We present and evaluate several models for the correlation and prediction of crystal solubility and purity when solid solution formation is the dominant mechanism that leads to impure crystals. The models include our previously published relationship based on measurements of the purity of amino acid crystals obtained from aqueous solutions of two amino acids, and a new thermodynamic model based on dilute solution theory. An interesting result of our analysis is that the crystal phase exhibits significant nonidealities even when the dominant mechanism that leads to impure crystals is lattice substitution by isomorphic or near-isomorphic compounds. Potential causes for these nonidealities are explored.

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