Application Note No. 191/2015 TKN determination in water and waste water

Application Note No. 191/2015
TKN determination in water and waste
water
KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376:
Colorimetric determination of TKN (Total Kjeldahl Nitrogen) in water and waste water according to
the Kjeldahl method
1 Introduction
A reliable method for the determination of total Kjeldahl nitrogen (TKN) in water and waste water,
according to ISO 5663, DIN EN 25 663, AOAC 973.48 and EPA 351.3, is introduced [1] [2] [3].
The samples are digested using the KjelDigester K-449. The distillation and boric acid titration are
performed with the KjelMaster K-375 with KjelSampler K-376. Equivalent to the international
norms, the detection principle of the boric acid titration is colorimetric. I.e. a mixed indicator
according to Sher is added to the boric acid solution and the KjelMaster K-375 is equipped with
the colorimetric sensor. The fast heating and cooling of the KjelDigester K-449, the synchronized
process steps of the KjelMaster system K-375 / K-376 combined with the titration mode “Online”
increases the sample throughput per work day.
2 Equipment








KjelDigester K-449 (the parameters used are also valid for the K-446)
User protection shield (BUCHI 11057889)
Scrubber K-415 TripleScrub ECO with TKN Set (BUCHI 11057333)
KjelMaster K-375 with colorimetric sensor
KjelSampler K-376 (the parameters used are also valid for the K-377)
Analytical balance (accuracy ± 0.1 mg)
Digestion rods, boiling aid (BUCHI 043087)
Volumetric pipettes
3 Chemicals and Materials
Chemicals:
 Sulfuric acid conc. 98 %, analytical reagent, Beijing Chemical Works
 Titanium, BUCHI Kjeldahl Tablet (11057980)
 Sodium hydroxide 32 %, analytical reagent, Sinoharm chemical reagent
 Boric acid 2 %: 100 g boric acid, (analytical reagent, Tianjin guangfu Fine chemical research
institute) diluted to 5 L with deionized water, dissolve completely, add 12.5 mL of mixed Sher
indicator, BUCHI (003512), adjust pH to 4.65
 Sulfuric acid 0.01 mol/L, Aladdin (S128556-1L)
 Neutralization solution for the Scrubber: 600 g sodium carbonate (analytical reagent, Sinoharm
chemical reagent) about 2 mL ethanol and a spatula tip of bromthymol blue, (analytical reagent,
Tianjin Kemiou Chemical Reagent) diluted to 3 L with distilled water
Urea, assay 99.999 %, Aladdin (U111899-25g)
For a safe handling please pay attention to all corresponding MSDS.
Samples:
 Urea stock solution, 1.0720 g Urea diluted to 1000 mL with deionized water  0.499 mg N/mL
 Surface water of a river (city moat, slightly turbid), Beijing China
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4 Procedure
The determination of nitrogen in water and waste water includes the following steps:
 Acidify the water sample with 0.5 mL sulfuric acid (conc.) per liter for preservation
 Digestion of the sample, using the KjelDigester K-449 (K-446 respectively)
 Distillation and colorimetric titration of the sample, using KjelMaster system K-375 / K-376
4.1 Digestion method – urea stock solution (verification of the method)
1. Start the KjelDigester K-449 according to the parameters listed in Table 1 and 2
2. Place different volumes of the urea stock solution in a 300 mL sample tube and dilute with
deionized water to the required volume (e.g. 4 mL of stock solution diluted with 21 mL
deionized water to total volume of 25 mL)
3. Add 1 Titanium Kjeldahl Tablet, 8 mL of sulfuric acid (conc. 98 %) and 1 digestion rod to
each tube (immerge the end with the void into the sample) to avoid bumping (!)
4. Prepare additional blanks, chemicals without sample, with the corresponding volume of
deionized water
5. Attach the “user protection shield” to the rack (it protects the user in case of boiling
delays)
6. Connect the Scrubber K-415 to the K-449 for absorbing the acid fumes created during
digestion
7. Insert the rack with the samples into the cooling position and mount the suction module
onto the samples, immediately start the digestion according to the parameters listed in
Table 2.
8. Let the samples cool down when the digestion is completed.
4.2 Digestion method – samples
1. Start the KjelDigester K-449 according to the parameters listed in Table 2
2. Place each sample in a 300 mL sample tube as described in Table 1
Table 1: Nitrogen content depending sample volume
TKN [mg/L]
50 - 100
20 - 50
10 - 20
Volume [mL]
25
50
100
0 - 10
200
3.
4.
5.
6.
7.
8.
Digestion Time [min]
50
70
90
160
Add 1 Titanium Kjeldahl Tablet, 8 mL of sulfuric acid (conc. 98%) and 1 digestion rod to
each tube (immerge the end with the void into the sample) to avoid bumping (!)
Prepare additional blanks, chemicals without sample, with the corresponding volume of
deionized water
Attach the “user protection shield” to the rack (it protects the user in case of boiling
delays)
Connect the Scrubber K-415 to the K-449 for absorbing acid fumes created during
digestion
Insert the rack with the samples into the cooling position and mount the suction module
onto the samples, immediately start the digestion according to the parameters listed in
Table 2.
Let the samples cool down when the digestion is completed.
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June 2015
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Table 2: Temperature profile for digestion with the K-449
Step
1
2
Cooling
Temperature [°C]
250
420
–
Time [min]
0
See Table 1
35
NOTE: If the liquid inside the sample tube is not clear and blue-green just after the digestion
finished, digest for additional 15 min at 420 °C.
4.3 Distillation and titration
For colorimetric titration it is necessary to determine the setpoint of the boric acid solution in
advance to the blank and sample determinations. It is necessary to determine the setpoint every
day before starting sample determinations, and when the method is changed or fresh chemicals
are used to adjust the device to the current conditions.
The detailed procedure including the preparation of the sensor is described in the Technical Note
179/2015 “Colorimetric titration procedure using Sher indicator” [4].
The setpoint was measured three times.



1st setpoint  preheating
2nd setpoint  1st measurement
3rd setpoint  2nd measurement, confirms the 1st measurement
The last setpoint measurement is used as endpoint for all following determinations including
priming, blanks and samples.
1. Determine the setpoint and check it`s range and deviation:
Select all paramters for the setpoint determination according to Table 3.
Table 3: Parameters for setpoint determination
Parameter
Preheating before setpoint
Setpoint runs
Setpoint cycle
Boric acid
Indicator
Method
Setting
yes
3
Via sampler
2%
Sher
Select the same method as for sample
determination
NOTE: The selected method, boric acid and indicator for setpoint determination must be
identical to the method used for sample determination!
2. Check the setpoint range and deviation

The determined setpoints should be in a range of 700 – 900 mV

The deviation between the two last measured setpoints should be ≤ 20 mV
3. Perform a priming to remove all residues.
4. Determine blanks according to the parameters listed in Table 5.
5. Determine samples according to the parameters listed in Table 5.
Table 4: Setpoint measurements and deviation
Setpoint 1
Setpoint 2
Deviation
807.5 mV
816.0 mV
8.5 mV
Table 5: Distillation and titration with the KjelMaster system K-375 / K-376
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H2O volume
NaOH volume
Reaction time
Distillation mode
Distillation time
Stirrer speed distillation
Steam output
Titration type
Receiving solution vol.
50 mL
60 mL
5s
Fixed time
180 s
5
100 %
Boric acid
60 mL
Titration solution
Sensor type
Titration mode
Titration start time
Measuring mode
Stirrer speed titration
Titration start volume
Titration algorithm
H2SO4 0.01 mol/L
Colorimetric
Online
90 s
Setpoint
10
0 mL
Optimal
NOTE: The sample throughput for this application was increased by using the “Online” titration
mode: By applying the “Online” titration the time for the distillation and titration process is
reduced to about 5 minutes per analysis because titration starts during the distillation is still in
progress.
NOTE: The titration start time must not be less than 60 seconds
4.4 Calculation
The results are calculated as TKN (Total Kjeldahl Nitrogen). For the urea stock solution, the
recovery was caslculated (2).
TKN actual 
(VSample - VBlank ) z c
Recovery 
f MN
m Sample
1000
TKN actual
*100
TKN theoretical
(1)
(2)
TKNactual : Total Kjeldahl nitrogen [mg/L]
VSample
: amount of titrant for the sample [mL]
VBlank
: mean amount of titrant for the blank [mL]
z
: molar valence factor (1 for HCl, 2 for H2SO4)
c
: titrant concentration [mol/L]
f
: titrant factor (for commercial solutions normally 1.000)
MN
: molecular weight of nitrogen (14.007 g/mol)
mSample
: sample volume [mL]
1000
: conversion factor [mL/L]
Recovery: of the urea stock solution [%]
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June 2015
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4.5 Limit of Detection (LOD) and Limit of Quantification (LOQ)
The “blank method” from DIN 32645 [5] was used for the determination of the detection limit
(LOD) and the quantification limit (LOQ). Based on these limits the performance of the method
can be evaluated.
First, ten blanks were determined using 200 mL deionised water, one Titanium Kjeldahl Tablet
and 8 mL sulfuric acid. The blanks were digested and determined according to the parameters
listed in Table 2 and 3, the results are presented in Table 6.
Table 6: Results of the blank determination (Volume of deionized water 200 mL)
VBlank [mL]
0.442
0.429
0.439
0.428
0.425
0.464
0.462
0.461
0.469
0.479
1
2
3
4
5
6
7
8
9
10
Average [mL]
SD [mL]
0.450
0.019
RSD [%]
4.32
The following equation (3) was used to calculate the limit of detection (LOD).
LOD = ᶲn;α • SD
(3)
LOD = 0.058 mL
ᶲn;α
: factor 3.0; depending on the number of blanks (n=10)
and the level of significance (α=0.01)
SD
: standard deviation of the blank determination (SD=0.019 mL)
Depending on the LOD, the limit of quantification (LOQ) can be calculated, see equotation (4).
LOQ = k • ᶲn;α • SD
(4)
LOQ = 0.175 mL
k
: factor 3 (for more information refer to DIN 32645)
According to DIN 32645, the LOD =0.081 mg/L and LOQ = 0.25 mg/L for 200 mL of sample could
be estimated.
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5 Results
5.1. Recovery of urea stock solution
The results of TKN determination and recovery for urea stock solution with different sample
volumes are presented in Tables 7-8.
Table 7: Results of the recovery of TKN in urea stock solution with a total sample volume of 25 mL
25 mL
VSample [mL]
TKNtheoretical [mg/L]
Sample 1.1
6.017
59.9
Sample 1.2
5.925
59.9
Sample 1.3
5.926
59.9
Average [%]
RSD [%]
The mean blank volume (VBlank) was 0.456 mL (n = 3).
TKNactual [mg/L]
60.68
60.56
60.44
-
Recovery [%]
101.3
101.1
100.9
101.1
1.0
Sample 2.1
9.275
99.9
Sample 2.2
9.155
99.9
Sample 2.3
9.113
99.9
Average [%]
RSD [%]
The mean blank volume (VBlank) was 0.251 mL (n = 3).
101.11
99.77
99.30
-
101.2
99.9
99.4
100.2
0.9
Table 8: Results of the recovery of TKN in urea stock solution with a total sample volume of 200 mL
200 mL
VSample [mL]
TKNtheoretical [mg/L]
Sample 1
0.745
0.50
Sample 2
0.731
0.50
Sample 3
0.776
0.50
Average [%]
RSD [%]
The mean blank volume (VBlank) was 0.397 mL (n = 3).
TKNactual [mg/L]
0.49
0.47
0.53
-
Recovery [%]
98.2
94.2
106.2
99.5
6.1
Sample 1
1.060
0.99
Sample 2
1.056
0.99
Sample 3
1.062
0.99
Average [%]
RSD [%]
The mean blank volume (VBlank) was 0.339 mL (n = 3).
1.01
1.00
1.01
-
101.1
100.1
101.1
100.8
0.6
5.2 TKN determination in water samples
The results of the TKN determination in samples of river surface water (city moat, slightly turbid,
Beijing, China), are presented in Table 9.
Table 9: Results of the TKN determination in surface water of a pond (Beijing, China) with total sample volume of 200 mL
VSample [mL]
TKNactual [mg/L]
Sample 1
1.051
1.00
Sample 2
0.999
0.92
Sample 3
1.041
0.98
Average [mg/L]
–
0.97
Rsd [%]
–
5.0
The mean blank volume (VBlank) was 0.339 mL (n = 3).
Acording to the AOAC and EPA regulations, these results correspond well to the low content of
Total Kjeldahl Nitrogen (TKN) in water and waste water with low relative standard deviations
(Rsd).
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6 Comparison to Standard Methods
The differences between this application and the official methods ISO 5663, AOAC 973.48 and
EPA 351.3 are shown in Table 10. The AOAC and EPA regulations are similar with minor
differences as indicated in Table 10.
Table 10: Differences to ISO 5663 and AOAC 973.48
Application note
ISO 5663
DIN EN 25 663
AOAC 973.48
EPA 351.3
5 g mixture
Sulfuric acid
3.7 g Tablets
Composition:
94.4 % K2SO4
2.8 % TiO2
2.8 %
CuSO4*5H2O
8 mL
Water
50 mL
250 ±50 mL
Sodium
hydroxide
60 mL
(conc. 32 %)
50 mL
(conc. 36 %)
Na2S2O3
no
no
100 ml digestion The choice of catalyst
does not influence
solution cont.
the result. No use of
13 g K2SO4
toxix Se/Hg!
0.2 g Hg
Easy handling using
tablets.
10 mL
No impact, same
ration of sulfuric
acid/catalyst.
300 mL
The K-375 generates
steam in a separated
vessel; therefore it is
not necessary to add
such a high amount
of water
60 mL
No impact, same
(conc. 50 %)
ratio of sodium
(EPA: 100 mL)
hydroxide/sulfuric
acid.
yes
Only necessary when
Hg is used as
catalyst
colorimetric
No impact, the results
(EPA:
are equal.
colorimetric &
potentiometic)
Methyl red /
No impact, Sher
methylene blue indicator has even a
more accurate
transition point
H2SO4
0.01 mol/L
Catalyst
Composition:
99.0 % K2SO4
1.0 % Se
10 mL
Titration
colorimetric
measurement
method
colorimetric
Indicator
Mixed indicator
according to
Sher
Methyl red /
methylene blue
Titration
solution
H2SO4
0.01 mol/L
HCl
0.02 mol/L
Application Note 191/2015
June 2015
Notes / Impact
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7 Conclusion
The determination of TKN (Total Kjeldahl Nitrogen) in water using the KjelDigester K-449, the
KjelMaster system K-375 / K-376 and the colorimetric sensor provides reliable and reproducible
results, even for nitrogen contents close to the limit of quantification. The recovery with urea stock
solution was very good with low standard deviation.
The LOD is 0.081 mg/L and the LOQ is 0.25 mg/L for 200 mL sample volume according to
DIN 32 645.
With the KjelDigester K-449 the digestion process (including preheating, digestion and cooling) is
very fast and is fully automated. Together with the fully-automatic KjelMaster system K-375/K-376
and the novel “Online” titration mode, the time to results is significantly reduced and it offers fully
walk-away convenience.
8 References
[1] ISO 5663 Water quality – Determination of Kjeldahl nitrogen – Method after mineralization
with selenium
[2] AOAC 973.48 Nitrogen (Total) in Water
[3] EPA 351.3 Nitrogen, Kjeldahl, Total (colorimetric or titrimetric or potentiometric)
[4] Technical Note No.179/2015 Colorimetric titration procedure using Sher indicator
[5] DIN 32 645 Nachweis-, Erfassungs- und Bestimmungsgrenze
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