Determination of free oxalic acid and vanadium content in vanadyl oxalate solution by acid-base titration and quantitative addition of oxalic acid complexation-acid-base titration

Abstract

The present invention provides an acid-base titration method and a quantitative addition of oxalic acid complexation-acid-base titration method to realize the determination of free oxalic acid and vanadium content in vanadyl oxalate solution. The determination of free oxalic acid content includes the following steps: Using an automatic potentiometric titrator, directly titrate the solution to be tested with a sodium hydroxide standard solution, calculate the hydrogen ion content based on the volume of the sodium hydroxide standard solution consumed at the inflection point, and then calculate the free oxalic acid content. The determination of vanadium content includes the following steps: Add a certain amount of oxalic acid to the solution to be tested, use a sodium hydroxide standard solution as the titrant, select a pH electrode, and two pH inflections occur during the titration process. Calculate the vanadium content based on the amount of oxalic acid added, the volumes of the standard solution consumed corresponding to the two inflections that occur during the titration process, and the determination result of the free oxalic acid content. This method is also applicable to the determination of free oxalic acid content and metal ion content in oxalate solutions of other transition metals, such as titanium oxalate, cobalt oxalate and other solutions.

Description

Technical Field

[0001] The present invention relates to the measurement technology of the content of free oxalic acid and vanadium content in vanadyl oxalate solution, and in particular to the determination of free oxalic acid and vanadium content in vanadyl oxalate solution by acid-base titration method and quantitative addition of oxalic acid complexation-acid-base titration method. Background Art

[0002] Vanadyl oxalate is the main raw material for the production of denitration catalysts. The selective catalytic reduction (SCR) denitration efficiency can reach more than 90%. This technology is to convert nitrogen oxides (NO X ) into nitrogen and water naturally contained in the air by a reducing agent (such as NH3) at an appropriate temperature in the presence of a catalyst. The catalyst is made by supporting vanadyl oxalate solution with WO3, TiO2, etc. on a carrier and then undergoing post-treatment. Vanadyl oxalate is also widely used in the regeneration of denitration catalysts. Vanadyl oxalate can also be used to produce solid diesel vehicle exhaust purification catalysts. In the exhaust gas ejected after the diesel engine burns diesel, there are many harmful substances that pollute the atmospheric environment, such as carbon monoxide, carbon particles, hydrocarbons, nitrogen oxides, etc. The diesel engine exhaust purification catalyst can catalytically decompose these harmful substances and convert them into harmless substances for emission. Using cordierite honeycomb ceramics as the carrier, impregnating the catalyst with vanadyl oxalate as the main material, and then undergoing post-treatment to form a solid diesel vehicle exhaust purification catalyst. When in use, the exhaust gas flows through the catalytic layer, turning the harmful exhaust gas emitted by the diesel engine into harmless gas, meeting the national IV and national V emission standards. Vanadyl oxalate is also widely used in the synthesis of organic chemical products. In addition to being used as a chemical reagent, it is also used to prepare high-purity vanadium compounds, nanomaterials, glazes, electroplating materials, and high-energy battery material lithium vanadium phosphate.

[0003] If the content of free oxalic acid in vanadyl oxalate solution is too high, oxalic acid crystals are likely to precipitate in a low-temperature environment during storage and transportation. Therefore, it is necessary to strictly control the content of free oxalic acid in the solution. At present, there is no corresponding standard method for the determination of the content of free oxalic acid. The determination of vanadium content in vanadyl oxalate solution usually adopts the general detection method of vanadium content recommended by national standards or metallurgical standards, etc., that is, potassium permanganate (or ammonium persulfate) oxidation-ferrous ammonium sulfate titration method. Due to the presence of oxalic acid, the oxidation process is relatively complex and the consumption of reagents such as potassium permanganate is relatively large. Summary of the Invention

[0004] The purpose of the present invention is to establish a detection method to achieve rapid and accurate determination of the content of free oxalic acid and vanadium in vanadyl oxalate solution. At room temperature (25±5°C), oxalate ions and vanadium ions have a strong complexing ability. On the one hand, this complexation can avoid the precipitation of vanadium salts during the acid-base titration process. On the other hand, as Figure 1 shown, the complexing ratio of oxalate ions to vanadium ions is 2:1. When titrating with sodium hydroxide, only one pH jump appears (asFigure 2 As shown in Figure 2 , the "free" oxalic acid is in a completely ionized state, and the measured hydrogen ion content is twice the content of "free" oxalic acid. The determination of vanadium content is based on the determination result of free oxalic acid. Oxalic acid is quantitatively added to make the molar ratio of oxalate ion to vanadium ion in the solution greater than 2. While the oxalate ion and vanadium ion form a stable complex ion, the corresponding hydrogen ions are released. During the titration with a sodium hydroxide standard solution, the hydrogen ions released from the formation of the complex with vanadium and the hydrogen ions generated from the first ionization of oxalic acid that do not form a complex ion correspond to the first pH jump during the acid-base titration process, and the hydrogen ions generated from the second ionization of oxalic acid that do not form a complex ion correspond to the second pH jump during the acid-base titration process (as shown in Figure 2 ). According to the content of hydrogen ions generated from the second ionization, the determination result of free oxalic acid content, and the amount of oxalic acid added, the vanadium content of the vanadyl oxalate solution can be calculated. The analytical method of determining the free oxalic acid content by acid-base titration and determining the vanadium content by quantitative addition of oxalic acid complexation - acid-base titration is simple in operation and can quickly and accurately determine the free oxalic acid content and vanadium content in the vanadyl oxalate solution. Figure 2 As shown in Figure 2 .

[0005] To achieve the above object, the technical solution adopted by the present invention is to use acid-base titration and quantitative addition of oxalic acid complexation - acid-base titration to respectively determine the free oxalic acid content and vanadium content in the vanadyl oxalate solution, including the following steps:

[0006] Step (1) Determination of free oxalic acid content in the vanadyl oxalate solution:

[0007] (1-1) Accurately transfer a certain volume (V0) of the vanadyl oxalate solution to a beaker and dilute it with water (for example, accurately transfer a certain volume (V0) of the vanadyl oxalate solution to a 250 mL beaker and dilute it to 150 mL);

[0008] (1-2) Using a sodium hydroxide standard solution (C NaOH ) as the titrant, select a pH electrode and use an automatic potentiometric titrator for titration;

[0009] (1-3) When a pH jump occurs during the titration process, the volume of the sodium hydroxide standard solution consumed is V1. Calculate the free oxalic acid content in the vanadyl oxalate solution according to formula (1)

[0010]

[0011] In the formula:

[0012] —Free oxalate ion content in the vanadyl oxalate solution, unit: mole per liter, mol / L;

[0013] —Coefficient for converting hydrogen ion content to corresponding oxalate ion;

[0014] C NaOH — Standard concentration of sodium hydroxide solution, unit: mole per liter, mol / L;

[0015] V0 — Sampling volume of vanadyl oxalate solution, unit: milliliter, mL;

[0016] V1 — Consumption volume of sodium hydroxide standard solution corresponding to pH jump, unit: milliliter, mL;

[0017] Step (2) Determination of vanadium content in vanadyl oxalate solution:

[0018] (2-1) Accurately pipette a certain volume (V2) of vanadyl oxalate solution into a beaker (such as a 250 mL beaker);

[0019] (2-2) Accurately weigh a certain mass of oxalic acid dihydrate (C2H4O4·2H2O), and add it to the beaker containing vanadyl oxalate solution, then dilute with water (for example, dilute to 150 mL);

[0020] (2-3) Using sodium hydroxide standard solution (C NaOH ) as the titrant, select a pH electrode, and perform titration using an automatic potentiometric titrator;

[0021] (2-4) Two pH jumps occur during the titration process. The consumption volume of sodium hydroxide standard solution corresponding to the first pH jump is V3, and the consumption volume of sodium hydroxide standard solution corresponding to the second pH jump is V4. Calculate the vanadium content in the vanadyl oxalate solution according to formula (2)

[0022]

[0023] In the formula:

[0024] — Vanadium content in vanadyl oxalate solution, unit: mole per liter, mol / L;

[0025] — Free oxalic acid content in vanadyl oxalate solution, unit: mole per liter, mol / L;

[0026] C NaOH — Standard concentration of sodium hydroxide solution, unit: mole per liter, mol / L;

[0027] V2 — Sampling volume of vanadyl oxalate solution, unit: milliliter, mL;

[0028] V3 — Consumption volume of sodium hydroxide standard solution corresponding to the first pH jump, unit: milliliter, mL;

[0029] V4—the volume of the sodium hydroxide standard solution consumed corresponding to the second pH jump, in milliliters, mL;

[0030] —the mass of oxalic acid dihydrate weighed, in grams, g;

[0031] —the numerical value of the molar mass of oxalic acid dihydrate, 126.07, in grams per mole, g / mol;

[0032] Furthermore, in step (1), the concentration of the sodium hydroxide standard solution is 0.1 mol / L to 1.0 mol / L, preferably 0.4 mol / L to 0.6 mol / L.

[0033] Furthermore, in step (1), the two hydrogen ions corresponding to the free oxalic acid are completely ionized, and only 1 pH jump appears during the titration process.

[0034] Furthermore, in step (2), the amount of oxalic acid dihydrate added is estimated according to the vanadium content range and the free oxalic acid content in the vanadyl oxalate solution.

[0035] Furthermore, in step (2), the amount of oxalic acid dihydrate added is determined by gradually increasing the amount of oxalic acid dihydrate added until two obvious pH jumps appear in the acid-base titration curve.

[0036] Furthermore, in step (2), when oxalic acid dihydrate is quantitatively added and the ratio of the amount of substance of oxalate ions to vanadium ions in the solution is controlled at 3.0 to 4.5, the repeatability and accuracy of the detection results are better.

[0037] Furthermore, in step (2), the concentration range of the sodium hydroxide standard solution is: 0.1 mol / L to 1.0 mol / L, preferably 0.4 mol / L to 0.6 mol / L.

[0038] Furthermore, the detection processes of step (1) and step (2) need to be carried out at 25 ± 5 °C.

[0039] The present invention can achieve: 1) determining the content of free oxalic acid in the vanadyl oxalate solution by acid-base titration; 2) determining the vanadium content in the vanadyl oxalate solution by quantitative addition of oxalic acid complexation - acid-base titration. The present invention is also applicable to the determination of the content of free oxalic acid and metal ions in oxalate solutions of other transition metals, such as titanium oxalate solution, cobalt oxalate solution, etc.

[0040] The method for detecting the content of free oxalic acid and vanadium in the vanadyl oxalate solution of the present invention has scientific and reasonable steps, and has the following advantages compared with the prior art:

[0041] 1) The acid-base titration method provided by the present invention for determining free oxalic acid in the vanadyl oxalate solution is simple in operation, accurate in result, and excellent in detection precision.

[0042] 2) The quantitative addition of oxalic acid complexation - acid-base titration method provided by the present invention for determining the vanadium content in vanadyl oxalate solution has fewer types of reagents, less reagent consumption, and is environmentally friendly compared to the redox titration method.

[0043] In summary, the acid-base titration method and the quantitative addition of oxalic acid complexation - acid-base titration method of the present invention respectively realize the determination of the free oxalic acid content and vanadium content in vanadyl oxalate solution, and are particularly suitable for the quality control detection in the production and application of vanadyl oxalate solution. Description of the Drawings

[0044] Figure 1 Schematic diagram of oxalic acid - vanadium complex ion;

[0045] Figure 2 Titration curve of directly determining free oxalic acid in vanadyl oxalate solution by acid-base titration method;

[0046] Figure 3 Titration curve of determining vanadium content in vanadyl oxalate solution by oxalic acid complexation - acid-base titration method. Detailed Embodiments

[0047] The present invention discloses an acid-base titration method for determining the free oxalic acid and vanadium content in vanadyl oxalate solution: using the acid-base titration method to determine the free oxalic acid content, and then using the quantitative addition of oxalic acid complexation - acid-base titration method to determine and calculate the vanadium content. The method includes the following steps: 1) Prepare and standardize the sodium hydroxide standard solution; 2) Determine the free oxalic acid in vanadyl oxalate solution by acid-base titration method; 3) Select the amount of oxalic acid added: the amount of oxalic acid added is 3.5 times the total amount of vanadium ions in the sample solution to be titrated minus the total amount of free oxalic acid. Using the sodium hydroxide standard solution as the titrant, select a pH electrode, and use an automatic potentiometric titrator for titration. 4) Calculate the hydrogen ion content generated by the second ionization of the uncomplexed oxalic acid from the volume difference of the sodium hydroxide standard solution consumed at the second and first inflection points in the titration in step 3). This value corresponds to the content of the uncomplexed oxalic acid (the oxalic acid remaining after the quantitative addition of oxalic acid forms a 2:1 complex with vanadium ions). Combining with the free oxalic acid content in step 1), use the formula to calculate the vanadium content in vanadyl oxalate solution.

[0048] Only reagents confirmed to be of analytical purity and water meeting the requirements of grade 3 water in GB / T 6682 "Specifications and Test Methods for Water for Analytical Laboratory Use" are used in the method of the present invention.

[0049] The following further illustrates the present invention with examples:

[0050] Example 1

[0051] 1. Preparation and standardization of sodium hydroxide standard solution:

[0052] 1A. Weigh 110 g of sodium hydroxide and dissolve it in 100 mL of carbon dioxide-free water. Shake well, transfer it to a polyethylene container, close it, and let it stand until the solution becomes clear. Use a plastic pipette to transfer 27 mL of the supernatant, dilute it to 1000 mL with carbon dioxide-free water, shake well, and wait for standardization.

[0053] 1B. Accurately weigh 1.0 g (accurate to 0.0001 g) of potassium hydrogen phthalate that has been dried to constant weight at 105 - 110 °C and place it in a 250 mL beaker. Add 150 mL of carbon dioxide-free water, stir to dissolve it, and use a pH electrode potentiometric titrator to titrate with the sodium hydroxide standard solution as the titrant. Record the volume of the sodium hydroxide standard solution consumed at the pH jump point. At the same time, conduct a blank test and calculate the concentration C of the sodium hydroxide standard solution according to formula (1). NaOH = 0.5158 mol / L.

[0054] C NaOH = 1000 × m KHP / M KHP / (V5 - V6)……(1)

[0055] Where:

[0056] 1000 - Conversion coefficient between milliliters (mL) and liters (L) of volume units;

[0057] C - Concentration of the sodium hydroxide standard solution, in mol / L;

[0058] m KHP —Mass of the potassium hydrogen phthalate reference reagent, in g;

[0059] M KHP —Numerical value of the molar mass of the potassium hydrogen phthalate reference reagent, 204.32, in g / mol;

[0060] V5 - Volume of the sodium hydroxide standard solution consumed by the potassium hydrogen phthalate reference reagent, in mL;

[0061] V6 - Volume of the sodium hydroxide standard solution consumed by the blank, in mL;

[0062] 2. Acid-base titration to determine free oxalic acid in the vanadyl oxalate solution:

[0063] 2A. Accurately pipette 2.0 mL of the vanadyl oxalate solution into a 250 mL beaker, dilute it to 150 mL with water, use the sodium hydroxide standard solution (C NaOH = 0.5158 mol / L) as the titrant, select a pH electrode, and use an automatic potentiometric titrator to titrate. There is a potential jump point on the titration curve. Record the volume V1 of the sodium hydroxide standard solution consumed, and conduct 3 parallel tests.

[0064] 2B. Calculate the free oxalic acid using formula (2). The test data and calculation results are shown in Table 1.

[0065]

[0066] In the formula:

[0067] — The content of free oxalate in the vanadyl oxalate solution, in moles per liter, mol / L;

[0068] — The coefficient corresponding to the conversion of hydrogen ion content to oxalate;

[0069] C NaOH — The calibrated concentration of the sodium hydroxide standard solution, in moles per liter, mol / L;

[0070] V0 — The sampling volume of the vanadyl oxalate solution, in milliliters, mL;

[0071] V1 — The volume of the sodium hydroxide standard solution consumed corresponding to the pH jump, in milliliters, mL;

[0072] Table 1 Detection results of free oxalic acid in vanadyl oxalate solution

[0073]

[0074] 3. Quantitative addition of oxalic acid complexation — Acid-base titration method for measuring vanadium content

[0075] 3A. Pipette 3 portions of 1.0 mL of vanadyl oxalate solution into a 250 mL beaker. Add 0.3 g, 0.4 g, and 0.5 g of oxalic acid to the beakers respectively, dilute and dissolve with 30 mL of water, and then dilute to 150 mL with water. Use the sodium hydroxide standard solution (concentration: C1 = 0.5158 mol / L) as the titrant and perform titration using an automatic potentiometric titrator. There are two potential jump points in the titration curve. The volume of the sodium hydroxide standard solution consumed corresponding to the first pH jump point is V3, and the volume of the sodium hydroxide standard solution consumed corresponding to the second pH jump point is V4. Conduct 3 parallel tests.

[0076] 3B. Calculate the vanadium content in the vanadyl oxalate solution using formula (3). The test and calculation results are shown in Table 2.

[0077]

[0078] In the formula:

[0079] — The vanadium content in the vanadyl oxalate solution, in moles per liter, mol / L;

[0080] — Content of free oxalic acid in vanadyl oxalate solution, unit: mole per liter, mol / L;

[0081] C NaOH — Standardized concentration of sodium hydroxide standard solution, unit: mole per liter, mol / L;

[0082] V2 — Sampling volume of vanadyl oxalate solution, unit: milliliter, mL;

[0083] V3 — Volume of sodium hydroxide standard solution consumed corresponding to the first pH jump, unit: milliliter, mL;

[0084] V4 — Volume of sodium hydroxide standard solution consumed corresponding to the second pH jump, unit: milliliter, mL;

[0085] — Mass of oxalic acid dihydrate weighed, unit: gram, g;

[0086] — Numerical value of molar mass of oxalic acid dihydrate, 126.07, unit: gram per mole, g / mol;

[0087] Table 2 Detection results of vanadium content in vanadyl oxalate solution

[0088]

[0089] Example 2

[0090] 1. Preparation and standardization of sodium hydroxide standard solution:

[0091] 1A. Weigh 110 g of sodium hydroxide and dissolve it in 100 mL of carbon dioxide-free water. Shake well, transfer it to a polyethylene container, and close it. Let it stand until the solution becomes clear. Use a plastic pipette to pipette 27 mL of the supernatant and dilute it to 1000 mL with carbon dioxide-free water. Shake well and wait for standardization;

[0092] 1B. Accurately weigh 1.0 g (accurate to 0.0001 g) of potassium hydrogen phthalate dried to constant weight at 105 - 110 °C into a 250 mL beaker. Add 150 mL of carbon dioxide-free water, stir to dissolve it, and use a pH electrode potentiometric titrator to titrate with the sodium hydroxide standard solution as the titrant. Record the volume of the sodium hydroxide standard solution consumed at the pH jump. At the same time, do a blank test and calculate the concentration of the sodium hydroxide standard solution, C according to formula (1) NaOH = 0.5447 mol / L.

[0093] C NaOH = 1000 × m KHP / M KHP / (V5 - V6)......(1)

[0094] In the formula:

[0095] 1000—the conversion factor between the volume units of milliliters (mL) and liters (L);

[0096] C NaOH —the concentration of the sodium hydroxide standard solution, in mol / L;

[0097] m KHP —the mass of the potassium hydrogen phthalate primary standard reagent, in g;

[0098] M KHP —the numerical value of the molar mass of the potassium hydrogen phthalate primary standard reagent, 204.32, in g / mol;

[0099] V5—the volume of the sodium hydroxide standard solution consumed by the potassium hydrogen phthalate primary standard reagent, in mL;

[0100] V6—the volume of the sodium hydroxide standard solution consumed by the blank, in mL;

[0101] 2. Acid-base titration for the determination of free oxalic acid in the vanadyl oxalate solution:

[0102] 2A. Accurately pipette 1.0 mL of the vanadyl oxalate solution into a 250 mL beaker, dilute it with water to 150 mL, use the sodium hydroxide standard solution (C NaOH = 0.5477 mol / L) as the titrant, select a pH electrode, and perform titration using an automatic potentiometric titrator. There is a potential jump point on the titration curve. Record the volume of the sodium hydroxide standard solution consumed, V1, and conduct 3 parallel tests.

[0103] 2B. Calculate the free oxalic acid using formula (2). The test data and calculation results are shown in Table 3.

[0104]

[0105] In the formula:

[0106] —the content of free oxalate in the vanadyl oxalate solution, in moles per liter, mol / L;

[0107] —the conversion coefficient of hydrogen ions corresponding to oxalate;

[0108] C NaOH —the calibrated concentration of the sodium hydroxide standard solution, in moles per liter, mol / L;

[0109] V0—the sampling volume of the vanadyl oxalate solution, in milliliters, mL;

[0110] V1—the volume of the sodium hydroxide standard solution consumed corresponding to the pH jump, in milliliters, mL;

[0111] Table 3 Detection Results of Free Oxalic Acid in Vanadyl Oxalate Solution

[0112]

[0113] 3. Quantitative addition of oxalic acid complexation - acid-base titration method for measuring vanadium content

[0114] 3A. Pipette 3 portions of 1.0 mL of vanadyl oxalate solution into a 250 mL beaker. Add 0.3 g, 0.4 g, and 0.5 g of oxalic acid to the beakers respectively. Add 30 mL of water to dilute and dissolve, then further dilute to 150 mL with water. Use sodium hydroxide standard solution (concentration: C1 = 0.5477 mol / L) as the titrant and perform titration using an automatic potentiometric titrator. There are two potential jump points in the titration curve. The volume of sodium hydroxide standard solution consumed corresponding to the first pH jump point is V3, and the volume of sodium hydroxide standard solution consumed corresponding to the second pH jump point is V4. Conduct 3 parallel tests.

[0115] 3B. Calculate the vanadium content in the vanadyl oxalate solution using formula (3). The detection and calculation results are shown in Table 4.

[0116]

[0117] Wherein:

[0118] — Vanadium content in vanadyl oxalate solution, unit is mole per liter, mol / L;

[0119] — Free oxalic acid content in vanadyl oxalate solution, unit is mole per liter, mol / L;

[0120] C NaOH — Standardized concentration of sodium hydroxide standard solution, unit is mole per liter, mol / L;

[0121] V2 — Sampling volume of vanadyl oxalate solution, unit is milliliter, mL;

[0122] V3 — Volume of sodium hydroxide standard solution consumed corresponding to the first pH jump, unit is milliliter, mL;

[0123] V4 — Volume of sodium hydroxide standard solution consumed corresponding to the second pH jump, unit is milliliter, mL;

[0124] — Mass of oxalic acid dihydrate weighed, unit is gram, g;

[0125] — Numerical value of molar mass of oxalic acid dihydrate, 126.07, unit is gram per mole, g / mol;

[0126] Table 4 Detection Results of Vanadium Content in Vanadyl Oxalate Solution

[0127]

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. The free oxalic acid and vanadium content in the vanadyl oxalate solution are determined by acid-base titration and quantitative addition of oxalic acid complexation-acid-base titration, characterized in that, It includes the following steps: Step (1): Determination of the free oxalic acid content in the vanadyl oxalate solution (1-1) Accurately pipette a certain volume V0 of the vanadyl oxalate solution into a beaker and dilute it with water; (1-2) Using sodium hydroxide standard solution C NaOH as the titrant, select a pH electrode and use an automatic potentiometric titrator for titration; (1-3) The volume of the sodium hydroxide standard solution consumed corresponding to the pH jump occurring during the titration is V1, and the free oxalic acid content in the vanadyl oxalate solution is calculated according to formula (1). In the formula: — The content of free oxalate in the vanadyl oxalate solution, unit: mole per liter, mol / L; — coefficient of conversion corresponding to hydrogen ions containing oxalate radicals; C NaOH — Standardized concentration of sodium hydroxide standard solution, unit: mole per liter, mol / L; V0—the sampling volume of the vanadyl oxalate solution, in milliliters, mL; V1—the consumed volume of the sodium hydroxide standard solution corresponding to the pH jump, in milliliters, mL; Step (2): Determination of the vanadium content in the vanadyl oxalate solution (2-1) Accurately pipette a certain volume V2 of the vanadyl oxalate solution into a beaker; (2-2) Weigh accurately a certain mass of oxalic acid dihydrate H2C2O4·2H2O, add it to the beaker containing the vanadyl oxalate solution in step (2-1), and dilute with water; (2-3) Using sodium hydroxide standard solution C NaOH as the titrant, select a pH electrode and use an automatic potentiometric titrator for titration; (2-4) There are two pH jumps during the titration process. The volume of the sodium hydroxide standard solution consumed corresponding to the first pH jump is V3, and the volume of the sodium hydroxide standard solution consumed corresponding to the second pH jump is V4. Calculate the vanadium content in the vanadyl oxalate solution according to formula (2). In the formula: — The vanadium content in the vanadyl oxalate solution, in moles per liter, mol / L; — The content of free oxalic acid in the vanadyl oxalate solution, unit: mole per liter, mol / L; C NaOH — Standard concentration of sodium hydroxide standard solution, unit is mole per liter, mol / L; V2—the sampling volume of the vanadyl oxalate solution, in milliliters, mL; V3—the consumed volume of the sodium hydroxide standard solution corresponding to the first pH jump, in milliliters, mL; V4—the consumed volume of the sodium hydroxide standard solution corresponding to the second pH jump, in milliliters, mL; — Mass of oxalic acid dihydrate, unit: gram, g; — The numerical value of the molar mass of oxalic acid dihydrate, 126.07, unit grams per mole, g / mol.

2. The determination of the free oxalic acid and vanadium content in the vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation - acid-base titration method according to claim 1, characterized in that, In step (1), the concentration of the sodium hydroxide standard solution is 0.1 mol / L to 1.0 mol / L.

3. The method for determining the contents of free oxalic acid and vanadium in vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation-acid-base titration method according to claim 1, wherein In step (1), the two hydrogen ions corresponding to the free oxalic acid are completely ionized, and only 1 pH jump appears during the titration process.

4. The determination of free oxalic acid and vanadium content in vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation - acid-base titration method according to claim 1, characterized in that, Step (2): Estimate the amount of oxalic acid dihydrate added according to the vanadium content range and the free oxalic acid content in the vanadyl oxalate solution.

5. The determination of free oxalic acid and vanadium content in vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation-acid-base titration method according to claim 1, characterized in that, Step (2): Determine the amount of oxalic acid dihydrate added by gradually increasing the amount of oxalic acid dihydrate added until two obvious pH jumps appear in the acid-base titration curve.

6. The determination of the free oxalic acid and vanadium content in the vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation-acid-base titration method according to claim 1, characterized in that, Step (2): Quantitatively add oxalic acid dihydrate to control the ratio of the amount of oxalate ion to the amount of vanadium ion in the solution to be 3.0 to 4.

5.

7. The method for determining the contents of free oxalic acid and vanadium in vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation-acid-base titration method according to claim 1, wherein In step (2), the concentration range of the sodium hydroxide standard solution is 0.1 mol / L to 1.0 mol / L.

8. The method for determining the contents of free oxalic acid and vanadium in vanadyl oxalate solution by the acid-base titration method and the quantitative addition of oxalic acid complexation-acid-base titration method according to claim 1, wherein The detection processes of step (1) and step (2) are carried out at 25 ± 5 °C.

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