Screen crystallization solvents for H2O, as a small amount of H2O in solvents or reagents can affect not only the formation of hydrates but also polymorph formation.  For an example of how small amounts of water in a starting material affected polymorph formation, see Gross, T. D.; Schaab, K.; Ouellette, M.; Zook, S.; Reddy, J. P.; Shurtleff, A.; Sacaan, A. I.; Alebic-Kolbah, T.; Bozigian, H. Org. Process Res. Dev. 2007, 11, 365.
