Determination of Total Kjeldahl Nitrogen (TKN) in Water and Waste Water using 500 ml Sample Tubes

Transcription

1 049/2010 SpeedDigester K-436, K-439 / Kjeldahl Sampler System K-370/K-371 Determination of Total Kjeldahl Nitrogen (TKN) in Water and Waste Water using 500 ml Sample Tubes

2 049/2010 SpeedDigester K-436 / K-439 Kjeldahl Sampler System K-370/K-371 SHORT NOTE Determination of Total Kjeldahl Nitrogen (TKN) in Water and Waste Water using 500 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 , 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 Introduction 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: - 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 < 5 mg N/l 400 ml 185 min 5 10 mg N/l 250 ml 135 min mg N/l 100 ml 80 min mg N/l 50 ml 60 min mg N/l 25 ml 50 min > 100 mg N/l 10 ml 45 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 > 15 min => Table 1 9 => Table 1 Cooling Table 3: Parameters for distillation and titration using the Kjeldahl Sampler System K-370/K-371 Distillation Water Sodium hydroxide Reaction time Distillation time 50 ml 50 ml 1 s 300 s Steam power 100% Sample tube Stirrer sp. dist. 5 Stirrer sp. titr. 7 Results 500 ml Titration Boric acid 2% with KCl 3 g/l Titration solution Min. titration time Max. titration vol. Titration method Type 50 ml H2SO mol/l 5 s 40 ml ph 4.65 Algorithm 1 Standard Endpoint 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 = 3) and recovery of spiking with glycine Surface water from Lake Constance Recovery of spiking with mg N K-436 K mg/l (2.7) 0.24 mg/l (0.9) 101.7% (3.1) 101.1% (2.0) Water entering the waste water treatment 57.8 mg/l (1.2) 58.8 mg/l (0.5) plant Recovery of spiking with mg N Conclusion 101.6% (1.0) 99.9% (1.1) 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 Method PAI-DK01, Nitrogen, Total Kjeldahl, Block Digestion, Steam Distillation, Titrimetric Detection. Revised December 22, 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 049/2010 Quality in your hands

3 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 (500 ml configuration; 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 for 500 ml tubes ( ) Digestion rods to prevent bumping (043087) Rack 12 complete for 500 ml sample tube (043970) Volumetric pipettes 10, 20, and 25 ml Adjustable micropipettes 1 and 5 ml Graduated cylinder 25, 100, 250, and 500 ml Volumetric flask 250 and 1000 ml Analytical balance (accuracy ± 0.1 mg) 3 Chemicals 2 and Materials Sulfuric acid conc. 98%, N %, Fluka (84727) Catalyst: Mixture of 100 g potassium sulfate, Merck ( ), and 3 g cupric sulfate anhydrous, Merck ( ) Note: The use of powder as catalyst is recommended; the use of Kjeldahl tablets causes foaming. Sodium hydroxide 32%, Brenntag ( ) Boric acid 2% with KCl, 100 g boric acid, Brenntag ( ), 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; ml sulfuric acid 0.25 mol/l, Fluka (35355), diluted to l with deionized water. Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical, Synopharm ( ) 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: g/l Glycine stock solution 2: 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 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 3/13

4 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 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 390 ml deionized water for a sample volume of 400 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 500 (250ml) method for the 250 ml sample or to the parameters listed in Tables 1 and 2 Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 4/13

5 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 [ml] Time [min] < 5 mg N/l mg N/l mg N/l mg N/l mg N/l > 100 mg N/l 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 500 (250ml) method for the 250 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 > See Table 1 9 See Table 1 Cooling 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 beside the SpeedDigester (cooling position of the SpeedDigester can only be used for racks with 300 ml sample tubes), 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. Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 5/13

6 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 uncleaned 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 H 2 SO 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. Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 6/13

7 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 , blank method, are calculated with equation (5) and (6). TKN= (V S-titration -V Blank) z c f M N V Sample 1000 (1) m N(total)-TKN =(V S-titration -V Blank ) z c f M N - TKN V Sample 1000 (2) m N(actual) =(V S-titration -V Blank ) z c f M N (3) Recovery= (m N(total)) or (m N(actual) ) m N(theoretical) 100 (4) LOD = 3.2 sd z c M N (5) LOQ = 3 LOD (6) VS-titration VBlank : amount of titrant for the sample [ml] : mean amount of titrant for the blank [ml] z : molar valence factor (1 for HCl, 2 for H 2 SO 4 ) c : concentration of the titrant [mol/l] f : titrant factor (for commercial solutions normally 1.000) M N : molecular weight of nitrogen ( 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) mn(theoretical) LOD sd LOQ : determined amount of nitrogen in a sample tube [mg] : theoretical amount of nitrogen from spiking or stock solution [mg] : limit of detection of nitrogen absolute in a sample tube [mg] : standard deviation for the blanks (4 blanks) [ml] : limit of quantification of nitrogen absolute in a sample tube [mg] 3.2 : factor, depending on number of blank and sample replicates Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 7/13

8 6 Results 6.1 Digestion with SpeedDigester K 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 % (see Application Note _370-03C Operational Quality Check Procedure). Table 4: Results for the recovery of nitrogen in glycine with K-439 (n = 4 for blank and n = 3 for sample) TKN [mg/l] VSample mn(theoretical) mn(actual) Recovery rsd [%] theoretical [ml] [mg] [mg] [%] 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml) => LOD = mg N (= 0.04 mg N/l); LOQ = 0.04 mg N (= 0.1 mg N/l) Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 8/13

9 Table 6: Results for the determination of nitrogen in a 400 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml). Stock solution 1: γ glycine = g/l => 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml). Stock solution 2: γ glycine = g/l => mg N absolute in each sample tube Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 9/13

10 6.2 Digestion with SpeedDigester K 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 % (see Application Note _370-03C Operational Quality Check Procedure). Table 9: Results for the recovery of nitrogen in glycine with K-436 (n = 4 for blank and n = 3 for sample) TKN [mg/l] VSample mn(theoretical) mn(actual) Recovery rsd [%] theoretical [ml] [mg] [mg] [%] 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 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml) => LOD = mg N (= 0.02 mg N/l); LOQ = 0.02 mg N (= 0.05 mg N/l) Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 10/13

11 Table 11: Results for the determination of nitrogen in a 400 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml). Stock solution 1: γ glycine = g/l => 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 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = 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 Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 4, sd = ml). Stock solution 2: γ glycine = g/l => mg N absolute in each sample tube Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 11/13

12 7 Comparison to Standard Methods This application note corresponds to the standard methods. The minor differences are as follows: Table 14: Deviatiom from the standard methods This application ISO 5663 Method PAI-DK01 Reason, impact note 4500-NH 3 B and C Catalyst 4.85 g K 2 SO 4 5 g of mixture K 2 SO 4 The choice of catalyst g CuSO 4 of 1 kg K 2 SO 4 CuSO4 does not influence the and 10 g Se- result. No toxic Hg/Se. Pellets => environmentallyfriendly Sulfuric 10 ml 10 ml Concentrated 10 ml H 2 SO 4 are acid sufficient. More H 2 SO 4 also requires more NaOH. Water 50 ml 200 ml For 4500-NH3 B and C, no digestion procedure is needed, sample is directly distilled => chemical savings 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 No impact for the AN solution (Conc.: 32%) NaOH solution is (500 g/l = 36%) and ISO 5663; used for ph No extra NaOH is adjustment to 9.5 needed during distillation. Na 2 S 2 O 3 no no no Na 2 S 2 O 3 addition only addition necessary when Hg is to NaOH used as catalyst. solution Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 12/13

13 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 1.2%, and of surface water from Lake Constance was 0.23 mg/l with a rsd of 2.7%. The recoveries of glycine were > 99% and are within the specifications of Application Note _370-03C Operational Quality Check Procedure (98 102%). The detection limit according to DIN is mg N (for 400 ml sample 0.04 mg/l) and the quantification limit is 0.04 mg N (for 400 ml Sample 0.1 mg/l). 9 References [1] German Drinking Water Ordinance (TVO) [TrinkwV, 1993] - ISO Method PAI-DK01, Nitrogen, Total Kjeldahl, Block Digestion, Steam Distillation, Titrimetric Detection. Revised December 22, OI Analytical - APHA Standard method 4500 Ammonia - DIN : Operation manual of SpeedDigester K-425 / K Operation manual of SpeedDigester K Operation manual of Scrubber B Operation manual of Kjeldahl Sampler System K-370/K Application Note _370-03C Operational Quality Check Procedure BÜCHI Labortechnik AG CH-9230 Flawil 1/Switzerland T F Quality in your hands Application Note 049/2010 Version B, Copyright 2010 Büchi Labortechnik AG 13/13