Boil gently on a preheated hot plate for 30 to 40 min or until a final volume of 10 mL is reached. Organophosphorus compounds such as AMP may require as much as 1.5 to 2 h for complete digestion. Cool, dilute to 30 mL with distilled water add 0.05 mL(1 drop) phenolphthalein indicator solution, and neutralize to a faint pink color with NaOH.