Page 1
Two & Six-Place (Macro)
Kjeldahl Digestion Systems
Models
2128401
2125611
2125612
2125613
Page 2
Copyright © 2017 Labconco Corporation. The information contained in this manual and the
Warranty
Labconco Corporation provides a warranty to the original buyer for the repair or replacement of parts and
reasonable labor as a result of normal and proper use of the equipment with compatible chemicals. Broken
glassware and maintenance items, such as filters, gaskets, light bulbs, finishes and lubrication are not
warranted. Excluded from warranty are products with improper installation, erratic electrical or utility supply,
unauthorized repair and products used with incompatible chemicals.
The warranty for Two and Six-Place Kjeldahl Digestion Systems will expire one year from date of installation
or two years from date of shipment from Labconco, whichever is sooner. Warranty is non-transferable and
only applies to the owner (organization) of record.
Buyer is exclusively responsible for the set-up, installation, verification, decontamination or calibration of
equipment. This limited warranty covers parts and labor, but not transportation and insurance charges. If the
failure is determined to be covered under this warranty, the dealer or Labconco Corporation will authorize
repair or replacement of all defective parts to restore the unit to operation. Repairs may be completed by 3rd
party service agents approved by Labconco Corporation. Labconco Corporation reserves the rights to limit this
warranty based on a service agent’s travel, working hours, the site’s entry restrictions and unobstructed access
to serviceable components of the product.
Under no circumstances shall Labconco Corporation be liable for indirect, consequential, or special damages
of any kind. This warranty is exclusive and in lieu of all other warranties whether oral, or implied.
accompanying products are copyrighted and all rights reserved by Labconco Corporation. Labconco
Corporation reserves the right to make periodic design changes without obligation to notify any
person or entity of such change.
Returned or Damaged Goods
Do not return goods without the prior authorization from Labconco. Unauthorized returns will not be
accepted. If your shipment was damaged in transit, you must file a claim directly with the freight
carrier. Labconco Corporation and its dealers are not responsible for shipping damages.
The United States Interstate Commerce Commission rules require that claims be filed with the
delivery carrier within fifteen (15) days of delivery.
Limitation of Liability
The disposal and/or emission of substances used in connection with this equipment may be governed
by various federal, state, or local regulations. All users of this equipment are required to become
familiar with any regulations that apply in the user’s area concerning the dumping of waste materials
in or upon water, land, or air and to comply with such regulations. Labconco Corporation is held
harmless with respect to user’s compliance with such regulations.
Contacting Labconco Corporation
If you have questions that are not addressed in this manual, or if you need technical assistance, contact
Labconco’s Customer Service Department or Labconco’s Product Service Department at 1-800-8215525 or 1-816-333-8811, between the hours of 7:30 a.m. and 5:30 p.m., Central Standard Time.
Part # 2081401, Rev. D
ECO L123
Page 3
T
T
AABBLLEE
O
O
FF
C
C
OONNTTEENNTTSS
CHAPTER 1: INTRODUCTION 1
Components Shipped 1
General Description 2
Performance 3
Component Identification 3
CHAPTER 2: INSTALLATION 6
General Installation Notes 6
Location 6
Electrical 7
Acid Fume Blower Exhaust System 8
Blower Airflow Exhaust Volume Requirement & Velocity Test 8
Digestion Rack Drip Shield 9
CHAPTER 3: NORMAL OPERATION & ROUTINE
Normal Operation Start-Up 10
Clean-Up and Cosmetic Guidelines 11
Fume Removal Manifold Suction Adjustment 11
Exhaust Systems Maintenance 11
Digestion Drip Shield 11
APPENDIX A: REPLACEMENT PARTS 12
APPENDIX B: DIMENSIONS & AIRFLOW REQUIREMENTS 13
APPENDIX C: TOTAL KJELDAHL NITROGEN METHOD 15
MAINTENANCE 10
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Components Shipped
Carefully check the contents of your Two-Place or Six-Place Kjeldahl
Digestion System for shipping damage while it is still on the shipping pallet.
Do not discard the packaging material until the contents have been checked
and the equipment has been approved for installation.
The Two-Place or Six-Place Kjeldahl Nitrogen Digestion System has been
shipped in one complete crate to minimize damage that may occur in transit.
Make sure to inspect the product thoroughly prior to installation and report
any damage that may have occurred in transit (see Warranty page for
instructions).
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Chapter 1: Introduction
2
General Description
Labconco’s Two and Six-Place (Macro) Kjeldahl Digestion Systems are
designed to facilitate the determination of Total Nitrogen content within
materials such as feeds, grains, soils, fertilizers, plant tissue, water, organic
wastes, and food products. The expression ‘Macro’ identifies the size of
sample vessel and the sample size that can be used with this equipment.
Other Kjeldahl systems are available, which use smaller sample vessels and
require smaller sample sizes. This apparatus can be used for the digestion of
all types of nitrogen containing samples. The Two and Six-place Digestion
Systems can be used to perform a standardized Macro-Kjeldahl Nitrogen
Determination Method.
The Kjeldahl Digestion Systems are not enclosed, excess heat and personnel
safety during the digestion reaction must be considered when selecting an
installation location. The digestion process involves boiling concentrated
sulfuric acid at high temperatures. Labconco recommends placing Two-Place
Digestion System inside a chemical fume hood for safety. Labconco
recommends locating the Six-Place Digestion System in a well ventilated
room away from normal laboratory operations.
2128401 & 2125601 Two-Place and Six-Place Kjeldahl Digestion Systems
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Chapter 1: Introduction
3
Performance
The Two and Six-Place Kjeldahl Digestion Systems have been designed for
use in the determination of nitrogen (or ammonia) concentrations. By
calculation, nitrogen can be converted to protein values for products such as
plant tissues, meats and other food substances. Nitrogen determinations with
the (Macro) Digestion Systems can accommodate sample sizes up to 5
grams, due to the size of the digestion flasks. However, high levels of
organic content within a specific sample type will require smaller sample
sizes due to the vigorous digestion reaction. Detailed procedures developed
by professional laboratory organizations such as American Association of
Analytical Chemists (AOAC) and American Association of Cereal Chemists
(AACC) should always be consulted for step by step analytical procedures
when operating this equipment. For methodology precision and accuracy
please consult the specific published method for the type of sample or
substance that will be analyzed.
Component Identification
1. Electric Heaters. 600-watt heaters are used in the digestion rack of the
units. Infinite control switches regulate each of the heaters. The curved
heater elements are provided and allow for the round base of the Kjeldahl
flasks, providing faster and more evenly distributed heat. The heating
element assembly on the digestion rack can be moved closer to or away
from the acid fume removal manifold to accommodate different size
Kjeldahl flasks. The six-place systems have heaters and temperature
controllers which are wired to a common subpanel electrical box. The
two-place systems come pre-wired with a three wire cord and plug, for
easy connection to a facility electrical circuit.
2. Fume Removal Manifold. Located in back of the digestion heaters, this
manifold is manufactured from chemical resistant chlorinated polyvinyl
chloride and features heat resistant PTFE nipples designed to prevent
leakage of the hot sulfuric acid fumes created during the digestion
process. The nipple design, extends in to the flask opening, and
efficiently removes excess acids fumes during the digestion process. The
flasks are oriented at a 45 degree angle to the fume removal manifold to
promote the condensation and refluxing of the acid fumes back down into
the digestion flask. The refluxing process prevents the samples from
boiling dry during the 45+ minute digestion process.
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Chapter 1: Introduction
4
1
2
Two Place Kjeldahl Digestion System
Six-Place Digestion System Fume Removal Options
3. Acid Fume Piping Exhaust System. Located on the left side of the unit,
this exhaust piping removes excess Sulfuric Acid fumes from the
digestion flasks. Attention - The Exhaust Piping must be connected to a
remote blower to remove acid fumes. The unit does NOT have an
integral blower. The remote blower draws the Sulfuric Acid fumes
through the CPVC digestion manifold and exhaust piping provided and
pulls the fumes under negative pressure out through a duct work system
connected to the remote blower and ultimately the building’s exterior.
The 6” exhaust stack of the blower must be connected to a Leak Tight –
PVC duck work system that vents 10’ above the building roof.
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Page 8
Chapter 1: Introduction
5
Six Place Kjeldahl Digestion System
1
2
3
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Page 9
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General Installation Notes
The Two and Six-Place Kjeldahl Digestion Systems are shipped fully assembled.
The Six-Place Kjeldahl Digestion System will require exhaust duct work which
should be made by a licensed contractor. The Two-Place Kjeldahl Digestions
System can be installed by the customer inside a fume hood and the fume removal
manifold connected to a common laboratory faucet water aspirator. The customer
will need to purchase the required Kjeldahl flasks,
Use caution when you remove the Six-Place Kjeldahl Digestion system protective
packaging material taped to the heater runways as fragile components are
involved. Levelers are shipped installed.
Location
The Two-Place Kjeldahl Digestion System can be carried by hand to its final
location inside a fume hood.
The Six-Place Kjeldahl Digestion System is heavy and cannot be disassembled to
reduce the overall size. Special arrangements should be made in advance for
moving the system to its final location. Special instruction tags are attached to the
apparatus; they must not be removed until the installation has been completed.
The Six-Place Kjeldahl Digestion Systems do not include a surrounding safety
enclosure. It is strongly advised to install this product in a segregated location
away from normal laboratory personnel traffic and inside a floor mounted
chemical fume hood. High heat and boiling acid present a safety hazard to
personnel in close proximity to the Six-Place Kjeldahl Digestion System and the
Six-Place Kjeldahl Digestion System should be placed inside a floor mounted
fume hood. Also recommended is spot exhaust ventilation for the final
installation location to remove excess heat produced by the digestion heaters.
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Page 10
Chapter 2: Installation
7
P/N
AC Electrical Code
Amps
-11
115v, single phase, 60 Hz
33
-12
230v, single phase, 60 Hz
18
-13
208/230v, three phase, 60 Hz
10
Electrical
The Two-Place Kjeldahl Digestion System comes with an 8-foot, 3 wire cord and
plug and can be connected to a 115 volt 60Hz 11 Amp circuit.
The Six-Place Digestion Kjeldahl System has been wired at the factory per the
product model number ordered by the customer. Labconco model numbers ending
in -11 are 115 volts. Model numbers ending in -12 are 230 volts. Models ending
in -13 are 230 volt, three phase. Amperage requirements vary depending on the
customer selected operational voltage of the digestion rack. The voltage and
amperage tables below provide the electrical requirements for the Six-Place
Digestion Systems. A qualified electrician will need to connect the facility
electrical supply to the main breaker box of the Digestion System.
1. Remove circuit breaker box cover panel.
2. Main line lead connection terminals are identified and connections
must be made accordingly.
3. Line leads to the apparatus must conform to local electrical codes.
4. The Remote Blower can be wired to the on/off switch to control the
remote blower from the unit. Remote Blower MUST be on before
using the unit.
5. Provide an electrical ground to the apparatus per code.
6. Before applying power to apparatus, check the electrical panel and
breakers for loose connections.
7. Reinstall box cover panel.
8. Power the breaker box and reset breakers to check circuits in the
apparatus.
9. All electrical wiring and connections must conform to local codes and
should be performed by qualified electricians. An earth ground must
be provided.
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Chapter 2: Installation
8
Drain Line
Damper
Flexible 6”
Coupling
Piping
Assembly
Acid Fume Blower Exhaust System
The exhaust duct connection on top of the damper is sized for use with 6" nominal
(6-5/8" O.D.) vent duct. The exhaust piping assembly is supplied with a short
piece of 6" PVC pipe and a flexible coupling with clamps to connect the
dedicated duct work exhaust. The flexible coupling must be sealed air-tight to the
exhaust duct work.
All exhaust duct work must be acid-resistant and sealed air-tight. Use only acidresistant sealant on duct work joints.
The bottom of the piping assembly is fitted with a condensate drain and corrosion
resistant tubing. This condensate (identified above and on the next page) can be
collected in a closed acid proof container, which should be emptied regularly or it
may be plumbed to an acid resistant drain line. The condensate is concentrated
acid and care must be exercised when handling it.
Blower Airflow Exhaust Volume Requirement &
Velocity Test
Cubic feet per minute = 25
Average airflow velocity = 117 feet per minute
The Kjeldahl’s airflow volume test is based on the "Pitot Traverse in Inlet Duct"
method described in the ANSI/ASHRAE 51-1999 standard. Duct velocity
measurements are taken at 48" from the flexible coupling using a 6 point duct
traverse pattern (Industrial Ventilation 24th Edition p. 9-10).
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Chapter 2: Installation
9
WARNING: Acid resistant duct must be used on the exhaust connection on
your Kjeldahl Apparatus.
Digestion Rack Drip Tube
Duct Acid Drip Tube
(Duct Area in Square Feet) x (Average Airflow Velocity in feet per minute)
= (Air Volume CFM)
(Pi) x (radius²) x (117 FPM) = 25 CFM
The exhaust stack rising from the piping assembly must be vented to the outside
atmosphere via acid-resistant, sealed duct work and a remote blower (not
included). If the remote blower pulls more than 25 CFM at the measurement
point (48” above flexible coupling), close the damper shown in Fig. 2-6 to reduce
airflow to the recommended 25 CFM.
Highly corrosive fumes produced from boiling sulfuric acid will flow through
the ductwork, so acid-resistant duct must be used. Polyvinyl chloride (PVC)
or fiberglass duct is recommended. The exhaust duct work must be supported
independent of the Kjeldahl System to avoid distortion of the blower housing.
Digestion Rack Drip Shield
The digestion heaters are located in a drip shield and constructed of stainless steel
which is resistant to acid attack. Liquid acid spills that occur can be drained to an
acid resistant container with the drip tube located on the left side of the digestion
rack.
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Normal Operation Start-Up
1. The KNA apparatus is designed for use with either 500 or 800-ml
Kjeldahl Flasks. The distillation heaters are free to move forward or
backward on the runway to accommodate either size flask.
2. Turn the heat controllers to the high setting on both the Digester and
Distillation racks if both racks will be used simultaneously. Wait
approximately 5 to 10 minutes to insure proper preheating.
3. Turn on either the acid fume removal blower or water aspirator
(depending upon model number purchased). Place a small piece of tissue
paper over the nipple on the digestion manifold. Suction should be
sufficient to hold 2" x 2" tissue paper in place.
A generalized Kjeldahl Total Nitrogen method is present in Appendix C. The
method does not detail sample preparation or preservation procedures, which
may be required depending on specific types of samples.
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Chapter 3: Normal Operation & Routine Maintenance
11
Clean-Up and Cosmetic Guidelines
Keeping the Kjeldahl Digestion System clean will preserve the
appearance and improve life of the equipment.
The equipment can be kept clean by washing with a weak solution of
sodium hydroxide and rinsing with clear water.
Sulphate, a salt of Sulfuric Acid, may build up between the channel
support and the fume removal manifold. Neutralize this with a weak
solution of sodium hydroxide periodically to keep the unit clean.
Fume Removal Manifold Suction Adjustment
The suction along the fume removal manifold is adjusted as follows:
Two Place Digestion System
Suction is adjusted by changes in the water supply flow rate. The suction
may be decreased by decreasing the flow rate.
Blower Exhaust System
No adjustment is required on the suction pressure at the manifold when used.
The remote blower should be inspected annually for debris, proper operation,
and checked for adequate airflow as specified on pages 11-12 of this manual.
Exhaust Systems Maintenance
Sometimes the suction in the nipples is reduced due to obstruction within the
exhaust system. These obstructions can usually be washed from the fume
pipe, piping assembly or water ejector by boiling water in a number of flasks
placed on the digestion heaters. The blower or water aspirator should be in
operation to draw the steam through the fume pipe.
Digestion Drip Shield
The drip shield is constructed of stainless steel and is resistant to most acid attack.
Acid spills can however cause discoloration and eventual marring of the surface
unless the following proper procedures are followed in their clean up.
1. Promptly wash down all major acid spills contained by the shield.
2. Periodically clean the stainless surface with any residential
stainless steel sink cleaning compound.
3. After cleaning, renew the bright shiny appearance by lightly
sanding with 220 grade emery paper.
The Two-Place Kjeldahl Digestion Systems does not feature a drip shield
with a drained tube.
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12
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Part Number
Description
1315400
Heater Control, Infinite (115Volts)
1315500
Heater Control, Infinite (230Volts)
2011500
Core Plate Casting, (package of 6)
2023200
Heater Base Casting (electric or gas)
2033100
Heater element, 115V, 600 W, (package of 6)
2033200
Heater element, 230V, 600 W, (package of 6)
2023100
Heater Top ceramic casting (electric or gas)
2031800
Heater lead wire assembly, (package of 6 pairs)
1870200
Knob for Infinite Heater Control
1317100
Blower Motor Switch
2056600
Ceramic nipple
2053000
Water ejector nozzle
1967000
Clamp, hose
2144600
Flex sleeving
1662200
Grommet 1/2 ID x 1-1/2 OD
2165800
PTFE nipple
1880128
Plastic screws for PTFE nipple
Part Number
Description
1620500
Tubing, rubber condenser connecting, per foot
2152407
Distillation manifold – 6 place stainless steel
2038800
Wire, No 14 Black, SEWF-2, 30 ft length
2038900
Wire No. 14 White, SEWF-2, 30 ft length
2031700
Heater terminal assembly, package of 24
2146400
Assembly inlet & outlet manifold (12 unit only)
2081300
Connector bulb/caustic trap, package of 6
2128800
Delivery tube, package of 6
2078800
Kit, Replacement Fume Pipe
Electrical Heaters and Motor/Blower Parts
Miscellaneous Replacement Parts
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Chapter 3: Normal Operation & Routine Maintenance
14
B-2
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Titrimetric Determination
Various scientific associations offer approved Kjeldahl methods. These
methods are available at their websites. The AOAC International (Official
Methods of Analysis), Association of American Cereal Chemists (Approved
Methods), American Oil Chemists Society, Environmental Protection Agency
(EPA Methods for Chemical Analysis of Water and Wastes), International Standards
Organization, The National Forage Testing Association, and United States
Department of Agriculture.
This methodology is for reference only. It is not a citable document. It is
based on data we believe to be reliable. It is offered in good faith but without
guarantee.
1. Scope and Application
1.1. This method determines the Total Kjeldahl Nitrogen (TKN) in water,
and organic substances. There are three steps within the method: 1.
the digestion procedure – which converts organic nitrogen to
ammonia, 2. the distillation procedure - transferring ammonia from
the digested sample to an ammonia trapping solution 3. sample
titration – the quantification of ammonia in the trapping solution.
NOTE 1: Some compounds containing nitrogen may not be converted;
such as amines, nitro-compounds, hydrazones, oximes,
semicarbazones and some tertiary amines.
1.2. This method is described for use with Kjeldahl equipment using
Macro-Size glassware (500 - 800 ml flasks).
2. Definitions
2.1. TKN is defined as the sum of free-ammonia and organic nitrogen
compounds, converted to ammonium sulfate (NH4)2SO4 by acid
digestion.
3. Apparatus
3.1. Digestion Apparatus: A Kjeldahl digestion apparatus with multiple
600 watt electric burners, using 500 - 800 ml flasks and a fume
removal manifold system.
3.2. Distillation Apparatus: A Kjeldahl distillation apparatus with
multiple 600 watt electric burners, using 500 - 800 ml flasks. The
distillation system includes connecting bulbs to prevent mechanical
carryover of NaOH during distillation, water cooled condensers and
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Appendix C: Total Kjeldahl Nitrogen Method
16
distillate receiving vessels, such as 250 ml Erlenmeyer flasks. The
equipment is available as a combined system, featuring both
digestion and distillation apparatus. The preheated burners should
bring 250 ml of water at 25° C to a rolling boil with in 5 minutes.
Condensation columns should cool distillate to 75° F (23.8°C).
3.3. Titration Class A Burets, 25-50 ml. for dispensing Standardized Acid
solution.
3.4. Analytical balance, sensitive to 0.0001grams.
4. Reagents
4.1. Purified water, nitrogen free.
4.2. Sulfuric acid, H2SO4 concentrated, Specific Gravity of 1.84. (95-
98%, nitrogen free)
4.3. Copper sulfate, CuSO4, anhydrous. Nitrogen free. (catalyst)
4.4. Potassium sulfate, K2SO4. Nitrogen free. (boiling point elevator)
4.5. Sodium hydroxide NaOH, reagent grade Specific Gravity 1.3,
nitrogen free. 45% solution, (dissolve pellets, 450g NaOH in distilled
water and dilute to 1 liter).
4.6. Boiling stones, Antibumping agent, aluminum oxide stones, (Hengar
granules).
4.7. Mixed color indicator: 0.75 g of Methyl Red and 0.5 g Methylene
Blue in 300 ml of 95% ethanol. (Prepare fresh every 30 days)
4.8. Boric acid H3BO
saturated solution, (dissolve granules, 40 g boric
3,
acid, in purified water and dilute to 1 liter). Add 3 ml of the mixed
color indicator solution to the 1 liter of boric acid solution.
4.9. Sulfuric acid, H2SO
standardized solution: (0.02 N) Prepare a stock
4,
solution of approximately 0.1 N sulfuric acid by diluting 3 ml of conc.
H2SO4 (sp. Gr. 1.84) to 1 liter with CO2-free distilled water. Dilute 200
ml of this solution to 1 liter with CO2-free distilled water.
Alternatively, premade standardized acid and base solutions with a
certified specification ranges are commercially available through most
Scientific Equipment Dealers. If the Sulfuric Acids standard solutions
are prepared, their normality must be determined analytically.
5. Digestion
5.1. The distillation condensation columns should be cleaned before use
by distilling a 1:1 mixture of distilled water and sodium hydroxide
until the distillate is ammonia-free. Then repeat this cleaning
procedure if the apparatus is out of service long enough to
accumulate ammonia (> 4 hours).
5.2. Digestion
5.2.1. Place a homogenous measured sample (0.250 – 1.000
gram) into a 500 - 800 ml Kjeldahl flask. Weigh or
measure the sample to the nearest 0.1 mg/ml.
5.2.2. Add to each flask:
20 ml sulfuric acid, 0.04 g CuSO4, 15g K2SO4 potassium
sulfate, 8-10 boiling stones. Place flask on the digestion
burner in inclined position (promotes acid refluxing), with
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Appendix C: Total Kjeldahl Nitrogen Method
17
the flask neck on the fume removal system. If necessary
lower the burner heat setting to prevent sample from
foaming up into neck of flask. Once, the sample comes to
a smooth rolling boil return heat setting to maximum.
When digestion sample clears (no black specs with pale
green tint color), continue to boil for an equal length of
time as required to reach the clear point, (total time
approx. 1 hr). If black specs occur in the flask neck, rotate
the flask 180 degrees to allow refluxing acid to rinse the
internal flask surface. Allow digestion mixture to cool and
carefully add 50 ml of distilled water to prevent K2S04 salt
solids from forming. All salt crystals must be dissolved
before proceeding to the next step. If necessary partially
reheat sample and agitate to dissolve crystals.
6. Distillation
6.1. The distillation burners should be preheated and condenser cooling
water turned on. The receiving flask should be in-place with enough
boric acid H3BO3 plus the mixed indicator solution to submerge the
tip of condenser delivery tube well below the level of the boric acid
receiving solution in the receiving flask. (Erlenmeyer 250 ml flasks
recommended)
6.2. In a fume hood, carefully adding of 100 ml of 45% sodium
hydroxide solution without mixing to make the digested sample
alkaline. Tilt the flask in to the hood while adding the sodium
hydroxide to the sulfuric acid digestion solution. Slowly add the
heavy sodium hydroxide solution down the neck of the flask. The
heavier sodium hydroxide solution will slip under the aqueous
sulfuric acid solution without mixing and with out loss of freeammonia. Do not mix until the flask has been connected to the
distillation apparatus.
6.3. Immediately connect the flask to a condenser using the rubber
stopper on the distillation connecting bulb attached to a condenser
column.
6.4. Vigorously swirl the Kjeldahl flask to mix contents thoroughly; heat
until all NH3 has been distilled (>8-10 minutes). During the ammonia
transfer the receiving solution with change color from purple to
green. Lower receiving flask and let liquid drain from condenser tip.
Turn off distillation burners.
6.5. For nitrogen concentrations above 1 mg/1, the ammonia can be
determined titrimetrically. For concentrations below this level,
colorimetric or potentiometric determination methods are
recommended.
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Appendix C: Total Kjeldahl Nitrogen Method
18
Organic N + H2SO4 →
(NH4)SO4 + H2O + CO4 + other sample matrix byproducts
7. Titration
7.1. Depending on the samples expected nitrogen level and the sensitivity
of the titration desired, select the appropriate standardized H2SO
4
solution (0.02 or 0.10 normal). High or low nitrogen concentrations
will require standardized H2SO4 solutions with stronger or weaker
normalitys. Titrations should require at least 15 ml of titrant to be
accurate. Fill a class A. buret to the zero line with the standardized
H2SO4 solution. Titrate the H3BO3 receiving solution with
standardized H2SO4 solution to first trace of the original purple
color. A white stirring plate will aid color visualization of end
point. Record ml H2SO4 titrated to the at least nearest 0.05 ml. Match
the endpoint against a sample blank containing the same volume of
distilled water and H3BO3 solution.
8. Calculation
For Dry Samples% Nitrogen =
% Nitrogen, = (ml H2SO4, sample - ml H2SO4, blank ) x Normality H2SO
x
4
1.4007/ weight of sample in grams. (1.4007 = a single factor that takes into
account the molecular weight of Nitrogen, the conversion of milli-equivalent results of
V*N, and the conversion to %)
% Nitrogen, = (ml H2SO4, sample - ml H2SO4, blank) x normality H2SO4 x
1400.7 x 100/ weight of sample in milligrams.
For Liquid Samples: calculate Total Kjeldahl Nitrogen, in mg/1, in the
original sample as follows:
Milligrams Total Nitrogen per Liter = (ml H2SO4, sample - ml H2SO4,
blank) x normality H2SO4 x 14.007x 1000/ volume of sample in milliliters.
If desired to determine % protein instead of % nitrogen, the calculated % N is
multiplied by a factor, the magnitude of the factor depending on the sample
matrix.
Common Protein Factors
5.7 – bread, wheat and wheat flour
6.25 – other grains
6.38 – milk
6.25 – unknown source
Total Kjeldahl Nitrogen Method Overview
Digestion is accomplished by boiling a homogeneous sample in concentrated
sulfuric acid. The end result is an ammonium sulfate solution. The general
equation for the digestion of an organic sample is shown below:
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Appendix C: Total Kjeldahl Nitrogen Method
19
ammonium
sulfate
heat
ammonia
gas
(NH4)2SO4 + 2NaOH
→
2NH3 + Na2SO4 + 2H2O
ammonia
gas
boric
acid
ammoniumborate complex
excess
boric acid
NH3 +
H3BO3
→
NH4 + H2BO
3
+
H3BO3
(color change occurs)
ammoniumborate
complex
sulfuric
acid
ammonium
sulfate
boric
acid
2NH4
+
H2BO
3
+
H2SO4 (NH4)2SO4
+
2H3BO3
(color change occurs in reverse)
Distillation: Excess base is added to the digestion product to convert NH4 to
NH3 as indicated in the following equation. The NH3 is recovered by
distilling the reaction product.
Titration quantifies the amount of ammonia in the receiving solution. The
amount of nitrogen in a sample can be calculated from the quantified amount
of ammonia ion in the receiving solution.
There are two types of titration—back titration and direct titration. Both
methods indicate the ammonia present in the distillate with a color change.
In the direct titration, boric acid is used as the receiving solution instead of a
standardized mineral acid, the chemical reaction is:
The boric acid captures the ammonia gas, forming an ammonium-borate
complex. As the ammonia collects, the color of the receiving solutions
changes.
The boric acid method has the advantages that only one standard solution is
necessary for the determination and that the solution has a long shelf life.
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