Phosphate stabilization solubility products

The charged group introduced into products by the aldol donors (phosphate, carboxylate) facilitates product isolation and purification by salt precipitation and ion exchange techniques. Although many aldehydic substrates of interest for organic synthesis have low water solubility, at present only limited data is available on the stability of aldolases in organic cosolvents, thus in individual cases the optimal conditions must be chosen carefully. 

The stabilizing effect of buffers that have multiple charged species in solution should also be investigated to determine the potential reaction between excipients and API. For example, buffers that use carbonates, citrate, tartrate, and various phosphate salts may precipitate with calcium ions by forming sparingly soluble salts. However, this precipitation is dependent upon the solution pH. Because phosphate can exist in mono-, di-, and tribasic forms, each calcium salt has its own solubility product, and precipitation will only occur when one of the solubility product is exceeded. Calcium ions may also interact or chelate with various amino acids, and other excipients, which may also lower the effective concentration of calcium that is capable of interacting with phosphate ions. Finally, the activity of phosphate ions may be lowered due to interactions with other solution components. 

To gain an insight into the sulfide stabilization, examine the solubility product constants for the sulfides and phosphates of hazardous metals listed in Table 16.4. In this table, except for barium sulfide, other sulfides as well as phosphates have very high pK p, indicating that their aqueous solubility is almost negligible. In particular, the pA sp of HgS and Ag2S is very high, and these two sulfides are insoluble in water. Therefore, when a waste stream contains one of these two, sulfide pretreatment followed by phosphate ceramic formation is an ideal way to treat the waste stream. 

Eq. 10.34) is more rapid with hydrochloric acid than with sulfuric acid, and yields soluble products from both the calcium and phosphate. Thus, there is neither a need to heat the mixture to speed up the reaction, nor are there any crystal form problems requiring temperature stabilization. 

The reactions (5.3a) and (5.3b) lead to local ionic supersaturation that causes precipitation of calcium phosphate phases with low solubility product and high thermodynamic stability (Drevet and Benhayoune, 2012). The type of calcium phosphate precipitated is a function of the pH of the solution adjacent to the cathode. At a pH value below about 6.5 brushite (calcium diphosphate dihydrate) is stable according to... 

The chemistry of phosphate complexation in inorganic systems is important in agriculture and soil science and more recently in remediation technology because of the low solubility product of many of the metal phosphate salts. A large number of inorganic phosphate phases have been identified [1,2]. In the area of catalysis the aluminophosphates have been investigated both as catalysts and catalyst supports due to their surface acidity and thermal stability. Re-... 

Polyphosphates do not precipitate metal ions from solution, but hold them in solution as soluble complexes, provided the ratio of cations to phosphates does not exceed some value related to a metastable solubility product. It should be obvious that a true solubility product does not exist for the same reasons a true solubility does not exist. If polyphosphates become overwhelmed, a precipitate will form and condensed phosphates function as orthophosphates in removing metal ions from solution. Rather than attempting to include an in-depth study of complexing agents and stability constants, a general statement will be made and readers are referred to published literature for a more comprehensive view. Perhaps the most perplexing issue about complexing, and equilibrium constants used to study and represent it, is the number of different constants used to represent a system. 

The complexing ability of the phosphate ion on metal cations is much more pronounced than that of the sulfate. The complex stability constants and solubility products are much higher than with the sulfate [4-6], and phosphate complexation occurs over the entire accessible pH range. 

Forrester Environmental Services, Inc., has developed a group of technologies for the stabilization of wastes containing heavy metals, such as lead, cadmium, arsenic, mercury, copper, zinc, and antimony. These technologies have been used in both industrial pollution prevention and remediation applications. One version of the technology involves the use of water-soluble phosphates and various complexing agents to produce a less soluble lead waste. This process results in a leach-resistant lead product. 

Some hazardous metals such as chromium (Cr) and radioactive fission products such as technetium (Tc) exhibit exactly opposite solubility characteristics as compared to the metals discussed above. These metals in higher oxidation states, e.g., chromates (Cr ) and pertechnetate (Tc ), are more soluble than their counterparts, e.g., chromium and technetium oxide (Cr and Tc " "). Chromium is a hazardous metal and technetium ( Tc) is a radioactive isotope. As we shall see in Chapters 16 and 17, one way to reduce their dispersibility is to reduce their solubility in ground water and reduce them into their lower oxidation state, and then encapsulate them in the phosphate ceramic. Thus, the reduction approach is also useful in stabilization of hazardous metal oxides of high oxidation states. Because of these reasons, a good understanding of the reduction mechanism of oxides... 

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. 

Again, as in the case of hazardous contaminants discussed in Chapter 16, the solubility of a radioactive contaminant plays a major role in its stabilization in a phosphate matrix. Therefore, one needs to understand the aqueous behavior of a radioactive contaminant prior to selecting the acid-base reaction that will form the CBPC used for fabricating the waste form matrix. In this respect, actinides, fission products, and salts have unique solubility behavior. This behavior is discussed below. 

Typical suspension stabilizers for the production of EPS are water-soluble, surface-active macromolecules, such as poly (vinyl alcohol) (PVA), hydroxyethyl-cellulose (HEC) and polyvinylpyrrolidone (PVP), or natural products, such as gelatin [36-40], and insoluble inorganic powders, such as tricalcium phosphate (TCP), also called picketing stabilizer , mostly in combination with surfactants called extenders [33-35,44], or a combination of these [129]. The differences and specialties of these stabilizing mechanism are described briefly below ... 

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