A microscale method for colorimetric determination of urea in soil extracts

The diacetyl monoxime colorimetric method of determining urea in soil extracts was modified for microplate format. A 100‐μL aliquot of extract was treated with color reagent in a disposable plastic microtiter plate (96 wells/plate), and color was developed by heating the plate in a low‐temperature oven at 87°C for 55 min. After cooling for 20 min at ambient temperature, absorbance measurements were simultaneously performed on all 96 wells using a microplate reader. The microscale method was faster and more convenient than the conventional method; moreover, the volume of waste was markedly reduced. Studies to compare the two methods showed very little difference in accuracy, precision, or sensitivity.