Nitrogen content determination method based on efficient catalytic digestion

Abstract

The invention discloses a nitrogen content determination method based on efficient catalytic digestion. The method comprises the following determination steps: 1, preparing a catalyst; 2, digesting the sample; 3, distilling and absorbing; and 4, automatic titration. According to the method, the chromium-free mercury-free environment-friendly efficient composite catalyst (nano titanium dioxide and copper sulfate compound) is adopted, the digestion time is shortened to be within 30 min, the digestion efficiency is improved, the reagent dosage is reduced, microwave heating and temperature programming are combined, sample blasting boiling is avoided, and the reaction uniformity is improved; a potentiometric method and a micro titration pump are adopted, automatic terminal point judgment and real-time data processing are achieved, the nitrogen content detection precision reaches + / -0.05%, and the device is suitable for nitrogen content measurement of complex matrix samples such as protein, fertilizer and waste water.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical analysis, and particularly relates to a method for determining nitrogen content based on high-efficiency catalytic digestion, which is used for quickly and accurately determining the nitrogen element content in organic or inorganic samples. Background Art

[0002] The Kjeldahl method is to heat and digest the sample together with concentrated sulfuric acid and a catalyst to decompose proteins, in which carbon and hydrogen are oxidized to carbon dioxide and water and escape, while the organic nitrogen in the sample is converted into ammonia and combined with sulfuric acid to form ammonium sulfate. Under alkaline conditions, ammonium sulfate decomposes to generate ammonia, which is distilled out with water vapor and absorbed by boric acid solution. The absorbent is titrated with a standard solution of hydrochloric acid or sulfuric acid with a known concentration, and the nitrogen content in the sample is calculated according to the consumption of the acid.

[0003] However, the above Kjeldahl method has the following problems: 1. Digestion stage: It takes too long (more than 12 hours). When dealing with stubborn samples with high fiber or high fat, mercury or chromium powder will be used, which has a certain pollution to the environment; 2. Low distillation efficiency: The ammonia absorption is incomplete, resulting in deviation of the results. Summary of the Invention

[0004] In view of the problems raised in the background art, the present invention has studied and designed a method for determining nitrogen content based on high-efficiency catalytic digestion, and its purpose is to provide a method for determining nitrogen content with high-efficiency catalytic digestion that can improve digestion efficiency, detection accuracy, and is applicable to samples with complex matrices.

[0005] The technical solution of the present invention: A method for determining nitrogen content based on high-efficiency catalytic digestion includes the following determination steps: Step 1. Preparation of catalyst Mix nano-TiO2 and CuSO4·5H2O in a certain proportion and grind them to a particle size ≤ 50 μm to obtain a composite catalyst; Step 2. Sample digestion Weigh 0.5 g - 2 g of the sample to be tested into a digestion tube, add 10 mL - 20 ml of concentrated sulfuric acid and 0.2 g - 0.4 g of the composite catalyst, and then place it in a microwave digestion instrument for gradient temperature rise; Step 3. Distillation and absorption Transfer the digestion solution in Step 2 to the distillation chamber, add 40% NaOH solution, and the ammonia gas is absorbed by 2% boric acid solution after multi-stage condensation; Step 4. Automatic titration Use a micro titration pump to titrate with a standard solution of sulfuric acid or hydrochloric acid with a concentration of 0.05 mol / L - 0.5 mol / L, and use potentiometry to monitor the change of potential in real time during the titration process. The titration end point is determined by the potential mutation slope.

[0006] In Step 1, the TiO2 nanoparticles have a particle size of 20 nm, and nano-TiO2 and CuSO4·5H2O are mixed at a mass ratio of 3:1.

[0007] In Step 2, the model of the microwave digestion instrument is TANK 40, produced by Shanghai New Instrument Microwave Chemistry Technology Co., Ltd.

[0008] The specific gradient heating in Step 2 is as follows: heat up to 150 °C and hold for 10 min, then continue to heat up to 300 °C and hold for 15 min, and then continue to heat up to 400 °C and hold for 5 min.

[0009] Both Step 3 and Step 4 are completed in an automatic Kjeldahl distillation unit, and the model of the automatic Kjeldahl distillation unit is NKY6160, produced by Jinan Lure Instrument Co., Ltd.

[0010] Advantages of the present invention: The present invention adopts a chromium-free and mercury-free environmentally friendly and highly efficient composite catalyst (nano-titanium dioxide and copper sulfate composite), shortens the digestion time to within 30 min, improves the digestion efficiency, reduces the reagent dosage, combines microwave heating and programmed temperature rise, avoids sample boiling over and improves the reaction uniformity; adopts potentiometry and a micro-titration pump to realize automatic endpoint determination and real-time data processing, and the detection accuracy of nitrogen content reaches ±0.05%, which is applicable to the determination of nitrogen content in complex matrix samples such as proteins, fertilizers, and wastewater. Specific Embodiments

[0011] The following further illustrates the present invention with specific embodiments.

[0012] Example 1 (The sample taken is a standard sample with a known nitrogen content, and the content is 25%) I. Catalyst Preparation Mix nano-TiO2 (particle size 20 nm) and CuSO4·5H2O at a mass ratio of 3:1, and grind to a particle size ≤ 50 μm to obtain a composite catalyst; II. Sample Digestion Weigh 0.5 g of the sample into a digestion tube, add 10 ml of water, 10 mL of concentrated sulfuric acid and 0.2 g of the composite catalyst; Microwave gradient heating (microwave digestion instrument TANK 40): heat up to 150 °C and hold for 10 min, then continue to heat up to 300 °C and hold for 15 min, and then continue to heat up to 400 °C and hold for 5 min; III. Distillation and Absorption Transfer the digestion solution to the distillation chamber, add 20 ml of 40% NaOH solution, and the ammonia gas is absorbed by 2% boric acid solution after multi-stage condensation. IV. Automatic Titration Use a micro burette pump to titrate with a 0.05 mol / L - 0.5 mol / L sulfuric acid or hydrochloric acid standard solution, and use potentiometry to monitor the change of potential in real time during the titration process. The titration end point is determined by the potential mutation slope. The final detection result of nitrogen content is: 24.98%.

[0013] Comparative Example 1 (The sample taken is a standard sample with a known nitrogen content, and the content is 25%) I. Weigh 0.5 g of the sample into a digestion tube, add 10 ml of water, 1.2 g of chromium powder and 10 ml of hydrochloric acid, let it stand for 5 min, insert a long-necked funnel, place it on a digestion instrument, heat it to boiling and continue heating for 2 min - 3 min after a large amount of green foam is generated, cool it to room temperature, add 2 g of a mixed catalyst (obtained by mixing and grinding 1 kg of potassium sulfate and 50 g of copper sulfate) and 10 ml of concentrated sulfuric acid, and let it stand overnight; II. Distillation and absorption Transfer the digestion solution to the distillation chamber, add 20 ml of 40% NaOH solution, and the ammonia gas is absorbed by 2% boric acid solution after multi-stage condensation; III. Automatic titration Use a micro burette pump to titrate with a 0.05 mol / L - 0.5 mol / L sulfuric acid or hydrochloric acid standard solution, and use potentiometry to monitor the change of potential in real time during the titration process. The titration end point is determined by the potential mutation slope. The final detection result of nitrogen content is: 23.57%.

[0014] Example 2 (The sample taken is a standard sample with a known nitrogen content, and the content is 25%) I. Catalyst preparation Mix nano-TiO2 (particle size 20 nm) and CuSO4·5H2O in a mass ratio of 3:1, and grind it to a particle size ≤ 50 μm as a composite catalyst; II. Sample digestion Weigh 1 g of the sample into a digestion tube, add 10 ml of water, 20 mL of concentrated sulfuric acid and 0.4 g of the composite catalyst; Gradient heating: Heat to 150 °C and keep it warm for 10 min, then continue heating to 300 °C and keep it warm for 15 min, and then continue heating to 400 °C and keep it warm for 5 min; III. Distillation and absorption - Transfer the digestion solution to the distillation chamber, add 20 ml of 40% NaOH solution, and the ammonia gas is absorbed by 2% boric acid solution after multi-stage condensation; IV. Automatic titration Use a micro-titration pump to titrate with a 0.05 mol / L - 0.5 mol / L sulfuric acid or hydrochloric acid standard solution. Use potentiometry to monitor the change in potential during the titration process in real time. The titration endpoint is determined by the potential mutation slope. The final detection result of the nitrogen content is: 24.97%.

[0015] Comparative Example 2 (The sample taken is a standard sample with a known nitrogen content, and the content is 25%) I. Similarly, weigh 1 g of the sample into a digestion tube, add 10 ml of water, 2.4 g of chromium powder and 20 ml of hydrochloric acid, let it stand for 5 min, insert a long-necked funnel, place it on a digestion instrument, heat it to boiling, continue heating for 2 - 3 min after a large amount of green foam is generated, cool it to room temperature, add 4 g of a mixed catalyst (obtained by mixing and grinding 1 kg of potassium sulfate and 50 g of copper sulfate) and 20 ml of concentrated sulfuric acid, and let it stand overnight; II. Distillation and absorption Transfer the digestion solution to the distillation chamber, add 20 ml of 40% NaOH solution, and the ammonia gas is absorbed by a 2% boric acid solution after multi-stage condensation; III. Automatic titration Use a micro-titration pump to titrate with a 0.05 mol / L - 0.5 mol / L sulfuric acid or hydrochloric acid standard solution. Use potentiometry to monitor the change in potential during the titration process in real time. The titration endpoint is determined by the potential mutation slope. The final detection result of the nitrogen content is: 23.41%.

[0016] By measuring the standard samples with known nitrogen content using the two methods of Example 1, Example 2 (the measurement method of the present invention) and Comparative Example 1, Comparative Example 2 (the traditional measurement method), it is obtained that: (1) The measurement methods of the present invention can reach 24.97% and 24.98% respectively, and the results of the traditional measurement methods are 23.57% and 23.41% respectively. Therefore, the nitrogen content measured by the measurement method of the present invention is closer to the sample standard content of 25%; (2) The test time of the measurement method of the present invention is shorter, and the detection can be completed within 2 hours, and the detection result is closer to the target content; (3) The amount of the nano-mixed catalyst used in the measurement method of the present invention is less, and no heavy metal such as chromium powder is used as a catalyst, which is more environmentally friendly; (4) The determination method of the present invention using stepwise temperature increase compared with direct heating has the following advantages: (a) With direct heating, the temperature rises quickly, and the sample is prone to carbonization due to local overheating, resulting in nitrogen loss. However, with stepwise temperature increase, the sample is first decomposed at a low temperature, and then the temperature is gradually increased, allowing the sample to fully react with sulfuric acid and the catalyst, reducing the risk of carbonization, making the digestion more complete, and improving the efficiency of nitrogen conversion to ammonium salts; (b) Through stepwise temperature increase and insulation, the sample digestion is more uniform, and the reaction degree of each part is consistent, avoiding excessive or insufficient local reactions, reducing the test error caused by uneven digestion, and improving the precision and repeatability of the experiment; (c) Different temperature conditions are required for the decomposition of organic matter at different stages. Stepwise temperature increase can make the easily decomposable substances react first at the low temperature stage and the difficult-to-react substances continue to react at the high temperature stage, conforming to the reaction law and accelerating the overall digestion rate; (d) It is difficult to control the temperature with direct heating, which may cause some nitrides to escape in the form of gas, resulting in a lower result. Stepwise temperature increase can accurately control the temperature, make the reaction stable, reduce nitrogen loss, and ensure the accuracy of the result.

[0017] The present invention uses a chromium-free and mercury-free environmentally friendly and highly efficient composite catalyst (nano-titanium dioxide and copper sulfate composite), shortens the digestion time to within 30 minutes, improves the digestion efficiency, reduces the reagent dosage, combines microwave heating with programmed temperature increase, avoids sample boiling over and improves the reaction uniformity; uses potentiometry and a micro-titration pump to achieve automatic end point determination and real-time data processing, with a nitrogen content detection accuracy of ±0.05%, and is applicable to the determination of nitrogen content in complex matrix samples such as proteins, fertilizers, and wastewater.

[0018] The above-described embodiments only represent the preferred implementation modes of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention's patent. Any non-substantive changes and substitutions made by those skilled in the art based on the present invention fall within the scope claimed by the present invention.

Claims

1. A method for determining nitrogen content based on efficient catalytic digestion, characterized in that: The measurement steps include: Step 1: Catalyst preparation The composite catalyst is prepared by mixing nano-TiO2 and CuSO4·5H2O in a certain ratio and grinding them to a particle size of ≤50μm; Step 2: Sample digestion Weigh 0.5g-2g of the sample to be tested into a digestion tube, add 10mL-20ml of concentrated sulfuric acid and 0.2g-0.4g of the composite catalyst, and place it in a microwave digestion instrument to increase the temperature gradually; Step 3: Distillation and absorption The digestion solution in step 2 is transferred to the distillation chamber, 40% NaOH solution is added, and the ammonia gas is absorbed by 2% boric acid solution after multi-stage condensation; Step 4: Automatic titration A micro titration pump is used to titrate 0.05mol / L~0.5mol / L sulfuric acid or hydrochloric acid standard solution. The potential change during the titration process is monitored in real time using the potentiometric method. The titration endpoint is determined by the potential mutation slope.

2. A method for determining nitrogen content based on efficient catalytic digestion as claimed in claim 1, characterized in that: In the step 1, the particle size of the nano-particle TiO2 is 20 nm, and the nano-TiO2 and CuSO4·5H2O are mixed in a mass ratio of 3:

1.

3. A method for determining nitrogen content based on efficient catalytic digestion as claimed in claim 1, characterized in that: The model of the microwave digestion instrument in step 2 is TANK 40, produced by Shanghai Xinyi Microwave Chemistry Technology Co., Ltd.

4. A method for determining nitrogen content based on efficient catalytic digestion as claimed in claim 1, characterized in that: The gradient heating in step 2 is specifically as follows: heating to 150° C. and then keeping the temperature for 10 minutes, heating to 300° C. and then keeping the temperature for 15 minutes, and then heating to 400° C. and keeping the temperature for 5 minutes.

5. A method for determining nitrogen content based on efficient catalytic digestion as claimed in claim 1, characterized in that: The steps 3 and 4 are both completed in a fully automatic Kjeldahl nitrogen analyzer, the model of which is NKY6160, produced by Jinan Lure Instrument Co., Ltd.