Standard preparation 3.3.1. Undissolved residue is discarded. Concentrate methylene chloride solution to 5 to 10 mL. Add a small amount n-heptane to start precipitation. The MDI precipitate is filtered and dried under vacuum. The precipitate formed is filtered and redissolved in a minimal volume of methylene chloride.
Hexane is added to the solution to initiate precipitation.
The precipitated MDI urea derivative is filtered and washed with methylene chloride. This derivative can be recrystallized by redissolving in methylene chloride and reprecipitating with n-heptane. Yield is approximately 1 g. 3.3.3. A stock standard solution is prepared by dissolving 2,4-TDI and MDI urea derivatives in acetonitrile for reverse phase HPLC analysis, and in methylene chloride for normal phase HPLC analysis. To express the derivative as free diisocyanate, the amount of 2,4-TDI urea weighed is multiplied by the correction factor 0.310.