SpeedDigester K-436, K-439 / Kjeldahl Sampler System K-370/K … · 2018-07-17 · 040/2010 SHORT...

040/2010

SpeedDigester K-436, K-439 / Kjeldahl Sampler System K-370/K-371

Determination of Total Kjeldahl Nitrogen (TKN) in Water and Waste Water using 300 ml Sample Tubes

040/2010

SHORT NOTE SpeedDigester K-436 / K-439 Kjeldahl Sampler System K-370/K-371

Determination of Total Kjeldahl Nitrogen (TKN) in Water and Waste Water using

300 ml Sample Tubes

The determination of total Kjeldahl Nitrogen (TKN) in water and waste water is a routine procedure used to ensure water quality. A simple and fast procedure for TKN determination as described in ISO 5663-1984, Method PAI-DK01, and APHA Standard method 4500 Ammonia, is introduced below. The sample is digested with sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with the Kjeldahl Sampler System K-370/K-371. Both this application note and methods referenced comply with 40 CFR 136.3.

Introduction

Table 3: Parameters for distillation and titration using the Kjeldahl Sampler System K-370/K-371

Total Kjeldahl Nitrogen (TKN) is a parameter for the evaluation of water and its pollution. The samples require digestion with sulfuric acid to convert nitrogen into ammonium sulfate. After conversion to ammonia through the alkalization with sodium hydroxide, the ammonia is distilled into a boric acid receiver via steam distillation, followed by a titration with sulfuric acid solution.

Experimental

Instrumentation: SpeedDigester K-436, K-439, with suction tube with condensate trap, digestion rods, Kjeldahl Sampler System K-370/K-371

Samples:

Distillation

Water 50 ml

Sodium 50 ml hydroxide

Reaction time 1 s

Distillation time 300 s

Steam power 100%

Sample tube 500 ml

Stirrer sp. dist. 5

Stirrer sp. titr. 7

Titration

Boric acid 2% 50 ml with KCl 3 g/l

Titration solution H2SO4

0.01 mol/l

Min. titration time 5 s

Max. titration vol. 40 ml

Titration method Standard

Type Endpoint

pH 4.65

Algorithm 1

- Stock solution with glycine - Surface water from Lake Constance - Water entering the waste water treatment

plant Uzwil/Switzerland

Table 1: Amount of sample and digestion time depend on the expected nitrogen content

Vsample Digestion time

< 10 mg N/l 200 ml 125 min

10 – 20 mg N/l 100 ml 85 min

20 – 50 mg N/l 50 ml 65 min

50 – 100 mg N/l 25 ml 55 min

> 100 mg N/l 10 ml 50 min

Determination: 5 g of the catalyst were added to each sample tube. The required amount of sample (as specified in Table 1) was placed into the sample tube. A portion of 10 ml of sulfuric acid was added and the digestion was performed using the parameters in Table 1 and 2. After digestion, the ammonia of the sample was distilled into a boric acid solution by steam distillation and then titrated with sulfuric acid (Table 3).

The method was verified by using glycine as the reference substance.

Table 2: Temperature profile for digestion with the K-439 and K-436

K-439 K-436

Step Temp. [°C] Time [min] Level Time [min]

Preheat 380 8.75 > 15 min

1 510 => Table 1 8.75 => Table 1

Cooling - 30 - 30

Results The results of the determination of TKN in the samples and spiked samples are presented in Table 4. Table 4: Determined TKN values in the real samples (relative standard deviation [%] in brackets, n = 4) and recovery of spiking with glycine

K-436 K-439

Surface water from 0.22 mg/l (6.5)

0.23 mg/l (6.3)

Lake Constance

Recovery of spiking with 99.8% (1.7)

100.2% (1.8)

0.248 mg N

Water entering the waste water treatment 58.3 mg/l (0.3) 58.3 mg/l (1.5) plant

Recovery of spiking with 100.9% (1.0)

100.8% (1.4)

0.930 mg N

Conclusion The determination of TKN in water and waste water using SpeedDigester K-436, K-439 and Kjeldahl Sampler System K-370/K-371 provides reliable and reproducible results.

References ISO 5663-1984 Method PAI-DK01, Nitrogen, Total Kjeldahl, Block Digestion, Steam Distillation, Titrimetric Detection. Revised December 22, 1994. OI Analytical APHA Standard method 4500 Ammonia

Operation manual SpeedDigester K-425 / K-436 Operation manual SpeedDigester K-439 Operation manual Kjeldahl Sampler System K-370/K-371 For more detailed information please refer to Application Note 040/2010

www.buchi.com Quality in your hands

1 Introduction

An easy and reliable method for the determination of total Kjeldahl nitrogen in water

and waste water is introduced below. The samples are digested using the

SpeedDigester K-436 and K-439. The distillation and boric acid titration are performed

with the Kjeldahl Sampler System K-370/K-371. According to the German Drinking

Water Ordinance [1], the maximum allowed value for TKN is 1 mg/l.

2 Equipment

−SpeedDigester K-439, K-436 (the parameters used for K-436 are also valid for SpeedDigester K-425)

−Scrubber B-414 with condenser and 4 liter condensate vessel − Kjeldahl Sampler System K-370/K-371 (or any other Büchi Kjeldahl distillation

unit)1

Suction tube with condensate trap (11055865) −Digestion rods to prevent bumping (043087) −Volumetric pipettes 10, 20, and 25 ml

−Adjustable micropipettes 1 and 5 ml

−Graduated cylinder 25, 100, and 250 ml

−Volumetric flask 250 and 1000 ml

−Analytical balance (accuracy ± 0.1 mg)

3 Chemicals2 and Materials

−Sulfuric acid conc. 98%, N ≤ 0.0005%, Fluka (84727) −Catalyst: Mixture of 100 g potassium sulfate, Merck (1.05153.5000), and 3 g cupric

sulfate anhydrous, Merck (1.02791.0250) Note: The use of powder as catalyst is recommended; the use of Kjeldahl tablets

causes foaming.

−Sodium hydroxide 32%, Brenntag (81980-452)

−Boric acid 2% with KCl, 100 g boric acid, Brenntag (80948-155), and 15 g KCl, Fluka (60132), were diluted to 5 l with deionized water, pH adjusted to 4.65

−Sulfuric acid 0.01 mol/l; 40.00 ml sulfuric acid 0.25 mol/l, Fluka (35355), diluted to 1.000 l with deionized water.

−Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical, Synopharm (0179420) about 2 ml ethanol and a spatula tip of bromthymol blue,

Fluka (18460) diluted to 3 l with distilled water −Glycine, Riedel de Haën (33226; assay: 100%)

−Glycine stock solution 1: 1.3292 g/l

−Glycine stock solution 2: 9.9712 g/l

1 When using the K-370/371, the sample tubes and digestion rods can be used for the next digestion directly after distillation without any cleaning.

2 It is important that the same chemicals are used for blanks and samples, because of

the possibility of nitrogen contaminations (e.g., 10 ml sulfuric acid can have contaminations up to 0.1 mg N).

Application Note 040/2010 Version B, Copyright © 2010 Büchi Labortechnik AG 3/13

4 Samples

−Glycine stock solution 1 and 2 −Surface water from Lake Constance (contained suspended particles, 0.1 ml sulfuric

acid 98% was added to each liter of sample for preservation) −Water entering the waste water treatment plant Uzwil/Switzerland (contained

suspended particles, 0.5 ml sulfuric acid 98% was added to each liter of sample for preservation)

Figure 1: Sample taken at the entry of waste water treatment plant

5 Procedure

The determination of total Kjeldahl nitrogen includes the following steps:

−Homogenization of sample through intensive shaking

−Digestion of the sample using SpeedDigester K-436, K-439 −Distillation and titration of the sample using

Kjeldahl Sampler System K-370/K-371

5.1 Digestion method using glycine stock solution (method verification)

−Add 5 g of catalyst to each sample tube −Place one digestion rod in each sample tube to prevent bumping −Place 1 (for 0.25 mg N), 2 (for 0.5 mg N), 4 (for 1 mg N), or 10 (for 2.5 mg N) ml

glycine stock solution 1 into the sample tube using volumetric or micropipettes −Dilute the stock solution with deionized water to the required volume (e.g., dilute 10

ml of stock solution by adding 190 ml deionized water for a sample volume of

200 ml, see Table 1) −Add a portion of 10 ml of sulfuric acid (98%) −Prepare additional blanks, only chemicals and deionized water without stock solution −Suspend the sample carefully by gently swirling the tube −Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 to absorb the acid

fumes created during digestion

−Insert the rack containing the samples into the preheated unit −Digest the samples according to the “tkn 300 (100ml)” method for the 100 ml sample

or to the parameters listed in Tables 1 and 2


5.2 Sample digestion method

−Add 5 g of catalyst to each sample tube −Place one digestion rod in each sample tube to prevent bumping −Place the required sample amount (as specified in Table 1) in the sample tube using

a graduated cylinder (if the sample contains suspended particles) or volumetric

pipettes

Table 1: Amount of sample and digestion time depend on the expected nitrogen content

Sample amount Digestion time

[ml] [min]

< 10 mg N/l 200 125

10 – 20 mg N/l 100 85

20 – 50 mg N/l 50 65

50 – 100 mg N/l 25 55

> 100 mg N/l 10 50

−Add a portion of 10 ml of sulfuric acid (98%) −Prepare additional blanks, only chemicals and deionized water without sample

−Suspend the sample carefully by gently swirling the tube −Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 to absorb the acid

fumes created during digestion

−Insert the rack containing the samples into the preheated unit −Digest the samples according to the “tkn 300 (100ml)” method for the 100 ml sample

or to the parameters listed in Tables 1 and 2

Table 2: Digestion parameters for SpeedDigester K-436, K-439

K-439 K-436

Step Temperature Time Heating Level Time

[°C] [min] [min]

Preheating 380 8.75 > 15

1 510 See Table 1 8.75 See Table 1

Cooling - 30 - 30

−If the liquid inside the sample tube is not clear and blue-green, digest for an additional 30 min as described in step 1

−Let the samples cool down to ambient temperature

NOTE: When the samples are placed in the cooling position, it takes approx. 30 min to cool them down, when they are left in the heating chamber, it takes at least 60 min.

NOTE: If a sample tends to foam add a small amount of stearic acid to prevent

foaming.

NOTE: If samples have a high organic matrix, add an additional 1 to 5 ml of sulfuric

acid to the samples and blanks. In this case, you also need to increase the amount of

sodium hydroxide (for each ml sulfuric acid, add 4 ml sodium hydroxide 32%) for

distillation.


NOTE: Rinse the suction tube after each digestion with water and deionized water.

Acid residual in the suction tube can accumulate alkaline nitrogen (e. g., ammoniac)

from the air. Measurements showed a high impact on the blank value and/or the

samples, corresponding to an amount of 0.05 mg of nitrogen or more when using un-

cleaned suction tubes.

To spike samples with glycine stock solution, follow the procedure described in 5.2

“Sample digestion method”. Add the required amount of stock solution to the sample

tube after adding the sample.

5.3 Distillation and titration Distill the samples according to the parameters listed in Table 3 Table 3: Distillation and titration with the Kjeldahl Sampler System K-370/K-371

Distillation Titration

Water 50 ml Boric acid 2% with 50 ml

KCl 3 g/l

Sodium hydroxide 50 ml Titration solution H2SO4 0.01 mol/l

Reaction time 1 s Min. titration time 5 s

Distillation time 300 s Max. titration volume 40 ml

Steam power 100% Titration method Standard

Sample tube 500 ml Type Endpoint

Stirrer speed dist. 5 pH 4.65

Stirrer speed titr. 7 Algorithm 1

Set result units on the K-370 for “Result 1” to “mg N/l” so that you will get a result with 2 decimal places (Select Configurator/Configuration/Device Settings/Result Units).

NOTE: Distill the samples as soon as possible after digestion. Storage of digested

samples overnight can lead to higher variation of blanks and samples and higher

blank values due to airborne contamination.


5.4 Calculation

The results are calculated as milligram of nitrogen per liter sample. The following

equations (1), (2), (3), and (4) are used to calculate the results. Detection limit (LOD)

and quantification limit (LOQ) according to DIN 32 645, “blank method”, are calculated

with equation (5) and (6).

TKN=(V

S-titration-VBlank)∙ z ∙ c ∙ f ∙ MN

VSample∙1000 (1)

mN(total)-TKN=(VS-titration-VBlank)∙z ∙c ∙f ∙ MN-TKN∙VSample

1000 (2)

mN(actual)=(VS-titration-VBlank)∙z ∙c ∙f ∙ MN (3)

Recovery=(mN(total)) or (mN(actual))

mN(theoretical)∙100 (4)

LOD = 3.2·sd·z·c·MN (5)

LOQ = 3·LOD (6)

TKN : Total Kjeldahl Nitrogen [mg/l]

VS-titration : 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 : concentration of the titrant [mol/l]

f : titrant factor (for commercial solutions normally 1.000)

MN : molecular weight of nitrogen (14.007 g/mol)

VSample : sample volume [ml]

1000 : conversion factor [mg/g] or [ml/l]

mN(total)-TKN : determined amount of nitrogen from spiking [mg]

Recovery : recovery of the spiked amount of nitrogen [%]

mN(actual) : determined amount of nitrogen in a sample tube [mg]

mN(theoretical) : theoretical amount of nitrogen from spiking or stock solution [mg]

LOD : limit of detection of nitrogen absolute in a sample tube [mg]

sd : standard deviation for the blanks (4 blanks) [ml]

LOQ : limit of quantification of nitrogen absolute in a sample tube [mg]

3.2 : factor, depending on number of blank and sample replicates


6 Results

6.1 Digestion with SpeedDigester K-439

6.1.1 Recovery of glycine stock solution

The results of the nitrogen determination and recovery of glycine stock solution are

presented in Table 4. The recoveries are within the specification of 98 – 102% (see

Application Note 001-437_370-03C Operational Quality Check Procedure).

Table 4: Results for the recovery of nitrogen in glycine with K-439 (n = 4 for blank and sample)

TKN [mg/l] VSample mN(theoretical) mN(actual) Recovery rsd [%] theoretical [ml] [mg] [mg] [%]

4.96 200 0.992 0.991 99.9 0.50

9.92 100 0.992 0.993 100.1 0.17

12.4 200 2.48 2.48 99.9 0.16

19.8 50 0.992 0.989 99.7 0.40

24.8 100 2.48 2.48 100.0 0.18

39.7 25 0.992 0.989 99.6 0.47

49.6 50 2.48 2.48 100.1 0.34

99.2 10 0.992 0.992 100.0 0.07

99.2 25 2.48 2.48 100.0 0.12

248 10 2.48 2.48 100.0 0.20

6.1.2 TKN determination in water samples

The results of the determination of nitrogen in surface water from Lake Constance

and the spiked sample are presented in Tables 5 - 6.

Table 5: Results for the determination of nitrogen in surface lake water with K-439

Surface water from Lake VSample [ml] VS-titration [ml] TKN [mg/l] Constance

Sample 1 200 0.509 0.24

Sample 2 200 0.498 0.23

Sample 3 200 0.490 0.22

Sample 4 200 0.513 0.25

Average - - 0.23

Rsd [%] - - 6.3 The mean blank volume for this sample was 0.335 ml (n = 4, sd = 0.009 ml) => LOD = 0.008 mg N (= 0.04 mg N/l); LOQ = 0.024 mg N (= 0.1 mg N/l)


Table 6: Results for the determination of nitrogen in a 200 ml sample of surface lake water with K-439, spiked with glycine

Surface water from Lake VSpiking [ml] VS-titration [ml] mN(total) - TKN Recovery Constance [mg] [%]

Sample 1 1 1.386 0.248 99.8

Sample 2 1 1.390 0.249 100.2

Sample 3 1 1.371 0.243 98.1

Sample 4 1 1.410 0.254 102.5

Average - - 0.248 100.2

Rsd [%] - - 1.8 1.8 The mean blank volume for this sample was 0.335 ml (n = 4, sd = 0.009 ml). Stock

solution 1: γglycine = 1.3292 g/l => 0.248 mg N absolute in each sample tube

The results of the determination of nitrogen in water entering the waste water

treatment plant Uzwil (Switzerland) and the spiked sample are presented in Tables 7 -

8.

Table 7: Results for the determination of nitrogen in water entering the waste water treatment plant Uzwil with K-439

Water entering the waste VSample [ml] VS-titration [ml] TKN [mg/l] water treatment plant Uzwil

Sample 1 25 5.610 58.4

Sample 2 25 5.647 58.8

Sample 3 25 5.656 58.9

Sample 4 25 5.484 57.0

Average - - 58.3

Rsd [%] - - 1.5 The mean blank volume for this sample was 0.400 ml (n = 4, sd = 0.032 ml).

Table 8: Results for the determination of nitrogen in a 25 ml sample of water entering the waste water treatment plant Uzwil with K-439, spiked with glycine

Water entering the waste VSpiking [ml] VS-titration [ml] mN(total) - TKN Recovery water treatment plant Uzwil [mg] [%]

Sample 1 0.5 8.901 0.925 99.4

Sample 2 0.5 8.978 0.947 101.7

Sample 3 0.5 8.908 0.927 99.6

Sample 4 0.5 8.995 0.951 102.3

Average - - 0.937 100.7

Rsd [%] - - 1.4 1.4 The mean blank volume for this sample was 0.400 ml (n = 4, sd = 0.032 ml). Stock



6.2 Digestion with SpeedDigester K-436

6.2.1 Recovery of glycine stock solution

The results of the nitrogen determination and recovery of glycine stock solution are

presented in Table 9. The recoveries are within the specification of 98 – 102% (see

Application Note 001-437_370-03C Operational Quality Check Procedure).

Table 9: Results for the recovery of nitrogen in glycine with K-436 (n = 4 for blank and sample)

TKN [mg/l] VSample mN(theoretical) mN(actual) Recovery rsd [%] theoretical [ml] [mg] [mg] [%]

1.24 200 0.248 0.251 101.3 1.1

2.48 200 0.496 0.499 100.5 0.50

4.96 200 0.992 0.997 100.5 0.55

9.92 100 0.992 0.989 99.7 0.29

12.4 200 2.48 2.49 100.5 0.06

19.8 50 0.992 0.988 99.5 0.62

24.8 100 2.48 2.47 99.7 0.61

39.7 25 0.992 0.992 100.0 0.58

49.6 50 2.48 2.49 100.2 0.26

99.2 10 0.992 0.990 99.7 0.42

99.2 25 2.48 2.49 100.4 0.18

248 10 2.48 2.49 100.2 0.04

6.2.2 TKN determination in water samples

The results of the determination of nitrogen in surface water from Lake Constance and the spiked sample are presented in Tables 10 - 11.

Table 10: Results for the determination of nitrogen in surface lake water with K-436

Surface water from Lake VSample [ml] VS-titration [ml] TKN [mg/l] Constance

Sample 1 200 0.516 0.21

Sample 2 200 0.516 0.21

Sample 3 200 0.513 0.21

Sample 4 200 0.535 0.24

Average - 0.22

Rsd [%] - 6.5 The mean blank volume for this sample was 0.365 ml (n = 4, sd = 0.018 ml) => LOD = 0.016 mg N (= 0.08 mg N/l); LOQ = 0.048 mg N (= 0.2 mg N/l)


Table 11: Results for the determination of nitrogen in a 200 ml sample of surface lake water with K-436, spiked with glycine

Surface water from Lake VSpiking [ml] VS-titration [ml] mN(total) - TKN Recovery Constance [mg] [%]

Sample 1 1 1.386 0.243 97.8

Sample 2 1 1.417 0.251 101.3

Sample 3 1 1.414 0.250 101.0

Sample 4 1 1.397 0.246 99.1

Average - 0.248 99.8

Rsd [%] - 1.7 1.7 The mean blank volume for this sample was 0.365 ml (n = 4, sd = 0.018 ml). Stock


The results of the determination of nitrogen in water entering the waste water

treatment plant Uzwil (Switzerland) and the spiked sample are presented in Tables 12

- 13.

Table 12: Results for the determination of nitrogen in water entering the waste water treatment plant Uzwil with K-436

Water entering the waste VSample [ml] VS-titration [ml] TKN [mg/l] water treatment plant Uzwil

Sample 1 25 5.616 58.1

Sample 2 25 5.649 58.5

Sample 3 25 5.636 58.4

Sample 4 25 5.630 58.3

Average - 58.3

Rsd [%] - 0.3 The mean blank volume for this sample was 0.428 ml (n = 4, sd = 0.012 ml).

Table 13: Results for the determination of nitrogen in a 25 ml sample of water entering the waste water treatment plant Uzwil with K-436, spiked with glycine

Water entering the waste VSpiking [ml] VS-titration [ml] mN(total) - TKN Recovery water treatment plant Uzwil [mg] [%]

Sample 1 0.5 8.944 0.928 99.7

Sample 2 0.5 8.974 0.936 100.6

Sample 3 0.5 8.999 0.943 101.4

Sample 4 0.5 9.020 0.949 102.0

Average - 0.939 100.9

Rsd [%] - 1.0 1.0 The mean blank volume for this sample was 0.428 ml (n = 4, sd = 0.012 ml). Stock



7 Comparison to Standard Methods

This application note corresponds to the standard methods. The minor differences

are as follows:

Table 14: Deviation from the standard methods

This application ISO 5663 Method PAI-DK01 Reason, impact

note 4500-NH3 B and C

Catalyst 4.85 g K2SO4 5 g of mixture K2SO4 The choice of catalyst

+ 0.15 g CuSO4 of 1 kg K2SO4 CuSO4 does not influence the and 10 g Se- result. No toxic Hg/Se. Pellets => environmentally-

friendly

Sulfuric 10 ml 10 ml Concentrated 10 ml H2SO4 are

acid sufficient. More H2SO4

also requires more

NaOH.

=> chemical savings

Water 50 ml 200 ml For 4500-NH3 B and C, no digestion procedure is needed, sample is directly distilled

The K-370 generates

steam in a separate

vessel; therefore, it is

not necessary to add

such a high amount of

water to the digested

sample as described in

the standard methods.

NaOH 50 ml 50 ml 4500-NH3: 6 N NaOH solution is used for pH adjustment to 9.5

No impact for the AN

solution (Conc.: 32%) (500 g/l = 36%) and ISO 5663;

No extra NaOH is

needed during

distillation.

Na2S2O3 no no no Na2S2O3 addition only

addition necessary when Hg is

to NaOH used as catalyst.

solution


8 Conclusion

The determination of total Kjeldahl nitrogen in water samples using the SpeedDigester

K-436, K-439, and Kjeldahl Sampler System K-370/371 provides reliable and

reproducible results with low relative standard deviations. The determined TKN of

water collected at the entry of the waste water treatment plant (contains a big amount

of suspended particles, see Fig. 1 on Page 4) was 58.3 mg/l with a rsd of ≤ 1.5%, and

of the surface water from Lake Constance was 0.23 mg/l with a rsd of ≤ 6.5%. The recoveries of glycine were > 99% and are within the specifications of Application

Note 001-437_370-03C ‘Operational Quality Check Procedure’ (98 – 102%). The

detection limit according to DIN 32 645 is ≤ 0.016 mg N (for 200 ml sample ≤ 0.08 mg/l) and the quantification limit is ≤ 0.048 mg N (for 200 ml sample ≤ 0.2 mg/l).

9 References

[1] German Drinking Water Ordinance (TVO) [TrinkwV, 1993]

- ISO 5663-1984 (E) - Method PAI-DK01, Nitrogen, Total Kjeldahl, Block Digestion, Steam Distillation,

Titrimetric Detection. Revised December 22, 1994. OI Analytical - APHA Standard method 4500 Ammonia - DIN 32 645:2008-11

- Operation manual of SpeedDigester K-425 / K-436 - Operation manual of SpeedDigester K-439 - Operation manual of Scrubber B-414 - Operation manual of Kjeldahl Sampler System K-370/K-371 - Application Note 001-437_370-03C ‘Operational Quality Check Procedure’

BÜCHI Labortechnik AG

CH-9230 Flawil 1/Switzerland

T +41 71 394 63 63

F +41 71 394 65 65

www.buchi.com Quality in your hands


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