Solubility Product Constant and Its pH Dependence

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. 

When an alkaline oxide such as MgO is stirred in phosphoric acid, the pH of the solution rises slowly due to the neutralization of this acid. Initially, the phosphoric acid has pH 0, but initial dissolution of the oxide and reaction with phosphate anions precipitate phosphate salts. This neutralization of the acid raises the pH of the solution to 2. Even in this pH range, the acid dissolves sufficiently, and protons and H2PO4 anions are readily available to react with the ions produced by the dissolution of metal oxides. Subsequently, consolidation of the precipitate in the neutral solution leads to the formation of ceramics. 

Consider the dissolution of a metal M as given by reaction 5.3. The ionization constant K for this reaction is defined as 

As before, angle brackets indicate molar concentration of individual species. In Eq. 5.14, Kx is a quantitative measure of the amount of dissolution of a particular oxide and, hence, the extent of its ionization. This quantity determines whether a particular oxide satisfies the condition of slow dissolution and will form CBPC in a given pH range. We will explore K,. in detail using the thermodynamics of dissolution reactions in Chapter 6, but here we will study its pH dependence. 

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