Possible impurity is N,N'-di-tert-butyl urea which is quite insoluble in H2O. Recrystallise it from hot H2O, filter off insoluble material, and cool to 0o to -5o with stirring. Dry in vacuum at room temperature over KOH or H2SO4. If dried at higher temperatures, it sublimes slowly. It can be recrystallised from EtOH as long white needles or from 95% aqueous EtOH as plates. During melting point determination the bath temperature has to be raised rapidly as the urea sublimes slowly above 100o at 760mm. [Smith & Emerson Org Synth Coll Vol III 151 1955, Beilstein 4 IV 665.]
