Application Note No. 116 / 2013 Nitrogen determination in sodium nitrate

Transcription

1 Application Note No. 116 / 2013 Nitrogen determination in sodium nitrate KjelMaster K-375: Determination of nitrogen in sodium nitrate according the the Devarda method

2 1. Introduction An easy, quick and sensitive method for the analysis of nitrogen in sodium nitrate using the Devarda method is described in this application note. The distillation and boric acid titration are performed with the KjelMaster K-375. The Devarda method is used for the determination of nitrogen as sum parameter in nitrate and nitrite. However, this method is not suitable in the presence of organic compounds, calcium cyanamide or carbamide. Devarda s alloy reacts with sodium hydroxide to form hydrogen. The produced hydrogen reduces the nitrate and nitrite to ammonia. The ammonia is then trapped in boric acid solution and titrated with sulfuric acid. 2. Equipment KjelMaster K-375 Analytical balance (accuracy ± 0.1 mg) and Nitrogen-free weighing paper 300 ml glass tubes 3. Chemicals and Materials Chemicals: Sodium hydroxide 32%, Brenntag ( ) Boric acid 4% (ph 4.65), 200 g boric acid, Brenntag ( ) diluted to 5 L with deionized water and adjusted to ph 4.65 by using 10 % NaOH Sulfuric acid 0.25 mol/l, Fluka (35355) Sodium nitrate 99.5%, Fluka (31440) Devarda s alloy powder, Fluka (31385) For a safe handling please pay attention to all corresponding MSDS data sheets! Samples: Sodium nitrate with a purity of 99.5% is used. The theoretical content of nitrogen in sodium nitrate with this purity is 16.4 % g/g. 4. Procedure Approximately 0.25 g of sample was weighted with an accuracy of ± 0.1 mg in a digestion tube, and 2.0 g Devarda alloy were added. Additionally 4 blanks, only Devarda alloy without sample, were prepared. 4.1 Hints Devarda reagent consits of a mixture of ~45 % aluminium, ~50 % copper and ~5 % zinc. Under alkaline conditions aluminium and zinc are reduced and hydrogen gas is produced. The hydrogen gas reduces nitrate to ammonia. It`s an exotherm reaction therefore during reaction the temperature of the liquid increases. 2Al+ 2OH 3 + 6H O 2 3NO + 8Al+ 5OH 2 [ Al( OH) ] H O 3NH 2 + 3H [ Al( OH) ] 4 Application Note 116/2013 July /8

3 For these measurements 2.0 g Devarda alloy were weighed into each 300 ml glass tube. Even though it`s a catalyst the amount should be weight in correctly for at least one decimal digit. When using unequal amounts of catalyst for blanks and samples wrong results would be the consequence. An amount of Devarda alloy has to be selected in a way that AFTER the distillation still some H 2 bubbles are visible. This guarantees that sufficient H 2 for the reaction is available. Picture 1: Left: NO H 2 development AFTER distillation wrong: H 2 development may be too little for complete reaction Right: H 2 development AFTER distilation correct: sufficient H 2 for reaction The concentration of NaOH should be 32 % to make sure that the concentration of the OH - is sufficient for the reaction. The aspiration of the sample has to be switched off, because the residue from the Devarda alloy can block the valves! For blank determination at least 3 blanks with an RSD of 5% have to be determined. In case that blanks are out of the tolerance the measurement has to be repeated. The residue of the Devarda alloy is a potential risk for the environment! Don`t empty the remaining substance to the sink after distillation! Make sure that it is disposed correctly. In case that recovery rate is too low the thightness of the sealings between the glass parts in the distillation unit should be checked. Optional the connections can be wrapped with a teflon tape to avoid NH 3 loss during distillation. Picture 2: Connection between splash protector and condenser is sealed with a teflon tape Application Note 116/2013 July /8

4 5. What happens during distillation? 1. Blank: Devarda alloy in 300 ml glass tube 2. Reaction time: H 2 O and NaOH are dosed and reaction starts 3. Start distillation: strong reaction in a blank due to H 2 is not consumed by the sample 4. End of distillation: intensity of reaction decreases 5. After distillation: still some H 2 bubbles are visible Picture 3: Devarda Process Application Note 116/2013 July /8

5 5.1 Distillation and titration Distill the samples according to the parameters listed in Table 1. H 2 O volume 30 ml Titration solution H 2 SO mol/l NaOH volume 30 ml Sensor type Potentiometric Reaction time 120 s Titration mode Standard Distillation mode Fixed time Measuring mode Endpoint ph Distillation time 200 s Endpoint ph 4.65 Stirrer speed distillation Stirrer speed 5 titration 7 Steam output Titration start 100 % volume 0 ml Titration type Boric acid Titration algorithm Optimal Receiving solution vol. 100 ml Table 1: Method parameters for KjelMaster K Calculation The results are calculated as a percentage of nitrogen. In order to calculate the protein content of the results are calculated as a percentage of nitrogen. The following equations (1) and (2) are used to calculate the results. The purity of the NaNO 3 is considered in equation (3). w N (V = Sample -V m Blank Sample ) z c f M 1000 N (1) %N = wn 100 % (2) %NNaNO3 %N P = (3) 100 wn : weight fraction of nitrogen V Sample : amount of titrant for the sample [ml] V Blank : 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) MN : molecular weight of nitrogen ( g/mol) m Sample : sample weight [g] 1000 : conversion factor [ml/l] %N : percentage of weight of nitrogen %NaNO 3 : percentage of weight of nitrogen corrected for the purity of reference substance P : purity of the reference substance NaNO 3 (%)

6 6. Results 6.1 Blank determination The results of nitrogen determination and recovery for sodium nitrate analysis are presented in Table 2. The nominal value of sodium nitrate with a purity of 99.5 % is 16.4 % nitrogen. The recoveries are within the specification of % [1] Blank m Devarda Alloy [g] Titrated Volume [ml] Blank Blank Blank Blank Average [%] SD Rsd [%] Table 2: Results of the blank determination The mean blank volume (V Blank ) was ml (m = 4). 6.2 Determnation of nitrogen in sodium nitrate The results of the determination of nitrogen content in sodium nitrate are presented in Table 3. Sample m Sample [g] V Sample [ml] %N Recovery Rate [%] Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Average [%] SD Rsd [%] Table 3: Results of the determination of nitrogen in sodium nitrate The mean blank volume (VBlank) was ml (m = 4). Application Note 116/2013 July /8

7 7. Comparison to AOAC Parameter Application note AOAC DIN CEN/TS 15476:2006 DIN H28 Medium Alkaline reduction Alkaline reduction Alkaline reduction Acidic reduction Amount of sample 0.25 g 0.5 g g 50 ml (or diluted with water to 50 ml) Nitrite Amount of catalyst Reaction solution Reaction time Reaction temperature Receiver solution Titration NaOH Sum of nitrite and nitrate Sum of nitrite and nitrate 2.0 g Devarda's alloy 3 g Devarda's alloy 30 ml Water / 30 ml NaOH 32% 300 ml Water / 5 ml NaOH 42% Sum of nitrite and nitrate At first nitrite has to be oxidized to nitrate with potassium permanganate 4 g Devarda's alloy 2 g K 2 SO g Devarda's alloy 300 ml Water / 5 ml alcohol/ 30 ml NaOH ca. 30% 120 s - 30 min 1h 4 ml sulfuric acid >96% Ambient temperature - Heated Ambient temperature, succeeding a Kjeldahl digestion Boric acid 4%, ph 4.65, 100 ml Volumetric acid Volumetric acid Boric acid 2%, 50 ml Boric acid titration, Back titration Back titration Boric acid titration, colorimetric or photometric potentiometric titration See reaction solution See reaction See reaction solution 25 ml 320 g (NaOH)/L and 250 ml water solution Distillation 200 s 250 ml in 1h 200 ml in 30 min (additionally to 200 ml the reaction time) Notes Is equivalent to ISO "Water quality; Determination of nitrogen-catalytic digestion after reduction with Devarda's alloy" Application Note 116/2013 July /8

8 8. Conclusion The determination of nitrogen in sodium nitrate according to the presented method and using the KjelMaster K-375 provides reliable and reproducible results. These results correspond well to the given value of sodium nitrate. The recovery rate was 99.7 % (rsd = 0.5 %), which was within the specification of %. Simplicity and rapidity of the method makes the KjelMaster K-375 to a powerful instrument for a daily routine analysis of nitrogen according Devarda method. 9. References [1] Application Note No 1 Devarda_370-03D: Determination of Nitrogen in sodium nitrate according the Devarda method [2] AOAC Official Method Nitrogen (Ammoniacal and Nitrate) in Fertilizers [3] DIN CEN/TS 15476:2006 Fertilzers- Determination of nitric and ammoniacal nitrogen according to Devarda (German version) [4] DIN H28 Operation Manual of KjelMaster K-375 Application Note 116/2013 July /8