Allow the sample to cool, add 5 mL of concentrated HNO , replace the cover, and reflux for 30 minutes. If brown fumes are generated,indicating oxidation of the sample by HNO , repeat this step (addition of 5 mL of conc. HNO ) over and over until no brown fumes are given off by the sample indicating the complete reaction with HNO . 3 Using a ribbed watch glass or vapor recovery system, either allow the solution to evaporate to approximately 5 mL without boiling or heat at 95EC ± 5EC without boiling for two hours