Crystallization and precipitation equilibrium

We will deliberate first on the role of pH, dissociation, etc., on various crystaUization/precipitation equilibria. Consider aqueous solutions of amino acids. We have seen in Section 5.2.3.1.2 that amphoteric molecules of amino acids can exist in three forms in solution a cationic form Am at low pH an anionic form Am at very high pH and at intermediate pH, the zwitterionic form. Am, is important The relative ratio of Am or Am to Am depends on the pH with respect to pKi or pK2- Only the Am form exists at the pi of the amino acid. On the other hand, it is the neutral form Am which forms the crystal (j = x)  

For amino acids there is a band of about 2-3 pH units around the p/ where the net charge is zero. This is identified as the isoelectric band a pH value beyond this band on either side increases the amino acid solubility. For example, from equation (5.2.118a), we find that as Ch+ , increases, CAm+w increases therefore the amino acid solubility increases since CAm+w increases. Zumstein and Rousseau (1989) have illustrated that, for the amino acid L-isoleucine, the species dominates above pH = 2 leading to the 

As the pH is decreased below 2 by the addition of HCI acid, more and more of the acid form of the amino acid is present in solution. However, there is a solubility limit of this form of amino acid which is subject to an equilibrium with the hydrochloric acid salt (CL N+H3RCOOH H20)j in the solid phase via 

If the complication of the salt solubility limit being exceeded is not present, then the total concentration of the amino acid in solution in terms of the concentration of the zwitterionic species CAm+, w be obtained from equations (5.2.116a, b) as 

As HCI is added, initially CAm+u will increase, but so does Car, w Correspondingly, at some point of acid addition, the 

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