pH 6.8. This clearly indicated certain metal ions were retarding the dissolution of silica in tris buffer at pH 9.0. 

The nitric acid-cleaned silica, which was free from organic material as well as metal ions, was immersed in 4-10 mM solutions of selected metal salts at a series of pH values and washed, and the rate of dissolution was measured in a standard manner at pH 9.0. Metals with no effect at pH 2-9 were La, Mo. and Cr, and at pH 4-11 were Ca and Mg at pH 8, which is that of seawater. Al. Be, Fe. Ga, Gd. and Y all retarded dissolution. However. Al was unique in that when applied over the range from 5 to 9, it rendered the silica completely insoluble at pH 9. 

The amount of aluminum on the silica surface required to reduce the solubility of. silica has been measured by Her (181a). When only one aluminum atom was absorbed on the surface as an anionic aluminosilicate site per 2 nm, at which point 

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