A field determination method was developed for water-leachable arsenic in soil by ultrasonic extraction and Molybdenum Blue colorimetry. The extraction of arsenic from 2.0 g of soil sample with 20 ml of water was performed by ultrasonication (46 kHz, 18 W) for 30 min. The extraction was repeatable within a relative standard deviation of 10%. The water-leachable concentrations of arsenic were comparable to those obtained by the shaking extraction (6 h) used in Notification 46 of Environmental Agency of Japan. Absorbance of molybdenum blue was measured at 630 nm with a band width of 25 nm by a laboratory-made portable colorimeter with an RGB light-emitting diode. Detection limits with and without the enrichment by MIBK extraction was 0.006 mg/l and 0.012 mg/l, respectively. The proposed method was applicable for soil samples containing 0.01 to 25 mg/l of arsenic.
