The publication can be downloaded from Plant Analysis Reference Procedures for the Southern Region of the United States.
3.1 Concentrated sulfuric acid, 18 M.
3.2 Standardized hydrochloric acid, (0.01 M) or sulfuric acid, 0.005 M.
3.3 Salt/Catalyst mixture – Mix 200 g of K2SO4, 20 g of cupric sulfate pentahydrate (CuSO4 @
5H2O) and 2 g of Se. The K2SO4 and/or CuSO4 @ 5H2O may need to be ground using a mortar
and pestle if the crystals are too large. It is important that the salt and catalyst be well mixed.
3.4 Sodium hydroxide solution, 10 M – Add 1600 g of NaOH to 2 L of carbon dioxide (CO2)-free
deionized water in a 4-L narrow neck polypropylene container. Stopper and cool the solution
before bringing it to a volume of 4 L. Protect the solution from contamination by
atmospheric CO2 with an ascarite guard tube at the air inlet.
3.5 Mixed indicator – Dissolve 0.01 g of bromocresol green and 0.07 g of methyl red in 100 mL
of ethyl alcohol (90%).
3.6 Boric acid indicator solution, 4% – Dissolve 40 g of H3BO3 in 800 mL of boiling deionized
water in a 2-L volumetric flask. Cool the solution and dilute to 1900 mL. Add 40 mL of the
mixed indicator solution. Carefully adjust the pH of this mixture with 0.1 M NaOH and 0.05
M H2SO4 until the solution turns a reddish purple color (pH 5.0) and then bring the solution to
2 L volume.
4.1 The Kjeldahl method for determination of total N has been the subject of many modifications
according to the type of material to be analyzed. For instance, some plant materials do not
require a digestion temperature as high as that recommended in paragraph 4.4 for complete
digestion. The user may desire to alter digestion times, temperatures, or reagent proportions
for a specific application. Useful references for this purpose are Nelson and Sommers (1973,
1980) and Schuman et al. (1973).
4.2 Place sample (weight depends on N content) or standard in Kjeldahl digestion tube and add
1.1 g of salt/catalyst mixture.
4.3 Digest blanks containing only reagents with each set of samples.
4.4 Add 3 mL of concentrated H2SO4. Slowly heat to 200oC. Once the frothing has subsided,
bring the temperature up to 350 to 375oC and heat until the digest clears. Digest at 350 to
375oC for an additional 35 minutes to 1 hour past clearing.
4.5 Cool the digest and add 20 mL of deionized water. If solidification has occurred within the
digest, it is important to mix the tube contents using a vortex mixer to dissolve the solid.
4.6 Add 5 mL of H3BO3 indicator solution to a 50-mL flask and place the flask under the
condenser with the condenser tube below the surface of the indicator solution.
4.7 Add 20 mL of 10 M NaOH to the digested sample. Immediately transfer the tube to the
Kjeldahl distillation apparatus and begin distillation. Collect distillate until the level in the
H3BO3 flask has reached approximately 35 mL (usually 12 minutes).