ASTM D3228 Test Method for Total Nitrogen In Lubricating Oils and Fuel Oils By Modified Kjeldahl Method
8. Procedure
8.1 Transfer 1.0 to 1.5 g of sample, weighed to the nearest 0.1 mg, into a Kjeldahl flask. Avoid contact of the sample with the neck of the Kjeldahl flask. Add the catalyst reagent mixture to the Kjeldahl flask. Add two or three beads to prevent bumping.
8.2 Wash down the neck of the Kjeldahl flask with 20 mL of H2SO4(rel dens 1.84). Swirl the contents of the Kjeldahl flask to facilitate the mixing of the sample, catalyst reagent, and H2SO4.
8.3 Warm the contents of the Kjeldahl flask on the digestion rack and repeat the swirling. Apply low heat until the frothing has stopped. Samples that do not froth or char shall be subjected to a 20-min low-heating period. Careful periodic swirling of the solution in the Kjeldahl flask shall also be made. Gradually apply intermediate heat to raise the temperature of the solution to boiling.
8.4 Maintain a minimum volume of 15 mL of liquid in the Kjeldahl flask during the digestion period. Add volumes of 5 to 15 mL of H2SO4(rel dens 1.84) when the volume does not conform to this condition. Use the H2SO4 to wash down the neck of the Kjeldahl flask after the contents have been allowed to cool sufficiently so that sulfur trioxide (SO3) fumes have subsided. The volume of H2SO4(rel dens 1.84) added will depend upon the carbonaceous material in the Kjeldahl flask. After all of the carbonaceous material has been digested and the solution has cleared, continue the digestion for two more hours at rapid rate of boiling. The total volume of liquid remaining in the Kjeldahl flask after digestion approximates the volume in the Kjeldahl flask for the blank.
NOTE 6 - For some samples, a two hour digestion period may be unnecessary, if the solution has completely cleared.
8.5 Turn off the heat, but allow the Kjeldahl flask to remain in the fume duct or hood until the evolution of SO3 fumes has subsided. Remove the Kjeldahl flask from the rack and cool to approximately room temperature.
8.6 Place a 300-mL receiving flask containing 25 mL of H3BO3 solution and 5 drops of methyl purple indicator solution under the condenser with the delivery tube tip extending to the bottom of the receiving flask.
8.7 Measure approximately 275 mL of water and add a portion of this water to the Kjeldahl flask and swirl the contents until the salt cake has dissolved (Note 4). Add the remainder of the water and cool the contents of the Kjeldahl flask to room temperature.
NOTE 7 - It can be necessary to warm the contents in the Kjeldahl flask to facilitate solution of the salt cake.
8.8 Add 25 mL of Na2S solution to the cooled contents of the Kjeldahl flask, to precipitate the mercury, and swirl to mix.
NOTE 8 - Precaution: In addition to other precautions, when the Na2S solution is added to the cooled digestion flask, considerable hydrogen sulfide is evolved. Therefore, conduct 8.8 and 8.9 in a hood with a suitable draft.
NOTE 9 - Precaution - In addition to other precautions, care must be exercised in the disposal of the mercuric sulfide. Laboratories processing large volumes of Kjeldahl nitrogen determinations should consider the use of a recovery trap for mercury.
8.9 Place the Kjeldahl flask in a slurry of ice and water. Cool the contents in the Kjeldahl flask to approximately 40°F (4.5°C). Slowly add 75 mL of NaOH solution (1000 g/L) down the inclined neck of the Kjeldahl flask, without agitation, to form two layers.
8.9.1 Carefully remove the Kjeldahl flask from the ice bath so that mixing of the layers does not occur. Carefully place the Kjeldahl flask on the Kjeldahl distillation rack.
8.9.2 Immediately connect the Kjeldahl flask to the distillation apparatus and mix the contents of the Kjeldahl flask thoroughly by swirling. The digestion flask must be connected to the distillation apparatus immediately after the alkali has been added and layered, but before swirling to mix the acid and alkali. When any mixing is permitted to occur before the digestion flask is connected, the heat generated can be sufficient to release some of the ammonia which can be lost. This loss results in low recovery of ammonia, and thus low values for the nitrogen content of the sample.
8.10 Promptly apply full heat to the digestion flask. Reduce the heat just before the solution begins to boil and maintain at low boiling for 5 min. Heat must be applied promptly to prevent sucking of the H3BO3 solution into the condenser as the digestion solution cools. The initial distillation rate must not be too rapid because most of the ammonia is distilled during the first few minutes, and if too large an amount is present it can not all be absorbed in the H3BO3 solution. Increase the heat to rapid boiling, until the volume in the receiving flask reaches a volume of approximately 130 mL.
8.11 Lower the receiving flask to expose the condenser delivery tube tip. Rinse the tip with water. After approximately 1 min of additional distillation, turn off the heat and allow the condenser to drain.
NOTE 10 - The total volume in the receiving flask is approximately 150 mL. For convenience the receiving flask can be marked at the 130 and 150-mL volume points.
8.12 Titrate the contents in the receiving flask with standard H2SO4(0.005 M) to an end point where the gray color of the solution just disappears and only the purple color remains. If the titration exceeds 50 mL, continue the titration with standard H2SO4(0.05 M). Read the volume of the standard acid to the nearest 0.05 mL.
8.13 Determine a blank with every set of samples, identical in every way with the regular determinations, except 1.0 g of sucrose is added in place of the sample. The initial volume of 20 mL of H2SO 4 (rel dens 1.84) is all that is used for the digestion of the sucrose.
9. Calculation
9.1 Calculate the nitrogen content of the sample as follows:
Nitrogen content, mass % = [(A - B) x M1 + C x M2] x 2 x 0.01401 x 100/W
where:
A = millilitres of 0.005 M H2SO4 required to titrate the sample,
B = millilitres of 0.005 M H2SO4 required to titrate the blank,
C = millilitres of 0.05 M H2SO4 required to titrate the sample,
0.01 = normality of 0.005 M H2SO4,
0.1 = normality of 0.05 M H2SO 4,
0.01401 = equivalent weight, g/mL,
100 = factor to convert to percent, and
W = weight of sample used, g.
