A simple and fast field method was developed for the determination of leachable Cr(VI) in soil by ultrasonic extraction and colorimetry with 1,5-diphenylcarbonohydrazide. Extraction from 2.0 g of a sample with 20 mL of water was accomplished with 30 min under ultrasonication (46 kHz, 15 W). The extraction was repeatable with a relative standard deviation of 10%. The extraction characteristics were discussed based on elution kinetics. The water-leachable concentrations of Cr(VI) or the total Cr were comparable to those obtained by the shaking extraction (6 h) used in Notification 46 of Environmental Agency of Japan. A detection limit of 0.003 mg/L was obtained by a laboratory-made colorimeter with a RGB light-emitting diode. The proposed method is applicable for soil samples containing no Cr(VI)-reducing matters, e.g., humic substances.
