SpeedDigester K-436, K-439 / KjelFlex K-360 Nitrogen Determination in Nitrate-Free Fertilizer according to the micro-kjeldahl Method 074/2011
074/2011 SpeedDigester K-436 / K-439 KjelFlex K-360 SHORT NOTE Nitrogen Determination in Nitrate-Free Fertilizer according to the micro-kjeldahl Method The following short note introduces a simple and fast procedure which helps to determine the content of nitrogen in nitrate-free fertilizer. The samples are digested with sulfuric acid using the SpeedDigester K-436 or K-439 equipped with the new micro- Kjeldahl sample tubes, followed by distillation and titration with KjelFlex K-360 and Metrohm Titrator 848 Titrino plus. The micromethod following AOAC 960.52 makes it possible to work with a reduced amount of chemicals needed for the determination thereby saving money and resources. The method can be adapted to homogeneous samples with a high nitrogen content. Introduction Nitrogen determination is one of the key analyses performed in quality control. For the determination progress it is necessary to digest the samples together with sulfuric acid in order to convert nitrogen into ammonium sulfate. After the conversion to ammonia through the alkalinization with sodium hydroxide, the sample is distilled into a boric acid receiver by steam distillation, then titrated with a sulfuric acid solution. Experimental Instrumentation: SpeedDigester K-436, K-439, KjelFlex K-360, Metrohm 848 Titrino plus Samples: Solid fertilizer for balcony plants, labelled nitrogen content 5 %. Figure 1: New micro-kjeldahl tube Determination: Approx. 0.2 g of the homogenized sample were weight into a micro-kjeldahl tube (see Figure 1). A portion of 3 ml of sulfuric acid and 1 Kjeldahl Tablet Titanium Micro were added, and the digestion was performed using the parameters specified in Table 1. After digestion, the ammonia of the sample was distilled into a boric acid solution by steam distillation and titrated with sulfuric acid (Table 2). The method was verified by using 50 mg glycine as the reference substance. Table 1: Temperature profile for digestion with the K-436, K-439 www.buchi.com K-439 K-436 Step Temp. [ C] Time [min] Level Time [min] Preheat 480-8.5 10 1 480 10 8.5 10 2 500 5 9.5 10 3 480 45 8.5 40 Cooling - 15-15 Table 2: Parameters for distillation and titration with KjelFlex K-360 and Metrohm 848 Titrino plus Distillation Water 25 ml Titration Solution Metrohm 848 Titrino plus Quality in your hands H 2SO 4 0.05 mol/l NaOH 15 ml Endpoint ph 4.65 Boric acid 2% 50 ml Titration Rate Optimal Reaction Time 5 sec Stop Crit. Drift Steam Power 100 % Stop Drift 20 µl/min Dist. Time 240 sec Stop Volume 40 ml Titration Start 240 sec Stop Time Off Titration Type Boric Acid Filling Rate Max. ml/min Stirrer Sp. Dist. 5 Stirrer Sp. Titr. 7 Results The glycine recoveries (n=4) were 100.3 %, rsd 0.4 % (K-439) and 99.7 %, rsd 0.3 % (K-436). The determined nitrogen contents are presented in Table 3. Table 3: Determined nitrogen contents in fertilizer (relative standard deviation in brackets, n = 4) Nitrogen content K-439 [%] Nitrogen content K-436 [%] Fertilizer 5% N 5.3 (0.7 %) 5.2 (0.9 %) Conclusion The determination of nitrogen content in nitrate free fertilizer according to the micro-kjeldahl method using SpeedDigester K-436, K-439, and KjelFlex K-360 with Metrohm Titrator 848 Titrino plus, provides reliable and reproducible results that correspond to the expected values with low relative standard deviations. The total digestion time is approx. 60 min. Compared to the standard Kjeldahl method 042/2010, the micro-kjeldahl method is an excellent alternative. It helps to speed up the digestion time and reduces the amount of chemicals needed, thereby saving money and resources. References Operation manual SpeedDigester K-425 / K-436 Operation manual SpeedDigester K-439 Operation manual KjelFlex K-360 For more detailed information please refer to Application Note 074/2011.
1 Introduction The micro-kjeldahl method according to AOAC 960.52 [1] helps to reduce the amount of chemicals used during the experiments and speeds up the digestion time significantly. It is suitable for homogeneous samples with a high nitrogen respectively protein content. With the micro-kjeldahl sample tubes, the insulation plate Micro, available for all SpeedDigesters, and the Kjeldahl tablets Titanium Micro, BUCHI now offers a complete system for micro-kjeldahl applications. Table 1 compares the micro-kjeldahl method to the standard Application Note 042/2010 [2]. Both application notes deal with nitrogen determination in fertilizer. Table 1: Comparison of standard-kjeldahl and micro-kjeldahl for nitrogen determination in nitrate free fertilizer Standard-Kjeldahl 042/2010 Micro-Kjeldahl 074/2011 Sample material 800 mg 200 mg Sulfuric acid (98 %) 20 ml 3 ml Titanium catalyst 2 x 5 g 1 x 1.56 g Digestion time 85-100 min 60 min Cooling time 30 min 15 min Water 80 ml 25 ml Sodium hydroxide (32 %) 80 ml 15 ml 2 Equipment SpeedDigester K-436, K-439 (the parameters used for K-436 are also valid for the SpeedDigester K-425) Insulation plate micro, BUCHI (11055204) Micro-Kjeldahl sample tubes (set of 4), BUCHI (11057442) KjelFlex K-360 Spacer to connect micro-kjeldahl tubes at the K-360, BUCHI (11058209) Titrator 848 Titrino plus (Metrohm) Connecting cable from Titrator to K-360 (11055333) Scrubber B-414 with condenser Analytical balance (accuracy ± 0.1 mg) 3 Chemicals and Materials Nitrogen free weighing paper Sulfuric acid conc 98 %, Fluka (84727) Kjeldahl Tablets Titanium Micro, BUCHI (11057981) Sodium hydroxide 32 %, Brenntag (81980-452) Boric acid 2 %, 100 g boric acid, Brenntag (80948-155) diluted to 5 l with deionized water, ph adjusted to 4.65 and 3 g/l potassium chloride (60132) Sulfuric acid 0.05 mol/l, Fluka (35358) standard solution 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, Merck (1.04201.0100; Assay: 99.7 %) Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 3/9
4 Samples Nitrate free fertilizer, labelled nitrogen content 5 % The samples were purchased at a local drugstore. 5 Procedure The determination of nitrogen in fertilizer includes the following steps: Homogenization of the sample with a mortar Digestion of the sample, using the SpeedDigester K-436, K-439 Distillation and titration of the sample, using KjelFlex K-360 and Metrohm 848 Titrino Plus 5.1 Digestion method - glycine (verification of the method) Place approx. 50 mg glycine in a micro sample tube (Use nitrogen free weighing paper) Add 1 Kjeldahl Tablet and 3 ml of sulfuric acid (98 %) Prepare additional blanks; chemicals without sample Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 in order to absorb the acid fumes created during digestion Insert the rack containing the samples into the preheated unit Digest the samples according to the parameters listed in Table 2 5.2 Digestion method samples Homogenization of the sample with a mortar Place 0.2 g of the sample into a micro sample tube (Use nitrogen free weighing paper) Add 1 Kjeldahl Tablet and 3 ml of sulfuric acid (98 %) Prepare additional blanks, chemicals without sample Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 in order to absorb the acid fumes created during digestion Insert the rack containing the samples into the preheated unit Digest the samples according to the parameters listed in Table 2 Table 2: Temperature profile for digestion with the K-436, K-439 Step K-439 K-436 Temperature [ C] Time [min] Heating Level Time [min] Preheating 480-8.5 10 1 480 10 8.5 10 2 500 5 9.5 10 3 480 45 8.5 40 Cooling - 15-15 If the liquid inside the sample tube is not clear and of a blue-green color, digest it for an additional 15 min as described in step 3 Let the samples cool down to room temperature NOTE: If the samples are placed in the cooling position it takes approx. 15 min to cool them down; if they are left in the heating chamber it takes at least 45 min. Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 4/9
5.3 Distillation and titration Distil and titrate the samples according to the parameters listed in Table 3. The determination of this note was carried out with the KjelFlex K-360 and Metrohm Titrator 848 Titrino plus. Table 3: Distillation and titration parameters Distillation Metrohm 848 Titrino plus Water 25 ml Titration Solution H 2SO 4 0.05 mol/l Sodium hydroxide 15 ml Endpoint ph 4.65 Boric acid 2 % 50 ml Titration Rate Optimal Reaction time 5 sec Stop Crit. Drift Distillation time 240 sec Stop Drift 20 µl/min Destillation start 240 sec Stop Volume 40 ml Steam power 100 % Stop Time Off Titration Type Boric acid Filling Rate Max. ml/min Stirrer speed dist. 5 Stirrer speed titr. 7 5.4 Calculation The results are calculated as percentage of nitrogen. The following equations (1) and (2) are used to calculate the results. For the reference substance, the purity of the glycine is considered in equation (3). w N (V = Sample - V m Blank Sample ) z c f M 1000 N (1) %N = w N 100 % (2) %NGly %N 100 P = (3) w N V Sample V Blank : weight fraction of nitrogen : 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 : titrant concentration [mol/l] f : titrant factor (for commercial solutions normally 1.000) M N m Sample : molecular weight of nitrogen (14.007 g/mol) : sample weight [g] 1000 : conversion factor [ml/l] %N : percentage of weight of nitrogen [%] Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 5/9
%N Gly : percentage of weight of nitrogen corrected for the purity of reference substance glycine [%] P : purity of the reference substance glycine [%] 6 Results 6.1 Digestion with SpeedDigester K-439 6.1.1 Recovery of glycine The results of the nitrogen determination and recovery in glycine are presented in Table 4. The nominal value of glycine (assay: 99.7 %) is 18.66 % of nitrogen. The recoveries are within the specification of 98 102 % [3]. Table 4: Results for the recovery of nitrogen in glycine with K-439 Glycine m Sample [g] V Sample [ml] %N Gly Recovery [%] Sample 1 0.0509 6.916 18.82 100.9 Sample 2 0.0519 7.006 18.70 100.2 Sample 3 0.0498 6.702 18.63 99.8 Sample 4 0.0530 7.150 18.69 100.2 Average - - 18.71 100.3 Rsd [%] - - 0.42 0.42 The mean blank volume for this sample was 0.098 ml (n = 4). 6.1.2 Nitrogen determination in nitrate free fertilizer The results of the determination of nitrogen in nitrate free fertilizer are presented in Table 5. Table 5: Results for the determination of nitrogen in fertilizer with K-439 (declared nitrogen content 5 %) Fertilizer m Sample [g] V Sample [ml] %N Sample 1 0.2117 8.032 5.25 Sample 2 0.2018 7.726 5.30 Sample 3 0.1956 7.372 5.21 Sample 4 0.2003 7.528 5.26 Average - - 5.25 Rsd [%] - - 0.68 The mean blank volume for this sample was 0.098 ml (n = 4). Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 6/9
6.2 Digestion with SpeedDigester K-436 6.2.1 Recovery of glycine The results of the nitrogen determination and recovery in glycine are presented in Table 6. The nominal value of glycine (assay: 99.7 %) is 18.66 % of nitrogen. The recoveries are within the specification of 98-102% [3]. Table 6: Results for the recovery of nitrogen in glycine with K-436 Glycine m Sample [g] V Sample [ml] %N Gly Recovery [%] Sample 1 0.0516 6.920 18.59 99.6 Sample 2 0.0567 7.592 18.58 99.6 Sample 3 0.0498 6.716 18.69 100.1 Sample 4 0.0493 6.612 18.58 99.6 Average - - 18.61 99.7 Rsd [%] - - 0.28 0.28 The mean blank volume for this sample was 0.092 ml (n = 4). 6.2.2 Nitrogen determination in nitrate free fertilizer The results of the determination of nitrogen in nitrate free fertilizer are presented in Table 7. Table 7: Results for the determination of nitrogen in fertilizer with K-436 (declared nitrogen content 5%) Fertilizer m Sample [g] V Sample [ml] %N Sample 1 0.1915 7.290 5.27 Sample 2 0.1977 7.564 5.29 Sample 3 0.1925 7.280 5.23 Sample 4 0.1968 7.378 5.19 Average - - 5.24 Rsd [%] - - 0.89 The mean blank volume for this sample was 0.092 ml (n = 4). Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 7/9
7 Comparison to AOAC 960.52 This application note is based on AOAC 960.52. Differences between this application note and AOAC 960.52 as well as their impact are discussed in Table 10. Table 10: Difference to the standard method AOAC 960.52 This application note AOAC 960.52 Notes/Impact Sample weight 0.05-0.2 g Weigh sample requiring 3-10 ml 0.01 or 0.02 N HCl The accuracy of the balance and the homogeneity of the sample affect the sample weight. Catalyst 1.56 g Tablets cont. - 94.3 % K 2SO 4-2.8 % TiO 2-2.8 % CuSO 4* 5 H 2O 1.9 ± 0.1 g K2SO4 40 ± 10 mg HgO Tablets are easy to handle especially in routine analytics. The choice of catalyst does not influence the result. No toxic mercury in use. Sulfuric acid 3 ml (Conc.: 98 %) 2 ± 0.1 ml (Conc.: 95-98 %) add additional 0.1 ml H 2SO 4 for each 10 mg dry organic matter >15 mg. The volume of sulfuric acid corresponds to the ratio of catalyst to sulfuric acid (1:2). Sodium hydroxide 15 ml (Conc.: 32 %) 8-10 ml sodium hydroxide-sodium thiosulfate solution (Conc. NaOH: 40 %) No impact. Same ratio of sodium hydroxide / sulfuric acid. Sodium hydroxide 32 % is more gentle to the pump than higher concentrated alkali. Without mercury catalyst, no sodium thiosulfate is necessary. Titration solution 0.05 mol/l H 2SO 4 0.02 mol/l HCl No impact. The consumption of titration solution is adapted. 8 Conclusion The determination of nitrogen in fertilizer with the help of the SpeedDigester K-436, K-439 and KjelFlex K-360 with 848 Titrino plus according to the micro-kjeldahl method provides reliable and reproducible results that correspond to the labelled values with low relative standard deviations. The results for the determination of nitrogen with K-439 were 5.25 % (rsd 0.68 %) and 5.24 % (rsd 0.89 %) for the K-436. The digestion is very fast taking 60 min for both digestion units. The recoveries with glycine were 100.3 % (K-439) and 99.7 % (K-436) respectively and are within the specification of 98 % [3]. Compared to the standard Kjeldahl method 042/2010, the micro-kjeldahl method is an excellent alternative for homogeneous samples with a high nitrogen content. It helps to speed up the digestion time and reduces the amount of chemicals needed for the determination thereby saving money and resources. Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 8/9
9 References [1] AOAC 960.52 [2] Application Note 042/2010 Nitrogen Determination in Nitrate Free Fertilizer according to AOAC 955.04-C (Kjeldahl Method) IR-Digester [3] Application Note 001-437_370-03C: Operational Quality Check Procedure Operation manual of SpeedDigester K-425 / K-436 Operation manual of SpeedDigester K-439 Operation manual of Scrubber B-414 Operation manual of KjelFlex K-360 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 Application Note 074/2011 Version 1, Copyright 2010 Büchi Labortechnik AG 9/9