Monoprotic Weak Acid, HA or Base, B

The protonation reactions for ionizable molecules have been defined in Section 3.1. When a solute molecule, HA (or B), is in equilibrium with its precipitated form, HA(s) (or B(s)), the process is denoted by the equilibrium expression 

By convention, [HA(s)] = [B(s)] = 1. Eqs. (6.1) represent the precipitation equilibria of the uncharged species, and are characterized by the intrinsic solubility equilibrium constant, Sq. The zero subscript denotes the zero charge of the precipitating species. In a saturated solution, the effective (total) solubility S, at a particular pH is defined as the sum of the concentrations of all the compound species dissolved in the aqueous solution  

In a saturated solution, the three relevant equilibria for the case of a diprotic ampholyte are Eqs. (3.3) and (3.4), plus 

Note that [XH(s)] by convention is defined as unity. For such a case, effective solubility is 

As a new rule of thumb [473], in 0.15 M NaCl (or KC1) solutions titrated with NaOH (or KOH), acids start to precipitate as salts above log (S/So) 4 and bases above log (S/.S o) 3. It is exactly analogous to the cliff 3-4 rule let us call the solubility equivalent the sdijf3-4 rule [473], Consider the case of the monopro-tic acid HA, which forms the sodium salt (in saline solutions) when the solubility product Ksp is exceeded. In additions to Eqs. (3.1) and (6.1), one needs to add the following reaction/equation to treat the case  

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