Analysis of NO3-N content in plant leaf sap

How will you separate the sap from the rest of the leaf? Will you use an extraction solution? Or? That will affect the colorimetric method. The Greiss reaction is quite well known for nitrate. You do though need to reduce the nitrate to nitrite to get the color reaction to work. That is usually done using cadmium reduction. You can find a method in Standard Methods for this. You may need to do some adjustments though, based upon your final sample matrix, if it is colored, etc. This is one method for the sample preparation portion, (may not apply): SAMPLE PREPARATION PROCEDURE 7.1.1. Weigh 0.10 g plant tissue into a sample container. Add 25 mL 2% acetic acid to the sample container. Shake vigorously for 15 minutes. Filter the sample through Whatman #2 filter paper. The analytical range for this method is 0.2 to 20 mg N/L as NO2+ NO3 in the extracts. It is an automated version (FIA) of the Greiss reaction with cadmium reduction.