• Pour 10 mL of clarified sample into a 30-mL beaker.
• Neutralize the sample to pH 8.0 using 1 N sodium hydroxide and pH meter.
• Transfer the treated sample into a 25-mL volumetric flask. Bring to volume with deionized water, and mix well.
• If the juice has not been clarified, filter the solution with a 5 µm syringe filter or equivalent.
• Transfer a 10-mL aliquot of the sample into a beaker, place calibrated pH/reference electrodes and a stirbar into the solution, mix, and re-adjust the pH to 8.0, if necessary.
• Add 2.5 mL of the previously neutralized formaldehyde (pH 8.0) to the aliquot, mix, and titrate to pH 8.0 using 0.05 N sodium hydroxide.
• The concentration of assimilable nitrogen is calculated as follows:
  • mg N/L (NH₄⁺ + amino nitrogen) = (mL of NaOH titrated) x 0.05N x 25/10 x 1000/10 x 14

Notes

The full equation for calculating assimilable nitrogen is:

mg N/L = (mL of NaOH) x (0.05meq OH^-/mL) x (1mmol N/meq OH^-) x (25mL/10mL- dilution factor) x 1000/10 (to convert to liters) x 14 mg/mmol N

If a different concentration of base is used, the equation requires an additional term: (0.05/N of NaOH used). The dilution factor in the equation (25 mL/10 mL) is changed if one uses a sample volume other than 10 mL and dilution to 25 mL.

Supply list:

• Sodium hydroxide solution, 1N
• Sodium hydroxide solution, 0.05 N, standardized against potassium
• hydrogen phthalate or equivalent
• Formaldehyde, reagent grade, 37% (vol./vol. or 40% wt/vol.) neutralized to
• pH 8.0 with 1N sodium hydroxide
• pH meter sensitive to 0.05 pH
• Electrode (Orion, epoxy body, sure flow, semi micro)
• Calibration buffers for the pH meter
• 5 µm syringe filters, or equivalent
• 10 ml syringes, or equivalent
• 25 ml volumetric flasks
• 10 ml buret (+/- 0.05)
• 30 ml beakers
• Mini stir bars