1. Introduction. 2. Equipment. 3. Chemicals and Materials

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Marjorie Mills
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2 1. Introduction Nitrogen and protein determination in food and feed are described in numerous standard procedures [1-3]. Therein, the titration specifications vary between boric acid and back titration. The aim of the following study was to determine the reproducibility and to compare the two different titration methods boric acid and back titration. Therefore, the nitrogen content of the dry reference substance ammonium dihydrogen phosphate monobasic was determined by distillation and titration according to Kjeldahl method applying the BUCHI KjelMaster System K-375 / K Equipment - KjelMaster System K-375 / K Dosing unit for back titration ( ) - Driving motor for dosing unit ( ) - Laboratory vessel (053203) - Analytical balance (accuracy ± 0.1 mg) 3. Chemicals and Materials - Sodium hydroxide 32 %, Brenntag ( ) - Boric acid (H3BO3) 4 %, 400 g boric acid, Brenntag ( ) diluted to 10 L with deionized water, ph adjusted to Sulfuric acid 0.1 mol/l, volumetric solution, VWR ( ) - Sodium hydroxide 0.5 mol/l, volumetric solution, Fluka (35257) - Ammonium dihydrogen phosphate monobasic 99.5%, Fluka (09709) was used as the reference substance. The theoretical total nitrogen content of the organic substance is wn = (12.18%) BUCHI (045527) For a safe handling please pay attention to all corresponding MSDS. Application Note 242/2016 August /5

3 4. Procedure 4.1. Boric acid titration Boric acid titration is used for the direct determination of the amount of distilled ammonia. Ammonia and boric acid form a complex which is directly titrated with sulfuric acid. An excess of boric acid guarantees that ammonia is bound in a complex. The determination of nitrogen includes the following steps: - Weigh in of ± 0.2 g of ammonium dihydrogen phosphate in each 300 ml sample tube. - Alkalization of the sample to free up the nitrogen (ammonia). - Steam distillation of the ammonia into boric acid receiver. - Determination of the nitrogen content by boric acid titration. - Perform at least 3 blank determinations before sample distillation. All distillation parameters are listed in Table 1. Table 1: Parameters for distillation and boric acid titration. H2O volume 20 ml Sensor type Potentiometric NaOH (32 %) volume 40 ml Titration mode Standard Reaction time 5 s Measuring mode Endpoint ph Distillation mode Fixed time Endpoint ph 4.65 Distillation time 240 s Stirrer speed titration 7 Stirrer speed distillation 5 Titration start volume 0 ml Steam output 100 % Titration algorithm Optimal Titration type Boric acid Aspiration sample tube Yes Receiving solution vol. 50 ml (Boric acid) Aspiration receiving vessel Yes Titration solution H2SO4 0.1 mol/l 4.2. Back titration Back titration is an indirect determination of the amount of distilled ammonia. An excess of sulfuric acid standard solution reacts with the NH3 in the distillate. The residual sulfuric acid is back titrated with sodium hydroxide standard solution. Consumption of sodium hydroxide solution indirectly correlates with amount of ammonia. First, the nitrogen content of the 3 blanks is determined. Subsequently, the nitrogen content of the reference sample is determined. NOTE: For back titration, a second burette is needed for accurate dosage of the receiving solution (sulfuric acid). The determination of nitrogen includes the following steps: - Weight in ± 0.2 g of ammonium dihydrogen phosphate in each 300 ml sample tube. - Alkalization of the sample to free up the nitrogen (ammonia). - Steam distillation of the ammonia into sulfuric acid receiver. - Determination of the nitrogen content by back titration. - Perform at least 3 blank determinations before sample distillation. All distillation parameters are listed in table 2. Table 2: Parameters for distillation and back titration. H2O volume 20 ml Titration solution NaOH 0.5 mol/l NaOH (32%) volume 40 ml Sensor type Potentiometric Reaction time 5 s Titration mode Standard Distillation mode Fixed time Measuring mode Endpoint ph Distillation time 240 s Endpoint ph 7.0 Stirrer speed distillation 3 Stirrer speed titration 5 Steam output 100 % Titration start volume 0 ml Titration type Back titration Titration algorithm Optimal Receiving solution H2SO4 0.1 mol/l Aspiration sample tube Yes Receiving solution volume 20 ml Aspiration receiving vessel Yes Application Note 242/2016 August /5

4 5. Results 5.1. Boric acid titration The recovery rates of the reference substance samples determined by boric acid titration are listed in Table 3. Table 3: Results of the boric acid titration. Expected N [%]; % (purity 99.5 %). Sample m Sample [g] V H2SO4 [ml] N [%] Recovery [%] Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Average [%] Rsd [%] An average nitrogen content of % (rsd 0.208) was determined, i.e. a recovery of 99.2 % was observed when using boric acid titration Back titration In Table 4 the results of the back titration are listed. Table 4: Results of the back titration. Expected N [%] was 12.18% (purity 99.5 %). Sample m Sample [g] V NaOH [ml] N [%] Recovery [%] Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Ammonium dihydrogen phosphate Average [%] Rsd [%] An average nitrogen content of % (rsd 0.013) was measured, yielding a recovery of 98.6 % using back titration. Application Note 242/2016 August /5

5 6. Discussion Boric acid titration yielded a higher recovery rate (99.2 %) than back titration (98.6 %). Nevertheless, both titration methods fulfilled the recovery requirements ± 2 % (98 % %) [4]. Depending the standard, boric acid or back titration methods are required. In general, both methods have their advantages and disadvantages Boric acid titration Advantages: + Direct titration. + No exact volume dosage for receiver solution required quick method and fewer measurement errors. + Depending on the regulation or method, either potentiometric or colorimetric titration can be chosen. + Time saving: Distillation and titration can be synchronized by means of the Online-Titration mode. Disadvantages: - Point of inflection is weaker leading to a potential loss in case of large amounts of NH 3.In this case choose a larger volume of boric acid solution and/or reduce the original sample weight. - The ph of the boric acid must be adjusted to Back titration Advantages: + Back titration can be used when boric acid should be avoided. + The acid immediately neutralizes the ammonia. The distillate receiver remains acidic, thus there are no losses even for large amounts of ammonia. + No ph adjustment of the receiving solution necessary. Disadvantages: - Indirect titration. - No colorimetric titration possible (potentiometric titration only). - The sulfuric acid concentration and volume must be precisely measured and is a potential source of errors second burette required. 7. Conclusion Depending on the standard followed, boric acid or back titration methods are required for nitrogen determination. Here, both titration methods fulfilled the recovery requirements of ± 2 % (98 % %) using the KjelMaster System K-375 / K References [1] AOAC , nitrogen in meat. Kjeldahl method. [2] AOAC , protein in macaroni products. [3] AOAC , baked products. [4] AOAC Protein (crude) in animal feed, forage (plant tissue), grain, and oilseeds. Application Note 242/2016 August /5

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

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