Reduced Volume Formol Titration for Fermentable Nitrogen
- Pour 10 mL of clarified sample into a 30-mL beaker.
- Neutralize the sample to pH 8.0 using 1 N sodium hydroxide and
- 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
- The concentration of assimilable nitrogen is calculated as
- mg N/L (NH4+ + amino nitrogen) = (mL of
NaOH titrated) x 0.05N x 25/10 x 1000/10 x 14
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.
- 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