Sparsely soluble

The major types of cytoskeletal filaments are 7-nm-thick microfilaments. 25-nm-thick microtubules, and 10-nm-thick intermediate filaments (IPs). These are respectively composed of actin, tubulin, and a variety of interrelated sparsely soluble fibrous proteins termed intermediate filament proteins. In addition, thick myosin filaments are present in large numbers in skeletal and heart muscle cells and in small numbers in many other types of eukaryotic cells. 

The behavior of two liquids that are sparsely soluble in each other is a familiar one to any experimental chemist. If we shake ether and water in a separatory funnel and observe the system at equilibrium, we see that the liquids have settled in two layers. The reader is asked to identify which liquid is the bottom layer. If water and chloroform are shaken, the system again equilibrates... 

Nonaqueous capillary electrophoresis is a new technique that matured over the last couple of years to complement the electrophoretic techniques performed under aqueous conditions. Consequently, it extends the applicability of CE to those analytes that are insoluble or only sparsely soluble in water, including several classes of pharmaceutical compounds and long-chain fatty acids, vitamins, and surfactants, to name only a few. 

These electrodes are based on two equilibria the electrochemical equilibrium involving formation of the interfacial potential and the solubility equilibrium between the cation and its sparsely soluble salt. The most popular electrode of this type is the silver/silver chloride electrode. The electrochemical equilibrium is the same as for the Ag/Ag+ electrode described above (6.27) and the solubility equilibrium is... 

Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

Benzoic acid occurs naturally in many types of berries, plums, prunes, and some spices. As an additive, it is used as benzoic acid or as benzoate. The latter is used more often because benzoic acid is sparsely soluble in water (0.27 percent at 18°C) and sodium benzoate is more soluble (66.0 g/100 mL at 20°C). The undissociated form of benzoic acid is the most effective antimicrobial agent. With a pKa of 4.2, the optimum pH range is from 2.5 to 4.0. This makes it an effective antimicrobial agent in high-acid foods, fruit drinks, cider, carbonated beverages, and pickles. It is also used in margarines, salad dressings, soy sauce, and jams. 

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

To form a CBC, control over the dissolution of the bases is crucial. The bases that form acid-base cements are sparsely soluble, i.e., they dissolve slowly in a small fraction. On the other hand, acids are inherently soluble species. Typically, a solution of the acid is formed first, in which the bases dissolve slowly. The dissolved species then react to form the gel. When the gel crystallizes, it forms a solid in the form of a ceramic or a cement. Crystallization of these gels is inherently slow. Therefore, bases that dissolve too fast will rapidly saturate the solution with reaction products. Rapid formation of the reaction products will result in precipitates and will not form well ordered or partially ordered coherent structures. If, on the other hand, the bases dissolve too slowly, formation of the reaction products will be too slow and, hence, formation of the gel and its saturation in the solution will take a long time. Such a solution needs to be kept undismrbed for long periods to allow uninterrupted crystal growth. For this reason, the dissolution rate of the base is the controlling factor for formation of a coherent structure and a solid product. Bases should neither be highly soluble nor almost insoluble. Sparsely soluble bases appear to be ideal for forming the acid-base cements. 

Other silicophosphate cements that use cation-releasing silicates are based on wolla-stonite [33], and serpentinite [34,35]. Naturally occurring phosphate cements have also been known [36]. In these cements, silicates are sparsely soluble and release cations (Ca, and Mg ), which react with the phosphate anions to form hydrophosphates and eventually convert to phosphates. This process is similar to that involving zinc phosphate cements, in which hydrophosphates form first, then convert to phosphates during aging. 

These observations imply that, forming a phosphate ceramic requires either diluted phosphoric acid or a partially neutralized phosphate solution as a source of anions, and a sparsely soluble (slightly soluble) oxide or a mineral to provide cations. All ceramics are formed in an aqueous solution. In general, the following scheme seems to work best. 

Phosphoric acid may be diluted with water. This step provides the water fraction needed to form the ceramic. Monovalent alkali metal oxides, with their high aqueous solubility, may be used for partial neutralization of the acid, while sparsely soluble divalent oxides are good candidates for providing the cations. In particular, oxides of Mg, Ca, and Zn are preferred because they are inexpensive compared to similar oxides, and unlike oxides of Pb, Cr, Cd, Hg, and Ni, they are not environmentally hazardous (see Chapter 16). 

Aluminum oxide is the only trivalent oxide that has been used to form a ceramic some heat treatment is needed. Kingery claims to have observed a setting reaction between trivalent iron oxide and phosphoric acid, but this reaction may have been caused by traces of magnetite in the trivalent oxide. Pure trivalent iron oxide such as hematite (Fe203) does not react with phosphoric acid. Overall, trivalent metal oxides have a solubility that is only marginal and falls below that of even sparsely soluble divalent oxides, while the solubility of oxides of most quadrivalent metals (zirconium is an exception) is too low to form a ceramic. 

The various CBPC products discussed in the last chapter reveal that CBPC powder consists of one or more sparsely soluble oxides and an acid phosphate. When this mixture is stirred in water, the acid phosphate dissolves first and makes the solution acidic, in which the sparsely soluble alkaline oxides dissolve and an acid-base reaction is initiated. This reaction produces slurry that subsequently hardens and a ceramic hard product is formed. If the acid phosphate is phosphoric acid solution, the setting reaction is too rapid. Such a process becomes impractical for production of large ceramic objects because the rapid acid-base reaction is exothermic and that boils the reaction slurry. Therefore, less acidic acid phosphates (such as chhydrogen phosphates) are preferred for fabrication of practical ceramics. 

The literature review in Chapter 2 reveals that divalent metal oxides such as oxides of calcium, magnesium, and zinc (CaO, MgO, and ZnO) are the major candidates for forming phosphate ceramics. These oxides are sparsely soluble in acidic solution, and as we shall see in Chapter 4, they are the most suitable ones to form ceramics. In addition, following the methods discussed in subsequent chapters in this book, aluminum oxide (alumina, AI2O3) and iron oxide (Fe203), which are abundant in earth s crust have excellent potential to form low cost CBPCs. For this reason, we have provided relevant information on these oxides. Table 3.2 gives some details. 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

The solubility of such solids is only a fraction of that of the acid phosphates that we discussed in the last chapter. A sparsely soluble oxide (or its hydroxide) dissolves in acidic solution in two steps. The first step is ionization or dissociation. When stirred in water, the oxide decomposes into its cations and anions. This decomposition occurs because of collisions between the oxide molecules and the polar molecules of water. Second is a screening step in which the two charged ions resulting from this dissociation are kept separate by the water molecules. These steps are described in detail below. 

In forming CBPCs, this dissociation is essential. The cations formed by dissociation react with phosphate anions that are present in the aqueous solution and form phosphate salt molecules. These salt molecules connect to each other and form a network and consolidate into a crystalline phosphate ceramic. Thus, success in forming CBPCs lies mainly in successfully dissociating sparsely soluble oxides in acidic solutions and precipitating salt in crystalline form. We wiU discuss the fundamentals of this dissociation in the next several chapters and present methods of dissociating various oxides in phosphate solutions to form ceramics. 

In Chapter 4, the ionization constant (i.e., the reaction constant of dissolution) for weak acids and acid phosphates was defined. The concept of the ionization constant is very general and useful while discussing dissolution of sparsely soluble oxides in acid-base reactions. We assign the symbol K for this constant. 

Below we will discuss this constant when a sparsely soluble oxide is dissolved in a phosphoric acid solution. The same discussion may then be generalized to other phosphate solutions. 

As discussed in Chapter 3 (also see Ref. [23]), MgO and Mg(OH)2 are sparsely soluble and their dissolution reactions are given by... 

Formation of a ceramic with magnetite is possible because it is sparsely soluble. This may be seen by calculating the pi sp of magnetite using its Gibbs free energies and that of dissolution products (listed in Appendix B), and the aqueous solubility equation... 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

Note in Fig. 13.2 that the solubility of monocalcium silicate is higher than that of the corresponding aluminate at any pH > 3. In the acidic region that is of interest for forming CBPCs, both may be considered as sparsely soluble, and if they are reacted with a phosphate salt, ceramics may be formed. Thus, monocalcium silicate and aluminate are starter minerals to form calcium phosphate ceramics. 

The first component on the left-hand side of Eq. 13.10 is alkaline, and the second one is acidic. Thus, this reaction is an acid-base reaction that yields HAP at a near neutral pH in an aqueous environment, where both reacting components are sparsely soluble. 

To evaluate the leaching performance of the waste streams, we assume that soluble and sparsely soluble compounds will leach out and fail the TCLP and, hence, should be target contaminants for stabilization. These soluble or sparsely soluble components may directly be treated with phosphates and converted to their insoluble, nonleachable forms. The literature is full of studies on stabilization of such divalent hazardous metal contaminants (Pb, Cd, and Zn, in particular), where treatment with various phosphates has been elfective. These studies are summarized in Section 16.3. 

Hydroxides All are sparsely soluble, except Ba(OH)2 and AgOH, which are moderately soluble... 

Sulfates Sulfates of Cd, Ni, and Zn are soluble. Others are sparsely soluble. Hydrosulfates are more soluble than sulfates... 

Arsenates have very low solubility, but AS2O3 is sparsely soluble. Like HgO, its solubility in the entire acidic range is almost constant and relatively high. Because of its higher solubility, phosphate treatment should be able to stabilize this compound. However, because other arsenates are nearly insoluble, they can be encapsulated in the phosphate matrix and will not be available for leaching. 

As evident from Fig. 16.1, the dissolution behavior of Cr203 is similar to that of other divalent oxides. Its solubility is high in the acidic region, drops almost linearly as pH increases, has a minimum at almost pH = 7, and then increases with pH. Its overall behavior is that of a sparsely soluble oxide. As a result, Cr203 will react with acid phosphates and form insoluble hydrophosphates or phosphates. 

Arsenic has two oxidation states, 3 - - and 5 - -. Figure 16.1 shows the dissolution characteristics of As for its 3 - - state. Compared to other sparsely soluble oxides, this oxide is soluble, and up to pH 9 its solubility is constant and then increases with pH. Thus, As will dissolve easily in the CBPC solution and can be converted to its phosphate form. Unlike the chromates, however, arsenates are insoluble in water but soluble or sparingly... 

As discussed in Chapter 16, chemical stabilization is a result of conversion of contaminants in a radioactive waste into their insoluble phosphate forms. This conversion is solely dependent on the dissolution kinetics of these components. In general, if these components are in a soluble or even in a sparsely soluble form, they will dissolve in the initially acidic CBPC slurry and react with the phosphate anions. The resultant product will be an insoluble phosphate that will not leach into the groundwater. On the other hand, if a certain radioactive component is not soluble in the acid slurry, it will not be soluble in more neutral groundwater, because the solubility of such components is lower in neutral than in acidic solutions. Such a component will be simply microencapsulated in the phosphate matrix of the CBPC. Thus, the solubility of hazardous and radioactive components is key to chemical immobilization. 

Other isotopes (I, Co, etc.) or their oxides and soluble salts can also be present. Stabilization of such contaminants is handled in a manner similar to the stabilization of hazardous metals, as discussed in Chapter 16, because most of them are sparsely soluble compounds. 

The+utility of carboxylic acids (formic, acetic), which also form Pu complexes sparsely soluble in TBP solutions, was investigated by Germain (22) and McKay (23). The results were not encouraging. 

Prussian blue analogs are here defined as polynuclear transition metal cyanides of the composition M [M (CN)6]i xHzO a retallizing with a cubic unit cell. They are easily obtained as sparsely soluble precipitates by mixing solutions of a cyano complex M (CN)e with an appropriate salt of The compounds prepared by using the hexacyanometalate in the form of the most common potassium salt invariably contain different amounts of potassium, which in some cases can be exchanged by cesium... 

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