The Kjeldahl Process From sample preparation to result calculation www.buchi.com Quality in your hands The Kjeldahl Process Introduction At the time when Johan Kjeldahl published his method for the determination of nitrogen in 1883 the electric lamp was just patented and the technical age in its childhood. Seldom in human history has an invention remained basically unchanged for such a long time as Kjeldahl’s method for nitrogen determination. As in 1883 a Kjeldahl nitrogen determination starts with sample preparation, proceeds to the digestion followed by separation using distillation and subsequent volumetric determination of the amount of ammonia formed in the process. Kjeldahl’s visionary idea of providing a simple method for nitrogen and protein determinations, which can also be carried out by non-academic lab personnel, has been put into practice by BUCHI’s Kjeldahl systems since 1961. With this short guide BUCHI would like to support you in your daily work by not only providing high quality instrumentation but also offering theoretical background information and useful tables for your daily routine. The laminated tables can be taken out of the guide to be placed at the locations in the lab where they are needed. For more detailed information please refer to the BUCHI Kjeldahl Guide. www.buchi.com/kjeldahl/en/applications/literature/ In addition to this short guide you could find detailed information including our application database at: www.buchi.com/kjeldahl/applications And to download BUCHI’s practical tablet guide or Kjeldahl calculator for iOS, Android and Windows Phone 7: www.buchi.com/kjeldahl/applications 2 The Kjeldahl Process From sample preparation to result calculation ∙∙ ∙∙ ∙∙ Sample preparation ∙∙ Grinding Sample tube size Weighing table Kjeldahl Tablets Samples must be homogeneous. The actual weight of a sample depends on the nitrogen content as well as on the inhomogeneity of the sample. ∙∙ Digestion: (CHNO) + H2SO4 → CO2 + SO2 + H2O + NH4+ Digestion ∙∙ Digestion parameters ∙∙ Neutralization/Alkalinization: H2SO4 + 2 NaOH → 2 Na+ + SO4 2- + 2 H2O Distillation ∙∙ Distillation: NH4+ + OH ⇌ NH3 (gas) + H2O ∙∙ Receiver: B(OH)3+ NH3+ H2O ⇌ NH4+ + B(OH)4– Titration Result ∙∙ Titration: – B(OH)4– + HX → X + B(OH)3 + H2O ∙∙ Calculation ∙∙ LIMS ∙∙ KjelLink Organic matter is destroyed by boiling in concentrated sulfuric acid. Kjeldahl Tablets raise the boiling point and accelerate the process. The digestion mixture is alkalized with NaOH prior to distillation to free up the ammonia. The ammonia is steam distilled into an acidic receiver solution. The pH in the acidic receiver solution rises upon addition of ammonia. The nitrogen and protein content is then determined by titration of the borate complex. The nitrogen content is then calculated. To calculate the protein content the nitrogen is multiplied by a sample specific proteinfactor. 3 Sample preparation Sample tube selection and weight Micro Weight < 0.2 g Sample volume 2 – 3 mL H2SO4 2 – 5 mL Kjeldahl Tablets 1 (Micro) Recommendation Homogeneous samples high in nitrogen/protein Benefit Reduction of chemicals of ca. 80 % compared to 300 mL tubes 300 mL Weight 0.1 – 5 g Sample volume < 200 mL H2SO4 5 – 30 mL Kjeldahl Tablets 2 Recommendation Standard tube for most applications Benefit All round sample tube 500 mL 4 Weight >4g Sample volume < 400 mL H2SO4 > 10 mL Kjeldahl Tablets 2 Recommendation Especially for high sample volumes or strongly foaming samples Benefit Problem-free digestion of strongly foaming samples Sample preparation Weighing The actual weight depends on Sample preparation ∙∙ The protein or rather the nitrogen content of the sample ▶ the higher the N-content, the lower the weight can be ∙∙ The Homogeneity of the sample ▶ By increasing the sample amount the impact of the grain size is reduced ▶ Example: homogeneous samples < 1 g, inhomogeneous samples > 2 g ∙∙ The concentration of the titrant ▶ The consumption of the titrant should be in a range of 3 and 17 mL (optimal accuracy of measurement when using a 20 mL burette) 5 Sample preparation Usage of the weighing table Example for the usage of the weighing table 1 Expected %N of the sample must be selected (here 2 %) 2 Selection of the titrant concentration used (e.g. 0.05 mol/L) 3 Determination of the expected titrant consumption in mL ▶ here 3.6 and 14.3 mL 4 Result: For samples containing 2% N and with titrant concentration of 0.05 mol/L, the expected consumption should be in a range of 3 – 17 mL. Therefore the weight must be between 0.125 and 0.5 g. Sample: weight [g] 5 2 1 4 0.5 0.125 Titrant conc.: [mol/L] 0.01 N [mg] per glas N [%] 0.5 0.01 0.03 0.05 0.10 0.40 3.6 2.0 0.04 0.10 0.20 0.40 1.60 14.3 2.5 0.05 0.13 0.25 0.50 2.00 7.0 0.14 0.35 0.70 1.40 5.60 10.0 0.20 0.50 1.00 2.00 8.00 50.0 1.00 2.50 5.00 10.00 40.00 100.0 2.00 5.00 10.00 20.00 80.00 0.05 0.1 0.5 Titrant consumption for sample [mL] 1 1 2 2.9 3.6 10.0 1 14.3 3 3 X 1.8 5.0 7.1 1.4 7.1 14.3 3 The limit of determination is 0.02 mg N per sample tube. However, optimal would be nitrogen content of 1 - 200 mg per sample tube. 6 Sample preparation Addition of chemicals and Kjeldahl Tablets Sample preparation Addition of chemicals, rule-of-the-thumb: ∙∙ sulfuric acid: 2 mL H2SO4 per 1 g catalyst ∙∙ Kjeldahl Tablets: frequent practice 2 pieces per sample tube The aim of the Kjeldahl Tablets is the acceleration of the digestion process by means of: ∙∙ catalysis by metal salts ∙∙ raising the boiling point of the H2SO4 by sulfate salts (K 2SO4) The selection of Kjeldahl Tablet depends on: For problem-free samples: 1. 2. 3. 4. Ideal digestion conditions are ∙∙ Boiling point at 370 °C ∙∙ No nitrogen losses ∙∙ Minimal time needs Safety aspects Digestion time Ecological aspects Foam formation of the sample www.buchi.com/kjeldahl/application These are achieved with ▶ 2 mL H2SO4 to 1 g catalyst For more demanding samples or samples that are high in fat or carbohydrate see page 10. 7 Sample preparation Weighing table for solid and liquid samples Solid samples Sample: weight [g] 5 2 1 Titrant concentration [mol/L] 0.5 0.125 N [mg] N [%] 0.01 0.05 0.1 0.5 Titrant consumption sample [mL] 0.5 0.01 0.03 0.05 0.10 0.40 3.6 2.0 0.04 0.10 0.20 0.40 1.60 14.3 2.5 0.05 0.13 0.25 0.50 2.00 3.6 1.8 7.0 0.14 0.35 0.70 1.40 5.60 10.0 5.0 10.0 0.20 0.50 1.00 2.00 8.00 14.3 7.1 50.0 1.00 2.50 5.00 10.00 40.00 7.1 100.0 2.00 5.00 10.00 20.00 80.00 14.3 2.9 1.4 Procedure: A: Select N % of sample B: select titrant concentration C: Choose weight in order that the titrant consumption can be expected between 3 and 17 mL Liquid samples Sample [mL] Titrant N [%] N mg/L Titrant [N] 4 0.1 – 0.6 0.10 6 0.06 – 0.4 0.10 10 100 – 200 0.01 25 50 – 100 0.01 50 20 – 50 0.01 100 10 – 20 0.01 250 5 – 10 0.01 400 <5 0.01 8 Sample preparation Kjeldahl Tablets overview Article Composition Titanium # 11057980 3.5 g K2SO4 / 0.105 g CuSO4 • 5 H2O 0.105 g TiO2 Benefit: Recommendation: Time saving Optimal compromise between environmental and performance priorities. Titanium Micro 1.5 g K2SO4 / 0.045 g CuSO4 • 5 H2O 0.045 g TiO2 # 11057981 3.71 g 1.59 g Benefit: Recommendation: Reduced chemical amount Same as Titanium (11057980) but for semi-micro & microKjeldahl applications. Missouri # 11057982 4.98 g K2SO4 0.02 g CuSO4 • 5 H2O Benefit: Recommendation: Easy to use and universally applicable The digestion with Missouri is more eco-friendly ECO # 11057983 3.998 g K2SO4 0.002 g CuSO4 Benefit: Recommendation: Eco-friendly Most environmentally friendly catalyst, due to the very low copper content Antifoam # 11057984 0.97 g Na2SO4 0.03 g Silikon Schaumhemmer Benefit: Recommendation: Maximum foam reduction Used as general purpose foam suppressant. This tablet has to be combined with Titanium Micro (11057981) or Copper Micro (11057985). Copper Micro # 11057985 1.5 g K2SO4 0.15 g CuSO4 • 5 H2O Benefit: Recommendation: 9 Weight 5g 4g 1g 1.65 g Reduced chemical amount Combo tablets for Antifoam or micro Kjeldahl applications. Sample preparation Amount of sulfuric acid The amount of H2SO4 is given by: 1. Conversion of K2SO4 to KHSO4 (K2SO4 is a component of Kjeldahl Tablets) ca. 2 – 3 mL 2. Consumption by organic matter Organic matter H2SO4/ g [mL] Example: Salami e.g. for 1.5 g weight (weight ∙ org. matter): Fat 9.7 27.3 % 1.5 ∙ 9.7 ∙ 27.3 = 3.97 mL 100 Protein 4.9 20.6 % 1.5 ∙ 4.9 ∙ 20.6 = 1.51 mL 100 Carbohydrates 4.0 0.0 % 1.5 ∙ 4.0 ∙ 0.0 = 0.0 mL 100 3. Losses due to evaporation ca. 1 mL/h 4. Remaining volume > 10 mL H2SO4 volume = conversion + (total consumption by org. matter) + evaporation + remaining volume 3 mL + (3.97 + 1.51 + 0.00) mL + 1 mL + 10 mL = 18.48 mL ~18 mL 10 The digestion Hints Inlet for ambient air Safety zone (≈ 5 cm) Condensation zone Digestion Boiling /digesting sample ∙∙ Optimal conditions are achieved when the condensation zone remains 5 cm below the constriction of the sample tube. ∙∙ For foaming samples one Kjeldahl Tablet «Antifoam» or stearic. ∙∙ To reduce the digestion time H2O2 can be added. ∙∙ When samples crystalize despite optimal H2SO4/catalyst ratio ▶ the suction power of the Scrubber should be reduced. ∙∙ The Scrubber must only be placed on the left of the digester. ∙∙ Minimal use of chemicals due to micro-Kjeldahl (only SpeedDigester). ∙∙ For liquid samples boiling rods prevent boiling delays. ∙∙ Boiling rods can in contrast to boiling stones, also be used for the following automated analysis via the KjelSampler. ∙∙ Slowly increasing the digestion temp. helps to reduce foam formation of problematic samples. 11 Digestion Digestionparameter: SpeedDigester (IR) TKN using 500 mL sample tube Temp. [°C] Digestion First H2O has to evaporate before the temperature can increase █ T nominal █ T sample 1) Step Temp. [°C] Level Time [min] Vol. [mL] Preheating 380 7 Digestion 520 9 Time [min] Cooling 1) The sample is heated to boil as fast as possible. Temp. [°C] The sample is kept constantly boiling. █ T nominal █ T sample 45 10 50 25 60 50 80 100 135 250 185 400 30 The digestion time depends on the sample volume Standard Kjeldahl using 300 mL sample tube Step Temp. [°C] Level Time [min] Preheating 480 8.5 Digestion 480 550 490 8.5 9.5 8.5 Cooling 10 10 65 30 Temp. [°C] Time [min] The sample is heated to boil as fast as possible. Due to the low volume used for micro-Kjeldahl lower temperatures are required (480 compared to 490°C) to reach the boiling temperature and the digestion time can significantly reduced. █ T nominal █ T sample Time [min] 12 Micro Kjeldahl using 100 mL sample tubes Step Temp. [°C] Level Time [min] Preheating 480 8.5 Digestion 480 500 480 8.5 9.5 8.5 Cooling 10 5 45 15 Digestion Digestion parameter: Block digester TKN using 500 mL sample tubes Step Temp. [°C] Preheating 300 – 420 Digestion 420 1) Time [min] Cooling 1) Vol. [mL] 45 10 50 25 60 50 90 100 165 250 45 The digestion time depends on the sample volume. Standard Kjeldahl using 300 mL sample tubes Step Temp. [°C] Preheating 320 Digestion 420 Cooling Time [min] 120 45 13 Digestion Highest Sample throughput Reduce heat-up / cool-down periods █ Block: tablets █ IR: tablets Reduce digestion period █ Block: tablets + H2O2 █IR: H2O2 (operator’s presence required) 60 min 1 Time saving: 65 min Time saving: 135 min Use optimized method using Solution 1 to get the highest sample throughput (batch size: 20 samples) 3. Distillation Per batch: 100 min t [min] 14 batch #4 batch #6/7 of the day before batch #1 with K-377 Start 8:00 a.m. batch #6 batch #2 batch #7 cooling cooling batch #5 cooling batch #2 batch #3 cooling 2. Digestion (K-438 #2) Per batch: 140 min batch #1 cooling 2. Digestion (K-438 #1) Per batch: 140 min cooling batch batch batch batch batch batch batch #1 #2 #3 #4 #5 #6 #7 cooling 1. Sample preparation Per batch: 40 min batch #3 #4/5 with K-377 (unattended) Finish 05:00 p.m. Distillation Overview General overview 1. Dilution of the acidic digestion mixture ▶ 2. Alkalinization to convert NH4+ in NH3 ▶ 3. Steam distillation to drive out the NH3 ▶ 4. NH3 collection in acidic receiver ▶ H 2O NaOH H2O steam acid (H3BO3 or H2SO4) Distillation Step 4 can be varied as required for ∙∙ Boric acid titration ∙∙ Back titration 15 Distillation Parameter for boric acid titration Step Why How much Rule-of-the-thumb Step 1: Dilution H2O dist. Dilution of the strongly acidic 25 – 90 mL solution, prevents violent reactions 4 mL per mL used H2SO4 Step 2: Alkalinization NaOH 32 % Conversion of NH4+ in NH3 (gaseous) 15 – 90 mL 4.5 mL per mL used H2SO4 40 – 70 mL 2 % H3BO3 with KCl for low N contents 0.02 − 6.75 mg/L Step 3: Preparation of the receiver H3BO3 (pH 4.65) To collect the distilled NH3. NH3 is bound as borate complex (NH4BO4). 4 % H3BO3 for medium and high N content 6.75 – 125 mg/L Step 4: Distillation Water steam (100 %) Separation of NH3 by boiling of the sample 180 – 300 s Distillation time: 180 s with KjelMaster 240 s with others Step 5: Collection NH3 In boric acid receiver of pH 4.65 Condenser outlet tube and electrode must be completely immersed For the following analysis potentiometric as well as colorimetric titration can be chosen. 16 Distillation Parameter for back titration Step Why How much Rule-of-the-thumb Dilution of the strongly acidic solution, prevents from violent reactions 25 – 90 mL 4 mL per mL used H2SO4 15 – 90 mL 4.5 mL er mL used H2SO4 Step 1: Dilution H2O dist. Step 2: Alkalinization NaOH 32 % Conversion of NH4+ in NH3 (gaseous) Step 3: Preparation of the receiver H2SO4 (0.25 mol/L) To collect the distilled surplus 10 – 20 mL of NH3. Is titrated with H2SO4 Volume must be exactly dosed (usually 20 mL) Separation of NH3 by boiling of the sample Distillation time: 180 s with KjelMaster 240 s with others Step 4: Distillation Water steam (100 %) 180 – 300 s Step 5: Collection NH3 In sulfuric acid receiver Condenser outlet hose and electrode must be immersed completely. For the following analysis only potentiometric titration can be done. 17 Distillation Hints ∙∙ After digestion the samples must cool-down to 50 – 100 °C before they can be further processed. ∙∙ Optimal NaOH concentration = 32 % ∙∙ For very low N-contents 2% H3BO3 with KCI (3 g/L) should be chosen as receiving solution, to achieve lower detection limits. ∙∙ With KjelSampler samples can be processed overnight. ∙∙ With the «IntelliDist» function of the KjelMaster preheating of the instrument can be avoided. ∙∙ Distillation and titration can be synchronized by means of the «Online-Titration». ∙∙ Waste from the sample tube and the receiver can be collected separately with the KjelMaster. 18 Titration Overview Boric acid titration Potentiometric titration Colorimetric titration 1. Standard a) endpoint titration / b) startpoint titration 2. Online a) endpoint titration / b) startpoint titration 1. Standard a) endpoint titration / b) startpoint titration Start volume before titration can be used for all titration variants. Back titration Potentiometric Titration 1. Standard Advantage a) endpoint titration Disadvantage Potentiometric: direct pH measurement Lower detection limit / online titration possible / calibration required / relatively short lifetime shorter analysis time / „IntelliDist“ possible of electrode Colorimetric: detection of the color change longer analysis time / „IntelliDist“ impossible No calibration necessary / longer lifetime of the probe / end point visible / indicator required / more boric acid necessary (compared to pot.) Titration Standard: the titration starts after the distillation is finished Standardized process: Distillation → titration / easier Longer analysis time as for Online-Titration monitoring Online: the titration takes place whilst the distillation is still in progress Time saving: Synchronizes distillation and titration Useful for higher titration volume (> 9 mL) Fixed and known pH Adjustment of pH not needed Huge time saving, higher sample throughput Endpoint pH of boric acid must be adjusted to 4.65 Startpoint More boric acid required (compared to end point titration), to detect the startpoint Start volume Only for well-known titration volume 19 Titration Endpoint and Startpoint titration (potentiometric) Endpoint titration Parameter What Setting Rule-of-the-thumb 1. Algorithm Dosage steps for titration Optimal Optimal for Titrant < 0.5 N Slow for Titrant ≥ 0.5 N 2. Start volume Titrant HCI / H2SO4 Only for well-known samples / N-content and high titration volume 3. Titration Titrant Consumption should be between 3 and 17 mL 4. Endpoint pH-value pH 4.65 pH-value of the used H3BO3 must previously be adjusted to 4.65 Startpoint titration (only step 4 must be changed) Parameter What Setting Rule-of-the-thumb 4. Startpoint pH is detected pH-value before NaOH addition Adjustment of the pH not necessary Back titration (potentiometric) Endpoint titration 20 Parameter What Setting Rule-of-the-thumb 1. Algorithm Dosage steps for titration Optimal „Optimal“ for Titrant < 0.5 N, „Slow“ for Titrant ≥ 0.5 N 2. Start volume Titrant NaOH Only for well-known samples / N-content and high titration volume 3. Titration Titrant Consumption should be between 3 and 17 mL 4. Endpoint Titrant NaOH pH-value Back titration pH 7.00 Endpoint neutral pH 7.00 Titration Endpoint titration (Colorimetric) Endpoint titration Parameter What Setting Rule-of-the-thumb 1. Algorithm Dosage steps for titration Optimal Only „Optimal“, not „Slow“ 2. Start volume Titrant HCI / H2SO4 Only for well-known samples / N-content and high titration volume 3. Titration Titrant Consumption should be between 3 and 17 mL 4. Endpoint Color change Sher Indicator in H3BO3 Startpoint titration (only step 4 must be changed) Parameter What Setting Rule-of-the-thumb 4. Startpoint pH is detected pH before NaOH addition Adjustment of the pH not necessary 21 Titration Hints Endpoint Titration Boric Acid B(OH)3 + 2 H2O ⇌ B(OH)4– + H3O+ pKa = 9.24 B(OH)3 + 2 H2O Sher Indicator Endpoint pH 4.65 B(OH)4– + H3O+ Acid consumption [mL] ∙∙ Depending on the regulation or method, either potentiometric or colorimetric titration can be chosen. ∙∙ For endpoint and colorimetric titration the pH of the boric acid must be adjusted to 4.65. ∙∙ Back titration can be used when boric acid should be avoided. ∙∙ For well-known samples a start volume for titration might be used to accelerate the titration step. (only useful for high titration volumes) ∙∙ The distillation and titration process can be synchronized, by means of online titration. 22 Result calculation Hints ∙∙ The KjelMaster K-375 performs the calculation automatically. ∙∙ With the PC software KjelLink results/methods can be transferred to a PC for printing, storage or recalculation. ∙∙ To determine the optimal method parameters, the BUCHI Kjeldahl-calculator can be used as a helpful tool. [V(1) - V(Bl)] ∙ F ∙ c ∙ f ∙ M(N) w(N) =———————————————— m ∙ 1000 % N = w(N) ∙ 100 % % P = w(N) ∙ PF ∙ 100 % Example (Titrant is 0.25 M H2SO4,f=1.000): (7.5mL - 1mL) ∙ 2 ∙ 0.25 ∙ 1 ∙ 14 —————————————————— = 0.0607 g N 0.750 g ∙ 1000 0.0607 g N ∙ 100 % = 6.07 % N 0.0607 g N ∙ 6.25 ∙ 100 % = 37.94 % P weight fraction of N consumption of titrant, sample, [mL] average consumption of titrant, blank, [mL] molar reaction factor (1 = HCI, 2 = H2SO4) concentration of titrant [mol /L] factor of titrant molecular weight of (14,007) sample weight [g] conversion factor (mL in L) Protein factor % of weight of N % of weight of protein Result w(N): V(1): V(Bl): F: c: f: M(N): m: 1000: PF: % N: % P: 23 BÜCHI Labortechnik AG CH – 9230 Flawil 1 T +41 71 394 63 63 F +41 71 394 65 65 [email protected] www.buchi.com BUCHI UK Ltd. GB – Oldham OL9 9QL T +44 161 633 1000 F +44 161 633 1007 [email protected] www.buchi.co.uk BUCHI (Thailand) Ltd. 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