A method for determining the content of neptunium in neptunium dioxide standard material

Abstract

This invention relates to a method for determining the neptunium content in neptunium dioxide standard materials. The method involves using sulfuric acid to fumigate the sample solution, removing hydrofluoric acid and simultaneously converting the system into a sulfuric acid measurement system. Excess silver oxide is destroyed by heating, reducing the introduction of additional reagents compared to the use of sulfamic acid. The simultaneous use of sulfuric acid fuming and heating to destroy the silver oxide avoids the need for sulfamic acid and aluminum nitrate reagents, reducing the introduction of other reagents, ensuring a single reaction system, and improving measurement accuracy. The coulometric analysis method for neptunium content in neptunium dioxide standard materials established by this invention has a wide range of applications. Controlled potentiometric titration can directly determine the neptunium content in neptunium dioxide standard materials and samples. The instrument is easy to operate, minimizing human error while ensuring radiation protection. Furthermore, the analytical results are highly accurate and precise, with less material consumption and lower experimental costs.

Description

Technical Field

[0001] This invention relates to the field of nuclear fuel cycle analysis technology research, and in particular to a method for determining the neptunium content in a neptunium dioxide standard material. Background Technology

[0002] In routine analysis and monitoring, neptunium dioxide standard materials are needed for analytical method development, standard curve plotting, standard pan preparation, instrument calibration, and quality control to ensure the accuracy of neptunium content-related analytical methods and results. In addition, there is also a demand for neptunium dioxide standard materials in domestic environmental monitoring and other fields.

[0003] The neptunium content in neptunium dioxide standard materials is a key indicator for controlling whether the content and uniformity of neptunium dioxide standard materials meet the standards, and it is an analytical item that must be controlled and tested. Accurately determining the neptunium content in neptunium dioxide standard materials plays a crucial role in their development. Therefore, establishing an accurate and precise method for determining the neptunium content in neptunium dioxide standard materials is of paramount importance.

[0004] Stromat et al. used the controlled-potential coulometric method to measure the content of neptunium in various valence states. The drawback of this method is that the oxidation of platinum at the counter electrode is relatively slow, resulting in a large absolute error and a cumbersome and complex measurement process. Bai Chunyi et al. used the constant-current method to determine 1 mg–3 mg of neptunium. The drawback of this method is that it requires titration with ferric sulfate (III), but the preparation of the ferric sulfate (III) standard solution is complex. The China Institute of Atomic Energy and the Lanzhou Nuclear Fuel Plant respectively used the constant-current coulometric method and potentiometric titration method to determine the neptunium content, but the precision of these methods was poor. Zhu Haiqiao et al. of the China Institute of Atomic Energy established an automatic potentiometric titration method for the determination of neptunium. The method for determining the content of neptunium has the following drawback: it requires the ferrous ammonium sulfate solution to be calibrated before each measurement, and the ferrous ammonium sulfate solution needs to be prepared fresh for each use. If there is an error in the calibration result of the ferrous ammonium sulfate, it will directly lead to a decrease in the accuracy of the neptunium content determination. Sun Yuanyuan et al. disclosed an analytical method for the neptunium content in neptunium dioxide samples. Their method uses a gold mesh as the working electrode and a saturated calomel electrode as the reference electrode, and controls the reduction potential and oxidation potential to make neptunium oxidize and reduce between pentavalent and hexavalent. However, the drawback of this invention for determining the neptunium content of neptunium dioxide standard materials is that it relies on the previous standard and cannot be suitable for the testing requirements of primary standard materials. Summary of the Invention

[0005] The purpose of this invention is to overcome the deficiencies in existing technologies and provide a method for determining the neptunium content in neptunium dioxide standard materials. This invention uses sulfuric acid to fume the sample solution, removing hydrofluoric acid and simultaneously converting the system into a sulfuric acid measurement system. It employs heating to destroy excess silver oxide, reducing the introduction of additional reagents compared to adding sulfamic acid. The simultaneous use of sulfuric acid fuming and heating to destroy silver oxide avoids the addition of sulfamic acid and aluminum nitrate reagents, reducing the introduction of other reagents, ensuring a single reaction system, and improving measurement accuracy. The coulometric analysis method for neptunium content in neptunium dioxide standard materials established by this invention has a wide range of applications. Controlled potentiometric titration can directly determine the neptunium content in neptunium dioxide standard materials and neptunium dioxide samples. The instrument is easy to operate, reduces human error while ensuring radiation protection, and provides high accuracy and precision in analytical results, with less material consumption and lower experimental costs.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for determining the neptunium content in a neptunium dioxide standard substance, the specific determination steps are as follows:

[0008] S1. Prepare low-concentration sulfuric acid solution and high-concentration sulfuric acid solution;

[0009] S2. Dissolve the neptunium dioxide standard substance to obtain a mixed solution;

[0010] S3. Add the high-concentration sulfuric acid solution obtained in step S1 to the mixed solution obtained in step S2, heat and concentrate to obtain the evaporated sample;

[0011] S4. Dilute the low-concentration sulfuric acid solution obtained in step S1 with the sample that was evaporated to dryness in step S3 to obtain a diluted sample solution.

[0012] S5. Measure the blank charge in the electrolytic cell cup without added sample solution;

[0013] S6. Measure the charge of the diluted sample solution obtained in step S4;

[0014] S7. Based on the blank charge obtained in step S5 and the charge of the sample solution obtained in step S6, calculate the neptunium content in the neptunium dioxide standard material according to Faraday's law.

[0015] Further, in step S1, a certain amount of analytical grade sulfuric acid is measured and added to deionized water while stirring. After cooling, the mixture is transferred to a volumetric flask and diluted to volume with deionized water.

[0016] Further, in step S1, the concentration of the low-concentration sulfuric acid solution is 0.4–0.6 mol / L.

[0017] Further, in step S1, the concentration of the high-concentration sulfuric acid solution is 2.5–3.5 mol / L.

[0018] Further, in step S2, nitric acid solution and HF solution are added to a certain amount of neptunium dioxide standard substance, heated to dissolve, and cooled to obtain a mixed solution.

[0019] Furthermore, the ratio of neptunium dioxide standard substance: nitric acid solution: HF solution = 0.03-0.05g: 10-15mL: 0.10-0.15mL.

[0020] Furthermore, the nitric acid solution is an analytical grade nitric acid solution, and the HF solution is an analytical grade HF solution.

[0021] Furthermore, the heating temperature is 160–180°C.

[0022] Furthermore, in step S3, the heating temperature is 220–250°C.

[0023] Further, in step S4, the concentration of neptunium in the sample solution is 1–3 mg / g.

[0024] Furthermore, in step S5, the specific steps for measuring the blank charge are as follows:

[0025] Add a low-concentration sulfuric acid solution to the electrolytic cell, then add excess silver oxide. Heat to remove the excess silver oxide, then add more low-concentration sulfuric acid solution until the solution surface submerges the electrodes. Introduce argon gas and perform reduction at the reduction potential. Stop reduction when the residual current drops below 10 μA. Perform oxidation at the oxidation potential. Stop oxidation when the residual current drops below 10 μA. Record the electrolytic charge Q1, electrolyte potentials E1 and E2, electrolysis time t1, and residual current I of the blank solution. r1 .

[0026] Furthermore, the silver oxide mentioned above is analytical grade silver oxide.

[0027] Furthermore, in step S6, the specific steps for determining the charge of the diluted sample solution are as follows:

[0028] (1) Immerse the electrode in nitric acid solution, boil it and soak it in nitric acid solution overnight. Before use, clean the electrode with deionized water and perform oxidation-reduction treatment in the electrolyte until the background current meets the requirements.

[0029] (2) The diluted sample solution obtained in step S4 is transferred to the electrolytic cell cup, and the mass Ws of the neptunium sample in the electrolytic cell cup is calculated.

[0030] (3) Add a low-concentration sulfuric acid solution to the electrolytic cell containing the neptunium sample obtained in step (2), add excess silver oxide, heat to remove the excess silver oxide, then add another low-concentration sulfuric acid solution to submerge the electrode. Introduce argon gas and reduce at the reduction potential. Stop the reduction when the residual current drops below 10 μA. Oxidize at the oxidation potential. Stop the oxidation when the residual current drops below 10 μA. Record the electrolytic charge Q of the neptunium sample solution. S Electrolysis potentials E3 and E4, residual current I r2 and electrolysis time t2;

[0031] (4) Determine the standard electrode potential E0: Add neptunium solution to the electrolytic cell cup and operate according to step (3). Control the potential to reduce to the reduction potential until the residual current is less than 10 μA. Stop the reduction and adjust the control potential to oxidize to the oxidation potential until the residual current is less than 10 μA. Record the oxidation charge Q and stop the oxidation. Control the potential to reduce to the reduction potential. When the charge is close to Q / 2, stop the reduction and measure the standard electrode potential E0.

[0032] Furthermore, in step (1), the electrode includes a working electrode and a counter electrode.

[0033] Furthermore, in step (1), the concentration of the nitric acid solution is 8 mol / L.

[0034] Furthermore, in step (1), the boiling time is 20 minutes.

[0035] Furthermore, in step (1), the electrolyte is a 0.5 mol / L sulfuric acid electrolyte.

[0036] Furthermore, in step (1), the background current is required to be 5 to 10 μA.

[0037] Furthermore, in step (2), the diluted sample solution obtained in step S4 is transferred to a pre-weighed sample bottle, weighed, and then the sample solution in the sample bottle is transferred to an electrolytic cell cup. The remaining solution in the sample bottle is weighed, and the mass difference is the mass of the solution in the electrolytic cell cup. Based on the concentration of the neptunium solution, the mass Ws of neptunium in the electrolytic cell cup is calculated.

[0038] Furthermore, in step S7, the specific calculation formula for the neptunium content in the neptunium dioxide standard substance is shown in Equation I:

[0039]

[0040] In Formula I:

[0041] C Np(%) — Neptunium content (%) in neptunium dioxide standard material;

[0042] Q s —Integral charge of the coulombometer during the oxidation of the neptunium sample, C;

[0043] Q b — Blank energy, C, the specific calculation formula for this value is shown in Formula II;

[0044] C—the integral factor of the coulomb counter, and the specific calculation formula for this value is shown in Equation III;

[0045] A r —The relative atomic weight of neptunium (calculated based on isotopic abundance);

[0046] f—reaction fraction, the specific calculation formula for this value is shown in Equation IV;

[0047] F—Faraday constant, 96485.34;

[0048] W s —Mass of the neptunium sample in the electrolytic cell cup, g;

[0049] The specific formula for calculating blank power is shown in Equation II:

[0050]

[0051] In Formula II:

[0052] E1—The potential at the end of the reduction electrolysis of the blank solution, in V;

[0053] E2—The potential at the end of the oxidation electrolysis of the blank solution, in V;

[0054] E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V;

[0055] E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V;

[0056] Q1—Integral charge measured by coulometric meter at the end of the oxidation electrolysis of the blank solution, in C;

[0057] I r1 —Residual current after oxidation electrolysis of blank solution, A;

[0058] t1 — Oxidation electrolysis time of blank solution, s;

[0059] t2 — Oxidative electrolysis time of neptunium sample solution, s;

[0060] I r2 —Residual current after oxidation and electrolysis of neptunium sample solution, in A;

[0061] The specific formula for calculating the integral factor of the coulomb counter is shown in Equation III:

[0062]

[0063] In Formula III:

[0064] I c —The constant current used during the initial calibration of the coulomb counter, in A;

[0065] t c —Actual calibration time, seconds;

[0066] Q c — Output electrical quantity of the coulomb integrator during calibration, C;

[0067] The specific formula for calculating the reaction fraction is shown in Equation IV:

[0068]

[0069] In Formula IV:

[0070] E1—The potential at the end of the reduction electrolysis of the blank solution, in V;

[0071] E2—The potential at the end of the oxidation electrolysis of the blank solution, in V;

[0072] E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V;

[0073] E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V;

[0074] E0—The standard potential of Np(V) / Np(VI) in a low-concentration sulfuric acid solution, in V;

[0075] R – molar gas constant, 8.3145 Jmol·L -1 K -1 ;

[0076] T—The absolute temperature of the solution during electrolysis, in K (here, T = T0). c +273.15).

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] The controlled potential coulometric method for determining the neptunium content of neptunium dioxide standard material established by this invention has a precision (RSD) better than 0.1%, which meets the requirements for determining the content of standard material. The method is accurate, reliable, and has a wide range of applications. Detailed Implementation

[0079] The present invention will now be described in detail with reference to specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0080] Example 1

[0081] This embodiment provides a method for determining the neptunium content in a neptunium dioxide standard substance. The specific determination steps are as follows:

[0082] (1) Preparation of 0.5 mol / L sulfuric acid solution

[0083] Measure 30-35 mL of analytical grade sulfuric acid using a 50 mL graduated cylinder, add it to 500-550 mL of deionized water, cool it, transfer it to a 1 L volumetric flask, and dilute to volume with deionized water. The prepared sulfuric acid concentration is 0.5 mol / L.

[0084] (2) Preparation of 3 mol / L sulfuric acid solution

[0085] Measure 165-170 mL of analytical grade sulfuric acid using a 100 mL graduated cylinder, add it to 500-550 mL of deionized water, cool it, transfer it to a 1 L volumetric flask, and dilute to volume with deionized water. The prepared sulfuric acid concentration is 3 mol / L.

[0086] (3) Sample dissolution

[0087] Accurately weigh 0.03 g to 0.05 g of neptunium dioxide standard using a balance with an accuracy of 0.01 mg and place it in a 100 mL beaker. Add 10 mL to 15 mL of analytical grade nitric acid solution and 2 to 3 drops of analytical grade HF solution. Place the beaker on an electric furnace (temperature control range: 0℃ to 300℃) and allow the sample to dissolve completely at 160℃ to 180℃. After the solution has completely evaporated to dryness, cool it to 20℃ to 25℃.

[0088] Add 1 mL to 2 mL of 3 mol / L sulfuric acid solution, and continue heating at 220℃ to 250℃ to concentrate the sample until it is nearly dry. Continue heating until white smoke appears. After the white smoke stops appearing, cool the evaporated sample solution to 20℃ to 25℃, add a small amount of 0.5 mol / L sulfuric acid solution, and use a dropper to transfer the sample solution to a pre-weighed aliquot bottle. Dilute the sample with 0.5 mol / L sulfuric acid solution to a neptunium concentration of 1 mg / g to 3 mg / g.

[0089] (4) Sample separation

[0090] Weigh the neptunium solution transferred to the aliquot bottle using a balance with an accuracy of 0.01 mg. Gently squeeze the aliquot bottle to allow the sample solution to drip into the electrolytic cell cup, and weigh the remaining solution in the aliquot bottle. The mass difference is the mass of the solution in the electrolytic cell cup. Depending on the concentration of the neptunium sample solution, the mass of neptunium is 1 mg to 2 mg, which is the mass of neptunium in the electrolytic cell cup, Ws.

[0091] (5) Electrode pretreatment

[0092] Immerse the working electrode and counter electrode in 8 mol / L nitric acid solution, boil for 20 min, and then soak overnight in 8 mol / L nitric acid solution. Before use, rinse the working electrode and counter electrode three times with deionized water, and then perform redox treatment in 0.5 mol / L sulfuric acid electrolyte until the background current is 5 μA to 10 μA, then set aside.

[0093] (6) Measurement of blank charge

[0094] Preheat the coulometric apparatus for 30 minutes. Add 10-15 mL of sulfuric acid solution (1) to the electrolytic cell cup, then add analytical grade high-purity silver oxide (Ag(II)) while stirring until an excess of high-purity silver oxide is added and the black color does not disappear after 10 minutes. Heat the test solution at 190℃-200℃ to completely eliminate the excess black high-purity silver oxide powder. After cooling, add 15-20 mL of sulfuric acid solution (1) so that the sample liquid level just completely submerges the gold electrode. Purge with argon gas (purity 99.99%) for 10-15 minutes (argon flow rate 10-15 mL / min) for 10 minutes. Reduction was performed at a reduction potential of 0.665 V (SCE for a saturated calomel electrode), and the reduction was stopped when the residual current dropped below 10 μA. Oxidation was performed at an oxidation potential of 1.025 V (SCE), and the oxidation was stopped when the residual current dropped below 10 μA. The electrolytic charge Q1, electrolyte potential E2, electrolysis time t1, and residual current I were recorded. r1 .

[0095] (7) Sample determination

[0096] In the pre-separated 1mg-2mg neptunium sample, the sample was measured according to the blank charge determination in step (6), and the electrolytic charge Q was recorded. S Electrolysis potential E4, residual current I r2 And electrolysis time t2.

[0097] (8) Determination of standard electrode potential E0

[0098] Add a certain amount of neptunium solution to the electrolytic cell cup, and perform standard electrode potential measurement according to the blank charge measurement in step (6). Control the potential at 0.665V (SCE) until the residual current is less than 10μA, then stop the reduction. Adjust the control potential at 1.025V (SCE) until the residual current is less than 10μA, record the oxidation charge Q, and stop the oxidation. Control the potential at 0.665V (SCE) until the reduction is close to Q / 2, then stop the reduction. At this time, the concentrations of Np(V) and Np(VI) in the solution are equal. Measure Np 6+ / Np 5+ The potential value of the redox couple is E0.

[0099] (9) Calculation of results

[0100] The specific formula for calculating the neptunium content in the neptunium dioxide standard substance is shown in Equation I:

[0101]

[0102] In Formula I:

[0103] C Np (%) — Neptunium content (%) in neptunium dioxide standard material;

[0104] Q s —Integral charge of the coulombometer during the oxidation of the neptunium sample, C;

[0105] Q b — Blank energy, C, the specific calculation formula for this value is shown in Formula II;

[0106] C—the integral factor of the coulomb counter, and the specific calculation formula for this value is shown in Equation III;

[0107] A r —The relative atomic weight of neptunium (calculated based on isotopic abundance);

[0108] f—reaction fraction, the specific calculation formula for this value is shown in Equation IV;

[0109] F—Faraday constant, 96485.34;

[0110] W s —Mass of the neptunium sample in the electrolytic cell cup, g;

[0111] The specific formula for calculating blank power is shown in Equation II:

[0112]

[0113] In Formula II:

[0114] E1—The potential at the end of the reduction electrolysis of the blank solution, in V;

[0115] E2—The potential at the end of the oxidation electrolysis of the blank solution, in V;

[0116] E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V;

[0117] E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V;

[0118] Q1—Integral charge measured by coulometric meter at the end of the oxidation electrolysis of the blank solution, in C;

[0119] I r1 —Residual current after oxidation electrolysis of blank solution, A;

[0120] t1 — Oxidation electrolysis time of blank solution, s;

[0121] t2 — Oxidative electrolysis time of neptunium sample solution, s;

[0122] I r2 —Residual current after oxidation and electrolysis of neptunium sample solution, in A;

[0123] The specific formula for calculating the integral factor of the coulomb counter is shown in Equation III:

[0124]

[0125] In Formula III:

[0126] I c —The constant current used during the initial calibration of the coulomb counter, in A;

[0127] t c —Actual calibration time, seconds;

[0128] Q c — Output electrical quantity of the coulomb integrator during calibration, C;

[0129] The specific formula for calculating the reaction fraction is shown in Equation IV:

[0130]

[0131] In Formula IV:

[0132] E1—The potential at the end of the reduction electrolysis of the blank solution, in V;

[0133] E2—The potential at the end of the oxidation electrolysis of the blank solution, in V;

[0134] E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V;

[0135] E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V;

[0136] E0—The standard potential of Np(V) / Np(VI) in a low-concentration sulfuric acid solution, in V;

[0137] R – molar gas constant, 8.3145 Jmol·L -1 K -1 ;

[0138] T—The absolute temperature of the solution during electrolysis, in K (here, T = T0). c +273.15).

[0139] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for determining the neptunium content in a neptunium dioxide standard substance, characterized in that, The specific measurement steps are as follows: S1. Prepare low-concentration sulfuric acid solution and high-concentration sulfuric acid solution; S2. Nitric acid solution and HF solution are added to a certain amount of neptunium dioxide standard substance, heated to dissolve, and cooled to obtain a mixed solution; S3. Add the high-concentration sulfuric acid solution obtained in step S1 to the mixed solution obtained in step S2, heat and concentrate to obtain the evaporated sample; S4. Dilute the low-concentration sulfuric acid solution obtained in step S1 with the sample that was evaporated to dryness in step S3 to obtain a diluted sample solution. S5. Measure the blank charge in the electrolytic cell cup without added sample solution; S6. Measure the charge of the diluted sample solution obtained in step S4; S7. Based on the blank charge obtained in step S5 and the charge of the sample solution obtained in step S6, calculate the neptunium content in the neptunium dioxide standard material according to Faraday's law. In step S2, the ratio of neptunium dioxide standard substance: nitric acid solution: HF solution = 0.03–0.05 g: 10–15 mL: 0.10–0.15 mL; The nitric acid solution is an analytical grade nitric acid solution, and the HF solution is an analytical grade HF solution; The heating temperature is 160–180℃; In step S5, the specific steps for measuring blank charge are as follows: Add a low-concentration sulfuric acid solution to the electrolytic cell, then add excess silver oxide. Heat to remove the excess silver oxide, then add more low-concentration sulfuric acid solution until the solution surface submerges the electrodes. Introduce argon gas and perform reduction at the reduction potential. Stop reduction when the residual current drops below 10 μA. Perform oxidation at the oxidation potential. Stop oxidation when the residual current drops below 10 μA. Record the electrolytic charge Q1, electrolyte potentials E1 and E2, electrolysis time t1, and residual current I of the blank solution. r1 ; The silver oxide was analytical grade silver oxide. In step S6, the specific steps for determining the charge of the diluted sample solution are as follows: (1) Immerse the electrode in nitric acid solution, boil it and soak it in nitric acid solution overnight. Before use, clean the electrode with deionized water and perform oxidation-reduction treatment in the electrolyte until the background current meets the requirements. (2) Transfer the diluted sample solution obtained in step S4 to the electrolytic cell cup and calculate the mass Ws of the neptunium sample in the electrolytic cell cup; (3) Add a low-concentration sulfuric acid solution to the electrolytic cell containing the neptunium sample obtained in step (2), add excess silver oxide, heat to remove the excess silver oxide, then add another low-concentration sulfuric acid solution to submerge the electrode. Introduce argon gas and reduce at the reduction potential. Stop the reduction when the residual current drops below 10 μA. Oxidize at the oxidation potential. Stop the oxidation when the residual current drops below 10 μA. Record the electrolytic charge Q of the neptunium sample solution. S Electrolysis potentials E3 and E4, residual current I r2 and electrolysis time t2; (4) Determine the standard electrode potential E0: Add neptunium solution to the electrolytic cell cup and operate according to step (3). Control the potential to reduce to the reduction potential until the residual current is less than 10 μA. Stop the reduction and adjust the control potential to oxidize to the oxidation potential until the residual current is less than 10 μA. Record the oxidation charge Q and stop the oxidation. Control the potential to reduce to the reduction potential. When the charge is close to Q / 2, stop the reduction and measure the standard electrode potential E0.

2. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step S1, a certain amount of analytical grade sulfuric acid is measured and added to deionized water while stirring. After cooling, it is transferred to a volumetric flask and diluted to volume with deionized water. The concentration of the low-concentration sulfuric acid solution is 0.4–0.6 mol / L; The concentration of the high-concentration sulfuric acid solution is 2.5–3.5 mol / L.

3. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step S3, the heating temperature is 220–250°C.

4. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step S4, the concentration of neptunium in the sample solution is 1–3 mg / g.

5. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step (1), the electrode includes a working electrode and a counter electrode; The concentration of the nitric acid solution is 8 mol / L; Boiling time is 20 minutes; The electrolyte is a 0.5 mol / L sulfuric acid electrolyte; The required background current is 5–10 μA.

6. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step (2), the diluted sample solution obtained in step S4 is transferred to a weighed sample bottle, weighed, and then the sample solution in the sample bottle is transferred to an electrolytic cell cup. The remaining solution in the sample bottle is weighed, and the mass difference is the mass of the solution in the electrolytic cell cup. The mass Ws of neptunium in the electrolytic cell cup is calculated based on the concentration of the neptunium solution.

7. The method for determining the neptunium content in a neptunium dioxide standard substance according to claim 1, characterized in that, In step S7, the specific calculation formula for the neptunium content in the neptunium dioxide standard substance is shown in Equation I: In Formula I: C Np (%) — Neptunium content (%) in neptunium dioxide standard material; Qs—Integrated charge in coulometric meter during the oxidation of neptunium sample, C; Q b — Blank energy, C, the specific calculation formula for this value is shown in Formula II; C—the integral factor of the coulomb counter, and the specific calculation formula for this value is shown in Equation III; A r —The relative atomic weight of neptunium; f—reaction fraction, the specific calculation formula for this value is shown in Equation IV; F—Faraday constant, 96485.34; W s —Mass of the neptunium sample in the electrolytic cell cup, g; The specific formula for calculating blank power is shown in Equation II: In Formula II: E1—The potential at the end of the reduction electrolysis of the blank solution, in V; E2—The potential at the end of the oxidation electrolysis of the blank solution, in V; E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V; E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V; Q1—Integral charge measured by coulometric meter at the end of the oxidation electrolysis of the blank solution, in C; I r1 —Residual current after oxidation electrolysis of blank solution, A; t1 — Oxidation electrolysis time of blank solution, s; t2 — Oxidative electrolysis time of neptunium sample solution, s; I r2 —Residual current after oxidation and electrolysis of neptunium sample solution, in A; The specific formula for calculating the integral factor of the coulomb counter is shown in Equation III: In Formula III: I c —The constant current used during the initial calibration of the coulomb counter, in A; t c —Actual calibration time, seconds; Qc—Output electrical quantity of the coulomb integrator during calibration, in C; The specific formula for calculating the reaction fraction is shown in Equation IV: In Formula IV: E1—The potential at the end of the reduction electrolysis of the blank solution, in V; E2—The potential at the end of the oxidation electrolysis of the blank solution, in V; E3—Potential at the end of the reduction electrolysis of the neptunium sample solution, in V; E4—Potential at the end of the oxidative electrolysis of the neptunium sample solution, in V; E0—Standard potential of Np(V) / Np(VI) in low-concentration sulfuric acid solution, V; R—Molar gas constant, 8.3145 J / (mol·K); T – Absolute temperature of the solution during electrolysis, in K.

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