A field method for quantifying ammonium picrate and picric acid in soil

A simple field method for the determination of ammonium picrate and picric acid in soil was developed. Picric acid is a strong acid with a pKa = 0.80, and is colorless when dissolved in an organic solvent, whereas its anion (picrate) is bright yellow. Picric acid and picrate ions were extracted from undried soil by shaking with acetone; any picric acid extracted was rapidly converted to picrate in the wet acetone. Picrate was extracted from the acetone soil extracts by passing the solutions through a solid-phase anion exchanger to remove interferences. Acidified acetone was used to convert the picrate to picric acid and elute it from the ion exchanger. The absorbance of the solution at 400 nm was measured; then the picric acid was converted to the colored picrate ion by diluting the eluent with water. Absorbance at 400 nm was measured again and the concentration of picrate was obtained from the difference in the absorbance measurements, corrected for dilution. The method detection limit is 1.3 μg g−1 of soil. Field-contaminated soils were assayed, and the results compared favorably to those from HPLC analyses in the range of 10–4400 μg g−1. The method is simple to use, can be implemented under field conditions, and complements on-site methods for TNT, RDX, and 2,4-DNT. © 1997 John Wiley & Sons, Inc. Field Analyt Chem Technol 1:165–170, 1997.