Sparsely soluble bases

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. 

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 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. 

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. 

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. 

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. 

Metal salts, such as sodium and potassium, of adipic, and terephthallc, etc., acids are largely insoluble in such organic liquids as chloroform and carbon tetrachloride. Our initial attempts were directed at neutralizing diacids employing triethylamine as the neutralizing base. The resulting ditriethylammonium salt yielded a sparsely soluble product. For instance ditriethylammonium terephthalate is partially soluble in chloroform... 

It is apparent from the preceding discussion that there are many choices over solubility measures on which to base a computational model, and data sets and models are sparse beyond solubility in water or aqueous buffered solutions. 

Solubility data for these gases are sparse. The positions of the R-lines for temperatures in the range 0-25 C would require a base scale corresponding to JVa from 0 to 1 to plot them on a common diagram. In Fig. 140 I have given representative plots and indicate the positions of certain observed Na values on the horizontal at 760 mm Hg. Even when the vapor pressure data are not available, the... 

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