Nitrogen determination in nutrient cycling studies: an improved technique for handling multiple samples.
One ml Kjeldahl digest containing 1-25µg N is pipetted into a specimen tube. A nylon fabric disc is placed in a depression inside the plastic cap and wetted with a drop of 1 N H2SO4 to absorb NH3 Small (0.1g) pellets of NaOH are dropped into the tube to release NH3, and the cap is immediately fixed in place. The tube is placed horizontally on rollers which turn it at 5 rpm. The narrow neck of the tube prevents digest from reaching the nylon disc. After 2 hr the plastic cap is put in a tube containing 10 ml diluted Nessler's reagent (4 ml/100 ml de-ionized water) in which the nylon disc is shaken. Fifteen minutes are allowed for colour development before the optical density is read at 410 mµ using a spectrophotometer fitted with a flow-through cell or with a violet filter in a colorimeter.