ASTM D3228-08(2019)
(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 mass % to 0.10 mass %, and for the determination of nitrogen in fuel oils when present in the concentration from 0.015 mass % 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. For specific warning statements, see 6.6, 6.9, and 8.8.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation:D3228 −08 (Reapproved 2019)
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 2. Referenced Documents
1.1 This test method covers the determination of nitrogen in 2.1 ASTM Standards:
lubricating oils when present in the concentration from D1193 Specification for Reagent Water
0.03 mass % to 0.10 mass %, and for the determination of D4057 Practice for Manual Sampling of Petroleum and
nitrogen in fuel oils when present in the concentration from Petroleum Products
0.015 mass % to 2.0 mass %. This test method is also appli- D6299 Practice for Applying Statistical Quality Assurance
cable to the analysis of additive concentrates and additive and Control Charting Techniques to Evaluate Analytical
packages. Measurement System Performance
E200 Practice for Preparation, Standardization, and Storage
NOTE 1—This test method may not be applicable to certain materials
of Standard and Reagent Solutions for ChemicalAnalysis
containing N–O or N–N linkage. However, the samples used in the
cooperative program to establish the precision of the test method were
3. Summary of Test Method
compounded with currently available ashless additives containing nitro-
gen. Complete recovery of the nitrogen present in these additives was
3.1 The sample is digested in a mixture of concentrated
obtained.
sulfuric acid, potassium sulfate, mercuric oxide, and copper
1.2 The values stated in SI units are to be regarded as
sulfate. After digestion, sodium sulfide is added to precipitate
standard. No other units of measurement are included in this
the mercury, and the mixture is made alkaline with caustic.
standard.
Nitrogen, now in the form of ammonia, is distilled into a boric
acid solution. The ammonia is titrated with standard sulfuric
1.3 This standard does not purport to address all of the
acid using methyl purple as an indicator.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety, health, and environmental practices and deter-
4.1 The concentration of nitrogen is a measure of the
mine the applicability of regulatory limitations prior to use.
presence of nitrogen-containing additives. Knowledge of its
For specific warning statements, see 6.6, 6.9, and 8.8.
concentration can be used to predict performance.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard-
5. Apparatus
ization established in the Decision on Principles for the
5.1 Buret, 50 mL, graduated in 0.1 mLsubdivisions, one for
Development of International Standards, Guides and Recom-
each titrant. Other size burettes may also be used.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
5.2 Flask, Erlenmeyer, 300 mL. Other sizes are also accept-
able.
5.3 Heater, electrical or gas.
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.03 on Elemental Analysis. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved May 1, 2019. Published June 2019. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1973. Last previous edition approved in 2014 as D3228 – 08 (2014). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/D3228-08R19. 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 (2019)
5.4 Kjeldahl Distillation Apparatus. phenolphthalein indicator. Standardize the NaOH solution
against primary standard grade potassium hydrogen phthalate
NOTE2—CommerciallyavailablesemiautomaticKjeldahlapparatusare
(HOOCC H COOK). Use the procedure outlined in Sections
acceptable. In such cases manufacturer prescribed sizes of burettes and 6 4
14 to 19 of Practice E200.
flasks may be used.
5.5 Kjeldahl Flask, at least 500 mL volume.
NOTE 3—Commercially available pre-standardized H SO and NaOH
2 4
solutions may be used.
6. Reagents
6.11 Sulfuric Acid (0.005 M)—Prepare by tenfold dilution
6.1 Purity of Reagents—Reagent grade chemicals shall be of the standard 0.05 M sulfuric acid prepared and standardized
in 6.10.
used in all tests. Unless otherwise indicated, it is intended that
all reagents shall conform to the specifications of the Commit-
6.12 Quality Control (QC) Sample, preferably are portions
tee onAnalytical Reagents of theAmerican Chemical Society,
of one or more liquid petroleum materials that are stable and
where such specifications are available. Other grades may be
representative of the samples of interest. These QC samples
used, provided it is first ascertained that the reagent is of
can be used to check the validity of the testing process as
sufficiently high purity to permit its use without lessening the
described in Section 10.
accuracy of the determination.
7. Sampling
6.2 Purity of Water—Unless otherwise indicated, references
7.1 Take the sample in accordance with the instructions in
to water shall be understood to mean reagent water as defined
by Types II and III of Specification D1193. Practice D4057.
7.2 Ensure that the sample is thoroughly representative of
6.3 Boric Acid Solution (40 g⁄L)—Dissolve 40 g of boric
acid (H BO ) in 1 L of boiling water. thematerialtobetestedandthattheportionofthesampleused
3 3
4 for test is thoroughly representative of the whole sample.
6.4 Catalyst Reagent —For each test carefully weigh and
mix 9.9 g of potassium sulfate (K SO ), 0.41 g of mercuric
2 4
8. Procedure
oxide (HgO), and 0.08 g of copper sulfate (CuSO ).
8.1 Transfer 1.0 g to 1.5 g of sample, weighed to the nearest
6.5 Methyl Purple Indicator Solution —Aqueous solution
0.1 mg, into a Kjeldahl flask.Avoid contact of the sample with
containing approximately 0.1 % active constituent (not methyl
the neck of the Kjeldahl flask.Add the catalyst reagent mixture
violet). Other appropriate indicator solutions may also be used.
to the Kjeldahl flask. Add two or three beads to prevent
6.6 Sodium Hydroxide Solution (1000 g⁄L)—Dissolve bumping.
1000 g of sodium hydroxide (NaOH) in 1 L of water.
8.2 WashdowntheneckoftheKjeldahlflaskwith20 mLof
(Warning—Causes burns. Poison.)
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
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 H SO .
2 4
dilute to 1 L.
8.3 Warm the contents of the Kjeldahl flask on the digestion
6.8 Sucrose (NIST)—Primary standard grade. rack and repeat the swirling.Apply low heat until the frothing
has stopped. Samples that do not froth or char shall be
6.9 Sulfuric Acid (rel dens 1.84)—Concentrated sulfuric
subjected to a 20 min low-heating period. Careful periodic
acid (H SO ). (Warning—Causes severe burns. Strong oxi-
2 4
swirlingofthesolutionintheKjeldahlflaskshallalsobemade.
dizer.)
Gradually apply intermediate heat to raise the temperature of
6.10 Sulfuric Acid, Standard(0.05 M)—Slowlyadd3 mLof
the solution to boiling.
concentrated sulfuric acid (H SO , rel dens 1.84) to 500 mLof
2 4
8.4 Maintain a minimum volume of 15 mL of liquid in the
water in a suitable size beaker. Mix the acid and water; allow
Kjeldahl flask during the digestion period. Add volumes of
it to cool and transfer to a 1 L volumetric flask. Dilute to the
5mLto15mLofH SO (reldens1.84)whenthevolumedoes
2 4
mark with water; mix well. Standardize sulfuric acid to the
notconformtothiscondition.UsetheH SO towashdownthe
2 4
nearest 0.0005 mol⁄L against 0.1 mol⁄L NaOH solution using
neck of the 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
2 4
Reagent Chemicals, American Chemical Society Specifications, American
depend upon the carbonaceous material in the Kjeldahl flask.
Chemical Society, Washington, DC. For Suggestions on the testing of reagents not
listed by the American Chemical Society, see Annual Standards for Laboratory
After all of the carbonaceous material has been digested and
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
the solution has cleared, continue the digestion for two more
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
hours at rapid rate of boiling. The total volume of liquid
MD.
The sole source of supply of commercially prepared catalyst reagent mixture, remaining in the Kjeldahl flask after digestion approximates
brandnameKel-Pak#1,knowntothecommitteeatthistimeisMathesonScientific,
the volume in the Kjeldahl flask for the blank.
1850 Greenleaf Ave., Elk Grove Village, IL 60007. If you are aware of alternative
suppliers, please provide this information to ASTM International Headquarters.
NOTE 4—For some samples, a two hour digestion period may be
Your comments will receive careful consideration at a meeting of the responsible
unnecessary, if the solution has completely cleared.
technical committee, which you may attend.
8.5 Turn off the heat, but allow the Kjeldahl flask to remain
Fleisher Methyl Purple Indicator, U.S. Patent No. 241669, may be obtained
from any chemical supply company handling Fleisher Methyl Purple. in the fume duct or hood until the evolution of SO fumes has
D3228−08 (2019)
150 mL.Forconveniencethereceivingflaskcanbemarkedatthe130 mL
subsided. Remove the Kjeldahl flask from the rack and cool to
and 150 mL volume points.
approximately room temperature.
NOTE 7—Commercially available digestion–distillation apparatus may
8.6 Place a 300 mL receiving flask containing 25 mL of
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
H BO solutionand5dropsofmethylpurpleindicatorsolution
3 3
details of digestion and distillation sequences.
under the condenser with the delivery tube tip extending to the
bottom of the receiving flask. 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
8.7 Measure approximately 275 mL of water and add a
solution just disappears and only the purple color remains. If
portionofthiswatertotheKjeldahlflaskandswirlthecontents
thetitrationexceeds50 mL,continuethetitrationwithstandard
until the salt cake has dissolved (Note 2).Add the remainder of
H SO (0.05 M). Read the volume of the standard acid to the
2 4
the water and cool the contents of the Kjeldahl flask to room
nearest 0.05 mL.
temperature.
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
solution is added to the cooled digestion flask, considerable 2 4
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
mercuric sulfide. Laboratories processing large volumes of
9.1 Calculate the nitrogen content of the sample as follows:
Kjeldahl nitrogen determinations should consider the use of a
Nitrogen content, mass % 5 A 2 B 3 M11C 3 M2 32 30.01401
@~ ! #
recovery trap for mercury.)
3100/W (1)
8.9 PlacetheKjeldahlflaskinaslurryoficeandwater.Cool
where:
the contents in the Kjeldahl flask to approximately 40 °F
(4.5 °C). Slowly add 75 mL of NaOH solution (1000 g⁄L) A = millilitres of 0.005 M H SO required to titrate
2 4
down the inclined neck of the Kjeldahl flask, without agitation,
the sample,
to form two layers. B = millilitres of 0.005 M H SO required to titrate
2 4
8.9.1 Carefully remove the Kjeldahl flask from the ice bath the blank,
C = millilitres of 0.05 M H SO required to titrate the
so that mixing of the layers does not occur. Carefully place the
2 4
sample,
Kjeldahl flask on the Kjeldahl distillation rack.
M1 = 0.005 (molarity of 0.005 M H SO ),
8.9.2 Immediately connect the Kjeldahl flask to the distilla- 2 4
M2 = 0.05 (molarity of 0.05 M H SO ),
2 4
tion apparatus and mix the contents of the Kjeldahl flask
2 = number of equivalents of H SO ,
2 4
thoroughly by swirling. The digestion flask must be connected
0.01401 = equivalent weight, g/mL,
to the distillation apparatus immediately after the alkali has
100 = factor to convert to percent, and
beenaddedandlayered,butbeforeswirlingtomixtheacidand
W = weight of sample used, g.
alkali. When any mixing is permitted to occur before the
9.2 Alternatively, calculate the nitrogen content of the
digestion flask is connected, the heat generated can be suffi-
cient to release some of the ammonia which can be lost. This sample as follows:
loss results in low recovery of ammonia, and thus low values
Nitrogen content, mass % 5 @~A 2 B! 30.011C 30.1# 30.01401
for the nitrogen content of the sample.
3100/W (2)
8.10 Promptly apply full heat to the digestion flask. Reduce
where:
the heat just before the solution begins to boil and maintain at
A = millilitres of 0.005 M H SO required to titrate
2 4
low boiling for 5 min. Heat must be applied promptly to
the sample,
prevent sucking of the H BO solution into the condenser as
3 3
B = millilitres of 0.005 M H SO required to titrate
2 4
the digestion solution cools. The initial distillation rate must
the blank,
not be too rapid because most of the ammonia is distilled
C = millilitres of 0.05 M H SO required to titrate the
2 4
during the first few minutes,
...
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 (Reapproved 2014) D3228 − 08 (Reapproved 2019)
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
1.1 This test method covers the determination of nitrogen in lubricating oils when present in the concentration from
0.030.03 mass % to 0.10 mass %, 0.10 mass %, and for the determination of nitrogen in fuel oils when present in the concentration
from 0.0150.015 mass % to 2.0 mass %. 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. For specific warning statements, see 6.6, 6.9, and 8.8.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.03 on Elemental Analysis.
Current edition approved May 1, 2014May 1, 2019. Published July 2014June 2019. Originally approved in 1973. Last previous edition approved in 20082014 as
D3228 – 08.D3228 – 08 (2014). DOI: 10.1520/D3228-08R14.10.1520/D3228-08R19.
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 (2019)
5. Apparatus
5.1 Buret, 50-mL,50 mL, graduated in 0.1-mL0.1 mL subdivisions, one for each titrant. Other size burettes may also be used.
5.2 Flask, Erlenmeyer, 300-mL.300 mL. Other sizes are also acceptable.
5.3 Heater, electrical or gas.
5.4 Kjeldahl Distillation 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-mL500 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 (40 g g/L)⁄L)—Dissolve 40 g 40 g of boric acid (H BO ) in 1 L 1 L of boiling water.
3 3
6.4 Catalyst Reagent —For each test carefully weigh and mix 9.9 g 9.9 g of potassium sulfate (K SO ), 0.41 g 0.41 g of
2 4
mercuric oxide (HgO), and 0.08 g 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(1000 g g/L)—Dissolve ⁄L)—Dissolve 1000 g of sodium hydroxide (NaOH) in 1 L 1 L
of water. (Warning—Causes burns. Poison.)
6.7 Sodium Sulfide Solution (40 (40 g g/L)⁄L)—Dissolve 40 g 40 g of sodium sulfide (Na S) in warm water 194°F (90°C);194 °F
(90 °C); cool and dilute to 1 L.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 (0.05 M)—Slowly add 3 mL 3 mL of concentrated sulfuric acid (H SO , rel dens 1.84) to
2 4
500 mL 500 mL of water in a suitable size beaker. Mix the acid and water; allow it to cool and transfer to a 1-L1 L volumetric
flask. Dilute to the mark with water; mix well. Standardize sulfuric acid to the nearest 0.00050.0005 mol mol/L ⁄L against
0.10.1 mol mol/L ⁄L NaOH solution using phenolphthalein indicator. Standardize the NaOH solution against primary standard
grade potassium hydrogen phthalate (HOOCC H COOK). Use the procedure outlined in Sections 14 to 19 of Practice E200.
6 4
NOTE 3—Commercially available pre-standardized H SO and NaOH solutions may be used.
2 4
6.11 Sulfuric Acid (0.005 (0.005 M)—Prepare by tenfold dilution of the standard 0.05 0.05 M sulfuric acid prepared and
standardized in 6.10.
6.12 Quality Control (QC) 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.
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 (2019)
8. Procedure
8.1 Transfer 1.01.0 g to 1.5 g 1.5 g of sample, weighed to the nearest 0.1 mg, 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 20 mL of H SO (rel dens 1.84). Swirl the contents of the Kjeldahl
2 4
flask to facilitate the mixing of the sample, catalyst reagent, and H SO .
2 4
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-min20 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 15 mL of liquid in the Kjeldahl flask during the digestion period. Add volumes of
55 mL to 15 mL 15 mL of H SO (rel dens 1.84) when the volume does not conform to this condition. Use the H SO to wash
2 4 2 4
down the neck of the 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
2 4
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 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-mL300 mL receiving flask containing 25 mL 25 mL of H BO solution and 5 drops of methyl purple indicator
3 3
solution under the condenser with the delivery tube tip extending to the bottom of the receiving flask.
8.7 Measure approximately 275 mL 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 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).40 °F (4.5 °C). Slowly add 75 mL 75 mL of NaOH solution (1000(1000 g g/L) ⁄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. 5 min. Heat must be applied promptly to prevent sucking of the H BO solution into the condenser as the
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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 H BO solution. Increase the heat to rapid
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boiling, until the volume in the receiving flask reaches a volume of approximately 130 mL.130 mL.
8.11 Lower the receiving flask to expose the condenser delivery tube tip. Rinse the tip with water. After approximately 1 min
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. 150 mL. For convenience the receiving flask can be marked at the
130130 mL and 150-mL150 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.
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8.12 Titrate the contents in the receiving flask with standard H SO (0.005 (0.005 M) to an end point where the gray color of
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the solution just disappears and only the purple color
...
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