ASTM D3228-08(2014)
(Test Method)Standard Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method
Standard Test Method for Total Nitrogen in Lubricating Oils and Fuel Oils by Modified Kjeldahl Method
SIGNIFICANCE AND USE
4.1 The concentration of nitrogen is a measure of the presence of nitrogen-containing additives. Knowledge of its concentration can be used to predict performance.
SCOPE
1.1 This test method covers the determination of nitrogen in lubricating oils when present in the concentration from 0.03 to 0.10 mass %, and for the determination of nitrogen in fuel oils when present in the concentration from 0.015 to 2.0 mass %. This test method is also applicable to the analysis of additive concentrates and additive packages.
Note 1: This test method may not be applicable to certain materials containing N–O or N–N linkage. However, the samples used in the cooperative program to establish the precision of the test method were compounded with currently available ashless additives containing nitrogen. Complete recovery of the nitrogen present in these additives was obtained.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific warning statements, see 6.6, 6.9, and 8.8.
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Designation:D3228 −08 (Reapproved 2014)
Standard Test Method for
Total Nitrogen in Lubricating Oils and Fuel Oils by Modified
Kjeldahl Method
This standard is issued under the fixed designation D3228; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope D6299 Practice for Applying Statistical Quality Assurance
and Control Charting Techniques to Evaluate Analytical
1.1 This test method covers the determination of nitrogen in
Measurement System Performance
lubricating oils when present in the concentration from 0.03 to
E200 Practice for Preparation, Standardization, and Storage
0.10 mass %, and for the determination of nitrogen in fuel oils
of Standard and Reagent Solutions for ChemicalAnalysis
when present in the concentration from 0.015 to 2.0 mass %.
This test method is also applicable to the analysis of additive
3. Summary of Test Method
concentrates and additive packages.
3.1 The sample is digested in a mixture of concentrated
NOTE 1—This test method may not be applicable to certain materials
sulfuric acid, potassium sulfate, mercuric oxide, and copper
containing N–O or N–N linkage. However, the samples used in the
sulfate. After digestion, sodium sulfide is added to precipitate
cooperative program to establish the precision of the test method were
the mercury, and the mixture is made alkaline with caustic.
compounded with currently available ashless additives containing nitro-
Nitrogen, now in the form of ammonia, is distilled into a boric
gen. Complete recovery of the nitrogen present in these additives was
obtained. acid solution. The ammonia is titrated with standard sulfuric
acid using methyl purple as an indicator.
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
4. Significance and Use
standard.
4.1 The concentration of nitrogen is a measure of the
1.3 This standard does not purport to address all of the
presence of nitrogen-containing additives. Knowledge of its
safety concerns, if any, associated with its use. It is the
concentration can be used to predict performance.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
5. Apparatus
bility of regulatory limitations prior to use. For specific
5.1 Buret,50-mL,graduatedin0.1-mLsubdivisions,onefor
warning statements, see 6.6, 6.9, and 8.8.
each titrant. Other size burettes may also be used.
2. Referenced Documents 5.2 Flask, Erlenmeyer, 300-mL. Other sizes are also accept-
2 able.
2.1 ASTM Standards:
D1193 Specification for Reagent Water 5.3 Heater, electrical or gas.
D4057 Practice for Manual Sampling of Petroleum and
5.4 Kjeldahl Distillation Apparatus.
Petroleum Products
NOTE2—CommerciallyavailablesemiautomaticKjeldahlapparatusare
acceptable. In such cases manufacturer prescribed sizes of burettes and
flasks may be used.
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
5.5 Kjeldahl Flask, at least 500-mL volume.
Subcommittee D02.03 on Elemental Analysis.
CurrenteditionapprovedMay1,2014.PublishedJuly2014.Originallyapproved
6. Reagents
in 1973. Last previous edition approved in 2008 as D3228 – 08. DOI: 10.1520/
D3228-08R14.
6.1 Purity of Reagents—Reagent grade chemicals shall be
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
used in all tests. Unless otherwise indicated, it is intended that
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
all reagents shall conform to the specifications of the Commit-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. tee onAnalytical Reagents of theAmerican Chemical Society,
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3228−08 (2014)
where such specifications are available. Other grades may be 6.12 Quality Control (QC) Sample, preferably are portions
used, provided it is first ascertained that the reagent is of of one or more liquid petroleum materials that are stable and
sufficiently high purity to permit its use without lessening the representative of the samples of interest. These QC samples
accuracy of the determination. can be used to check the validity of the testing process as
described in Section 10.
6.2 Purity of Water—Unless otherwise indicated, references
to water shall be understood to mean reagent water as defined
7. Sampling
by Types II and III of Specification D1193.
7.1 Take the sample in accordance with the instructions in
6.3 Boric Acid Solution (40 g/L)—Dissolve 40 g of boric
Practice D4057.
acid (H BO ) in 1 L of boiling water.
3 3
7.2 Ensure that the sample is thoroughly representative of
6.4 Catalyst Reagent —For each test carefully weigh and
thematerialtobetestedandthattheportionofthesampleused
mix 9.9 g of potassium sulfate (K SO ), 0.41 g of mercuric
2 4
for test is thoroughly representative of the whole sample.
oxide (HgO), and 0.08 g of copper sulfate (CuSO ).
6.5 Methyl Purple Indicator Solution —Aqueous solution
8. Procedure
containing approximately 0.1 % active constituent (not methyl
8.1 Transfer 1.0 to 1.5 g of sample, weighed to the nearest
violet). Other appropriate indicator solutions may also be used.
0.1 mg, into a Kjeldahl flask.Avoid contact of the sample with
6.6 Sodium Hydroxide Solution (1000 g/L)—Dissolve
the neck of the Kjeldahl flask.Add the catalyst reagent mixture
1000 g of sodium hydroxide (NaOH) in 1 L of water.
to the Kjeldahl flask. Add two or three beads to prevent
(Warning—Causes burns. Poison.)
bumping.
6.7 Sodium Sulfide Solution (40 g/L)—Dissolve 40 g of
8.2 WashdowntheneckoftheKjeldahlflaskwith20mLof
sodium sulfide (Na S) in warm water 194°F (90°C); cool and
H SO (rel dens 1.84). Swirl the contents of the Kjeldahl flask
2 4
dilute to 1 L.
to facilitate the mixing of the sample, catalyst reagent, and
H SO .
2 4
6.8 Sucrose (NIST)—Primary standard grade.
8.3 Warm the contents of the Kjeldahl flask on the digestion
6.9 Sulfuric Acid (rel dens 1.84)—Concentrated sulfuric
rack and repeat the swirling.Apply low heat until the frothing
acid (H SO ). (Warning—Causes severe burns. Strong oxi-
2 4
has stopped. Samples that do not froth or char shall be
dizer.)
subjected to a 20-min low-heating period. Careful periodic
6.10 Sulfuric Acid, Standard(0.05 M)—Slowlyadd3mLof
swirlingofthesolutionintheKjeldahlflaskshallalsobemade.
concentrated sulfuric acid (H SO , rel dens 1.84) to 500 mLof
2 4 Gradually apply intermediate heat to raise the temperature of
water in a suitable size beaker. Mix the acid and water; allow
the solution to boiling.
it to cool and transfer to a 1-L volumetric flask. Dilute to the
8.4 Maintain a minimum volume of 15 mL of liquid in the
mark with water; mix well. Standardize sulfuric acid to the
Kjeldahlflaskduringthedigestionperiod.Addvolumesof5to
nearest 0.0005 mol/L against 0.1 mol/L NaOH solution using
15 mL of H SO (rel dens 1.84) when the volume does not
2 4
phenolphthalein indicator. Standardize the NaOH solution
conform to this condition. Use the H SO to wash down the
2 4
against primary standard grade potassium hydrogen phthalate
neck of the Kjeldahl flask after the contents have been allowed
(HOOCC H COOK). Use the procedure outlined in Sections
6 4
to cool sufficiently so that sulfur trioxide (SO ) fumes have
14 to 19 of Practice E200.
subsided. The volume of H SO (rel dens 1.84) added will
2 4
NOTE 3—Commercially available pre-standardized H SO and NaOH
depend upon the carbonaceous material in the Kjeldahl flask.
2 4
solutions may be used.
After all of the carbonaceous material has been digested and
the solution has cleared, continue the digestion for two more
6.11 Sulfuric Acid (0.005 M)—Prepare by tenfold dilution
hours at rapid rate of boiling. The total volume of liquid
of the standard 0.05 M sulfuric acid prepared and standardized
in 6.10. remaining in the Kjeldahl flask after digestion approximates
the volume in the Kjeldahl flask for the blank.
NOTE 4—For some samples, a two hour digestion period may be
unnecessary, if the solution has completely cleared.
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, DC. For Suggestions on the testing of reagents not
8.5 Turn off the heat, but allow the Kjeldahl flask to remain
listed by the American Chemical Society, see Annual Standards for Laboratory
in the fume duct or hood until the evolution of SO fumes has
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia 3
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
subsided. Remove the Kjeldahl flask from the rack and cool to
MD.
approximately room temperature.
The sole source of supply of commercially prepared catalyst reagent mixture,
brandnameKel-Pak#1,knowntothecommitteeatthistimeisMathesonScientific,
8.6 Place a 300-mL receiving flask containing 25 mL of
1850 Greenleaf Ave., Elk Grove Village, IL 60007. If you are aware of alternative
H BO solutionand5dropsofmethylpurpleindicatorsolution
3 3
suppliers, please provide this information to ASTM International Headquarters.
under the condenser with the delivery tube tip extending to the
Your comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend. bottom of the receiving flask.
Fleisher Methyl Purple Indicator, U.S. Patent No. 241669, may be obtained
8.7 Measure approximately 275 mL of water and add a
from Harry Fleisher Chemical Co., Benjamin Franklin Station, Washington, DC
20004, or from any chemical supply company handling Fleisher Methyl Purple. portionofthiswatertotheKjeldahlflaskandswirlthecontents
D3228−08 (2014)
until the salt cake has dissolved (Note 2).Add the remainder of thetitrationexceeds50mL,continuethetitrationwithstandard
the water and cool the contents of the Kjeldahl flask to room H SO (0.05 M). Read the volume of the standard acid to the
2 4
temperature. nearest 0.05 mL.
NOTE 8—Commercially available automated colorimetric titrators may
NOTE 5—It can be necessary to warm the contents in the Kjeldahl flask
be used instead of the manual titration described in 8.12.
to facilitate solution of the salt cake.
8.13 Determine a blank with every set of samples, identical
8.8 Add 25 mL of Na S solution to the cooled contents of
in every way with the regular determinations, except 1.0 g of
the Kjeldahl flask, to precipitate the mercury, and swirl to mix.
sucrose is added in place of the sample. The initial volume of
(Warning—In addition to other precautions, when the Na S
20 mL of H SO (rel dens 1.84) is all that is used for the
2 4
solution is added to the cooled digestion flask, considerable
digestion of the sucrose.
hydrogen sulfide is evolved. Therefore, conduct 8.8 and 8.9 in
a hood with a suitable draft.) (Warning —In addition to other
9. Calculation
precautions, care must be exercised in the disposal of the
9.1 Calculate the nitrogen content of the sample as follows:
mercuric sulfide. Laboratories processing large volumes of
Nitrogen content, mass% 5 @ A 2 B 3 M11C 3 M2# 32 30.01401
~ !
Kjeldahl nitrogen determinations should consider the use of a
recovery trap for mercury.)
3100/W (1)
8.9 PlacetheKjeldahlflaskinaslurryoficeandwater.Cool
where:
the contents in the Kjeldahl flask to approximately 40°F
A = millilitres of 0.005 M H SO required to titrate
2 4
(4.5°C).Slowlyadd75mLofNaOHsolution(1000g/L)down
the sample,
the inclined neck of the Kjeldahl flask, without agitation, to
B = millilitres of 0.005 M H SO required to titrate
2 4
form two layers.
the blank,
8.9.1 Carefully remove the Kjeldahl flask from the ice bath
C = millilitres of 0.05 M H SO required to titrate the
2 4
so that mixing of the layers does not occur. Carefully place the
sample,
Kjeldahl flask on the Kjeldahl distillation rack. M1 = 0.005 (molarity of 0.005 M H SO ),
2 4
M2 = 0.05 (molarity of 0.05 M H SO ),
8.9.2 Immediately connect the Kjeldahl flask to the distilla-
2 4
2 = number of equivalents of H SO ,
tion apparatus and mix the contents of the Kjeldahl flask 2 4
0.01401 = equivalent weight, g/mL,
thoroughly by swirling. The digestion flask must be connected
100 = factor to convert to percent, and
to the distillation apparatus immediately after the alkali has
W = weight of sample used, g.
beenaddedandlayered,butbeforeswirlingtomixtheacidand
9.2 Alternatively, calculate the nitrogen content of the
alkali. When any mixing is permitted to occur before the
sample as follows:
digestion flask is connected, the heat generated can be suffi-
cient to release some of the ammonia which can be lost. This
Nitrogen content, mass% 5 @ A 2 B 30.011C 30.1# 30.01401
~ !
loss results in low recovery of ammonia, and thus low values
3100/W (2)
for the nitrogen content of the sample.
where:
8.10 Promptly apply full heat to the digestion flask. Reduce
A = millilitres of 0.005 M H SO required to titrate
2 4
the heat just before the solution begins to boil and maintain at
the sample,
low boiling for 5 min. Heat must be applied promptly to
B = millilitres of 0.005 M H SO required to titrate
2 4
prevent sucking of the H BO solution into the condenser as
3 3
the blank,
the digestion solution cools. The initial distillation rate must
C = millilitres of 0.05 M H SO required to titrate the
2 4
not be too rapid because most of the ammonia is distilled
sample,
during the first few minutes, and if too large an amount is
0.01 = normality of 0.005 M H SO ,
2 4
present it can not all be absorbed in the H BO solution.
3 3
0.01401 = equivalent weight, g/mL,
Increase the heat to rapid boiling, until the volume in the
100 = factor to convert to percent, and
receiving flask reaches a volume of approximately 130 mL.
W = weight of sample used, g.
8.11 Lower the receiving flask to expose the condenser
10. Quality Control
delivery tube tip. Rinse the tip with water.After approximately
10.1 Confirm the performance of the instrument or the test
1 min of additional distillation, turn off the heat and allow the
procedure by analyzing a quality control (QC) sample (6.12).
condenser to drain.
10.1.1 When QC/Quality Assurance (QA) protocols are
NOTE 6—The total volume in the receiving flask is approximately 150
already established i
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D3228 − 08 D3228 − 08 (Reapproved 2014)
Standard Test Method for
Total Nitrogen in Lubricating Oils and Fuel Oils by Modified
Kjeldahl Method
This standard is issued under the fixed designation D3228; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*Scope
1.1 This test method covers the determination of nitrogen in lubricating oils when present in the concentration from 0.03 to 0.10
mass %, and for the determination of nitrogen in fuel oils when present in the concentration from 0.015 to 2.0 mass %. This test
method is also applicable to the analysis of additive concentrates and additive packages.
NOTE 1—This test method may not be applicable to certain materials containing N–O or N–N linkage. However, the samples used in the cooperative
program to establish the precision of the test method were compounded with currently available ashless additives containing nitrogen. Complete recovery
of the nitrogen present in these additives was obtained.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use. For specific warning statements, see 6.6, 6.9, and 8.8.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
E200 Practice for Preparation, Standardization, and Storage of Standard and Reagent Solutions for Chemical Analysis
3. Summary of Test Method
3.1 The sample is digested in a mixture of concentrated sulfuric acid, potassium sulfate, mercuric oxide, and copper sulfate.
After digestion, sodium sulfide is added to precipitate the mercury, and the mixture is made alkaline with caustic. Nitrogen, now
in the form of ammonia, is distilled into a boric acid solution. The ammonia is titrated with standard sulfuric acid using methyl
purple as an indicator.
4. Significance and Use
4.1 The concentration of nitrogen is a measure of the presence of nitrogen-containing additives. Knowledge of its concentration
can be used to predict performance.
5. Apparatus
5.1 Buret, 50-mL, graduated in 0.1-mL subdivisions, one for each titrant. Other size burettes may also be used.
5.2 Flask, Erlenmeyer, 300-mL. Other sizes are also acceptable.
5.3 Heater, electrical or gas.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.03 on Elemental Analysis.
Current edition approved Dec. 1, 2008May 1, 2014. Published December 2008July 2014. Originally approved in 1973. Last previous edition approved in 20052008 as
D3228D3228 – 08.–05. DOI: 10.1520/D3228-08.10.1520/D3228-08R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3228 − 08 (2014)
5.4 Kjeldahl Distillation Apparatus . Apparatus.
NOTE 2—Commercially available semiautomatic Kjeldahl apparatus are acceptable. In such cases manufacturer prescribed sizes of burettes and flasks
may be used.
5.5 Kjeldahl Flask, at least 500-mL volume.
6. Reagents
6.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where
such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high
purity to permit its use without lessening the accuracy of the determination.
6.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by
Types II and III of Specification D1193.
6.3 Boric Acid Solution (40 g/L)—Dissolve 40 g of boric acid (H BO ) in 1 L of boiling water.
3 3
6.4 Catalyst Reagent —For each test carefully weigh and mix 9.9 g of potassium sulfate (K SO ), 0.41 g of mercuric oxide
2 4
(HgO), and 0.08 g of copper sulfate (CuSO ).
6.5 Methyl Purple Indicator Solution —Aqueous solution containing approximately 0.1 % active constituent (not methyl
violet). Other appropriate indicator solutions may also be used.
6.6 Sodium Hydroxide Solution (1000 g/L)—Dissolve 1000 g 1000 g of sodium hydroxide (NaOH) in 1 L of water.
(Warning—Causes burns. Poison.)
6.7 Sodium Sulfide Solution (40 g/L)—Dissolve 40 g of sodium sulfide (Na S) in warm water 194°F (90°C); cool and dilute to
1 L.
6.8 Sucrose (NIST)—Primary standard grade.
6.9 Sulfuric Acid (rel dens 1.84)—Concentrated sulfuric acid (H SO ). (Warning—Causes severe burns. Strong oxidizer.)
2 4
6.10 Sulfuric Acid, Standard (0.05 M)—Slowly add 3 mL of concentrated sulfuric acid (H SO , rel dens 1.84) to 500 mL of
2 4
water in a suitable size beaker. Mix the acid and water; allow it to cool and transfer to a 1-L volumetric flask. Dilute to the mark
with water; mix well. Standardize sulfuric acid to the nearest 0.0005 mol/L against 0.1 mol/L NaOH solution using phenolphthalein
indicator. Standardize the NaOH solution against primary standard grade potassium hydrogen phthalate (HOOCC H COOK). Use
6 4
the procedure outlined in Sections 14 to 19 of Practice E200.
NOTE 3—Commercially available pre-standardized H SO and NaOH solutions may be used.
2 4
6.11 Sulfuric Acid (0.005 M)—Prepare by tenfold dilution of the standard 0.05 M sulfuric acid prepared and standardized in
6.10.
6.12 Quality Control (QC) Samples , Sample, preferably are portions of one or more liquid petroleum materials that are stable
and representative of the samples of interest. These QC samples can be used to check the validity of the testing process as described
in Section 10.
7. Sampling
7.1 Take the sample in accordance with the instructions in Practice D4057.
7.2 Ensure that the sample is thoroughly representative of the material to be tested and that the portion of the sample used for
test is thoroughly representative of the whole sample.
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 H SO (rel dens 1.84). Swirl the contents of the Kjeldahl flask
2 4
to facilitate the mixing of the sample, catalyst reagent, and H SO .
2 4
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For Suggestions on the testing of reagents not listed by
the American Chemical Society, see Annual Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
The sole source of supply of commercially prepared catalyst reagent mixture, brand name Kel-Pak #1, known to the committee at this time is Matheson Scientific, 1850
Greenleaf Ave., Elk Grove Village, IL 60007. If you are aware of alternative suppliers, please provide this information to ASTM International Headquarters. Your comments
will receive careful consideration at a meeting of the responsible technical committee, which you may attend.
Fleisher Methyl Purple Indicator, U.S. Patent No. 241669, may be obtained from Harry Fleisher Chemical Co., Benjamin Franklin Station, Washington, DC 20004, or
from any chemical supply company handling Fleisher Methyl Purple.
D3228 − 08 (2014)
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 H SO (rel dens 1.84) when the volume does not conform to this condition. Use the H SO to wash down the neck of the
2 4 2 4
Kjeldahl flask after the contents have been allowed to cool sufficiently so that sulfur trioxide (SO ) fumes have subsided. The
volume of H SO (rel dens 1.84) added will depend upon the carbonaceous material in the Kjeldahl flask. After all of the
2 4
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 4—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 SO 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 H BO solution and 5 drops of methyl purple indicator solution under
3 3
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 2). Add the remainder of the water and cool the contents of the Kjeldahl flask to room
temperature.
NOTE 5—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 Na S solution to the cooled contents of the Kjeldahl flask, to precipitate the mercury, and swirl to mix.
(Warning—In addition to other precautions, when the Na S 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.) (Warning —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 H BO solution into the condenser as the digestion
3 3
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 H BO solution. Increase the heat to rapid boiling,
3 3
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 6—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.
NOTE 7—Commercially available digestion–distillation apparatus may be used as long as the same chemical reactions occurring in Section 8 are
maintained. In such cases, follow the manufacturer’s instructions for the details of digestion and distillation sequences.
8.12 Titrate the contents in the receiving flask with standard H SO (0.005 M) to an end point where the gray color of the
2 4
solution just disappears and only the purple color remains. If the titration exceeds 50 mL, continue the titration with standard
H SO (0.05 M). Read the volume of the standard acid to the nearest 0.05 mL.
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NOTE 8—Commercially available automated colorimetric titrators may be used instead of the manual titration described in 8.12.
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 H SO (rel dens 1.84) is all that is used for the digestion
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of the sucrose.
D3228 − 08 (2014)
9. Calculation
9.1 Calculate the nitrogen content of the sample as follows:
Nitrogen content, mass %5 A 2 B 3M11C 3M2 3230.01401 3100/W (1)
@~ ! #
wh
...
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