A method for testing vanadium content and vanadium valence

Abstract

The present invention belongs to the field of vanadium element analysis and testing, and specifically relates to a method for testing vanadium content and vanadium valence. The testing method of the present invention comprises the following steps: adding a buffer solution to an excess oxidant solution to obtain a standby solution; adding a vanadium-containing electrolyte to be tested with a volume of V to the standby solution, standing, and obtaining an intermediate solution containing excess oxidant and pentavalent vanadium ions; using a potentiometric titrator to add a titrant to the intermediate solution until two sudden jump points appear; the first sudden jump point is a sudden jump in the reduction potential of the excess oxidant, and the volume of the titrant consumed corresponding to the reduction of the excess oxidant is V1; the second sudden jump point is a sudden jump in the reduction potential of pentavalent vanadium, and the total volume of the titrant consumed at this time is V2, and the volume of the titrant consumed corresponding to the reduction of pentavalent vanadium ions to tetravalent vanadium ions is V2-V1; the vanadium content and vanadium valence are calculated by formula. The detection method of the present invention is simple, fast and accurate.

Description

Technical Field

[0001] The invention belongs to the field of vanadium element analysis and testing, and in particular relates to a method for testing vanadium content and vanadium valence. Background Art

[0002] All-vanadium liquid flow batteries (vanadium batteries for short) are widely used in large-scale energy storage systems because of their high safety, long service life, deep charge and discharge, environmental friendliness, and independent capacity and power adjustment. Vanadium is a typical variable valence element, with the main valence states of V(II), V(III), V(IV) and V(V). There are only one or two adjacent valence states of vanadium ions in the vanadium electrolyte, which is a mixture of V(III) and V(IV) in the initial state. As charging and discharging proceed, the positive electrolyte transforms between V(IV) and V(V), and the negative electrolyte transforms between V(II) and V(III). The conversion between chemical energy and electrical energy is achieved through the transformation of vanadium valence states.

[0003] In order to effectively monitor the quality of electrolyte products and the operating status of vanadium batteries, it is necessary to detect the changes in the valence state of vanadium in the electrolyte at any time during the charge and discharge process, that is, the concentration of vanadium in each valence state. The methods for determining the concentration of vanadium are usually UV-visible spectroscopy and potentiometric titration. Since V (V) ions have no characteristic absorption peaks in the ultraviolet-visible region, UV-visible spectroscopy cannot be used to determine the valence state analysis of the positive electrolyte. The traditional potentiometric titration method usually uses potassium permanganate solution or ammonium ferrous sulfate solution as a titrant, and it is necessary to predict the valence state of the sample so as to adopt a targeted test method and the operation is cumbersome. For example, when titrating the positive electrolyte with potassium permanganate solution and the electrolyte of any valence state with ammonium ferrous sulfate solution, multi-step titration is required. In addition, the V (II) ions in the negative electrolyte are extremely unstable, and special inert gas protection and strict sealing devices are required from sampling to sampling, which increases the uncertainty of the measurement results.

[0004] Therefore, there is an urgent need to develop a rapid, accurate and applicable method for detecting the vanadium valence state and vanadium content. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the present invention pre-prepare an excess of strong oxidizing solution, mix it immediately when sampling, so that the low-valent vanadium ions in the electrolyte are quickly oxidized to the highest valence V (V), and the ammonium ferrous sulfate solution is used as a titrant to obtain the total vanadium concentration and the concentration of vanadium in each valence state through one-step potentiometric titration. The method is simple, fast, and accurate, does not require the valence state of the sample to be known in advance, greatly avoids the problem of uncontrollable oxidation of low-valent vanadium, and is suitable for monitoring the valence state and vanadium content of vanadium in any environment.

[0006] Specifically, the present invention provides a method for testing vanadium content and vanadium valence, the method comprising the following steps:

[0007] S1: adding a buffer solution to an excess oxidant solution to obtain a standby solution;

[0008] S2: adding a volume V of the electrolyte to be tested containing vanadium element to the reserve solution, and letting it stand to obtain an intermediate solution containing excess oxidant and pentavalent vanadium ions;

[0009] S3: Use a potentiometric titrator to add a titrant to the intermediate solution until two jump points appear; wherein the first jump point is a potential jump of excess oxidant reduction, and the volume of titrant consumed for reducing the excess oxidant is V1; the second jump point is a potential jump of pentavalent vanadium reduction, and the total volume of titrant consumed at this time is V2, and the volume of titrant consumed for reducing pentavalent vanadium ions to tetravalent vanadium ions is V2-V1; wherein the titrant is ammonium ferrous sulfate solution;

[0010] The total concentration of vanadium ions in the electrolyte to be tested is C 总 The calculation formula is: C 总 =C Fe (V2–V1) / V;

[0011] The calculation formula of the total concentration X of the valence change of vanadium ions in the electrolyte to be tested during the oxidation process is: X = (C ox V ox –C Fe V1) / V;

[0012] The calculation formula for the average valence state S of vanadium ions in the electrolyte to be tested is: S = 5-X / C 总 ;

[0013] Among them, C 总 The unit is mol / L;

[0014] C Fe is the concentration of ammonium ferrous sulfate solution, in mol / L;

[0015] The unit of V, V1 and V2 is L;

[0016] The unit of X is mol / L;

[0017] C ox It is the total concentration of the valence change of the oxidant solution during the oxidation process, in mol / L;

[0018] V ox is the volume of the oxidant solution, in L;

[0019] The calculation method of the valence state of vanadium ions and the concentration of the corresponding valence state in the electrolyte to be tested is:

[0020] If X = 3C 总, then the valence state of vanadium ions in the electrolyte to be tested is all +2;

[0021] If 2C 总 <X<3C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +2 to +3, and the concentration of +2 vanadium ions C V(II) For X-2C 总 , +3-valent vanadium ion concentration C V(III) For 3C 总 -X;

[0022] If X = 2C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +3;

[0023] If C 总 <X<2C 总 , then the average valence of vanadium ions in the electrolyte to be tested is +3 to +4, and the concentration of +3 vanadium ions C V(III) For XC 总 , +4 vanadium ion concentration C V(IV) 2C 总 -X;

[0024] If X=C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +4;

[0025] If 0<X <C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +4 to +5, and the concentration of +4 vanadium ions is C V(IV) =X, the concentration of +5-valent vanadium ions is C V(V) =C 总 -X;

[0026] If X=0, the valence states of all vanadium ions in the electrolyte to be tested are +5.

[0027] In one or more embodiments, the oxidant solution is a potassium permanganate solution or a potassium dichromate solution.

[0028] In one or more embodiments, when the oxidant solution is a potassium permanganate solution, C ox Equal to 5 times the concentration of potassium permanganate solution; when the oxidant solution is potassium dichromate solution, C ox Equivalent to 6 times the concentration of potassium dichromate solution.

[0029] In one or more embodiments, the concentration of the oxidant solution is 0.005-0.030 mol / L.

[0030] In one or more embodiments, the volume of the oxidant solution is 5-50 mL.

[0031] In one or more embodiments, the buffer solution is a sulfuric acid-phosphoric acid mixed solution or a phosphoric acid solution.

[0032] In one or more embodiments, the sulfuric acid-phosphoric acid mixed solution is prepared by mixing 98 wt % sulfuric acid, 85 wt % phosphoric acid and water in a volume ratio of (0.5-2):(0.5-2):(0.8-4).

[0033] In one or more embodiments, the volume of the buffer solution is 10-100 mL.

[0034] In one or more embodiments, the volume V of the electrolyte to be tested is 0.1-5.0 mL.

[0035] In one or more embodiments, the concentration of the ammonium ferrous sulfate solution is 0.02-0.20 mol / L. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the first potentiometric titration curve of Example 1 of the present invention, wherein the blue curve is the potential-volume change curve of the electrolyte to be tested, and the green curve is the first-order derivative of the potential change with volume.

[0037] Figure 2 This is the first potentiometric titration curve of Example 2 of the present invention, wherein the blue curve is the potential-volume change curve of the electrolyte to be tested, and the green curve is the first-order derivative of the potential change with volume.

[0038] Figure 3 This is the first potentiometric titration curve of Example 3 of the present invention, wherein the blue curve is the potential-volume change curve of the electrolyte to be tested, and the green curve is the first-order derivative of the potential change with volume.

[0039] Figure 4 It is the potentiometric titration curve of comparative example 1 of the present invention, wherein the blue curve is the potential-volume variation curve of the electrolyte to be tested, and the green curve is the first-order derivative of the potential variation with volume. DETAILED DESCRIPTION

[0040] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0041] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0042] Herein, “comprising”, “including”, “containing” and similar terms encompass the meanings of “consisting essentially of” and “consisting of”. For example, when “A comprises B and C” is disclosed herein, “A consists essentially of B and C” and “A consists of B and C” should be deemed to be disclosed herein.

[0043] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are only for brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0044] In this document, unless otherwise specified, percentage refers to mass percentage and ratio refers to mass ratio.

[0045] Herein, when describing embodiments or examples, it should be understood that they are not used to limit the present invention to these embodiments or examples. On the contrary, all substitutes, improvements and equivalents of the methods and materials described in the present invention can be included in the scope limited by the claims.

[0046] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0047] The present invention provides a method for testing vanadium content and vanadium valence, the method comprising the following steps:

[0048] S1: adding a buffer solution to an excess oxidant solution to obtain a standby solution;

[0049] S2: adding a volume V of the electrolyte to be tested containing vanadium element to the reserve solution, and letting it stand to obtain an intermediate solution containing excess oxidant and pentavalent vanadium ions;

[0050] S3: Use a potentiometric titrator to add a titrant to the intermediate solution until two jump points appear; wherein the first jump point is a potential jump of excess oxidant reduction, and the volume of titrant consumed for reducing the excess oxidant is V1; the second jump point is a potential jump of pentavalent vanadium reduction, and the total volume of titrant consumed at this time is V2, and the volume of titrant consumed for reducing pentavalent vanadium ions to tetravalent vanadium ions is V2-V1; wherein the titrant is ammonium ferrous sulfate solution;

[0051] The total concentration of vanadium ions in the electrolyte to be tested is C 总 The calculation formula is: C 总 =C Fe (V2–V1) / V;

[0052] The calculation formula of the total concentration X of the valence change of vanadium ions in the electrolyte to be tested during the oxidation process is: X = (C ox V ox –C Fe V1) / V;

[0053] The calculation formula for the average valence state S of vanadium ions in the electrolyte to be tested is: S = 5-X / C 总 ;

[0054] Among them, C 总 The unit is mol / L;

[0055] C Fe is the concentration of ammonium ferrous sulfate solution, in mol / L;

[0056] The unit of V, V1 and V2 is L;

[0057] The unit of X is mol / L;

[0058] C ox is the total concentration of the valence change of the oxidant solution during the oxidation process, in mol / L; in the present invention, C ox It is equal to the concentration of the oxidant in the oxidant solution multiplied by the valence change of each mole of the oxidant reacting with ferrous sulfate. For example, when the oxidant is potassium permanganate, potassium permanganate undergoes a redox reaction, in which +7-valent manganese ions are reduced to +2-valent manganese ions. At this time, the valence change of the oxidant is 5, C ox =Equal to the concentration of the oxidant multiplied by 5; when the oxidant is potassium dichromate, potassium dichromate undergoes an oxidation-reduction reaction, in which the two +6-valent chromium ions in potassium dichromate are reduced to two +3-valent chromium ions. At this time, the corresponding valence change of the oxidant is 6, C ox It is equal to the concentration of the oxidant multiplied by 6;

[0059] V ox is the volume of the oxidant solution, in L;

[0060] The calculation method of the valence state of vanadium ions and the concentration of the corresponding valence state in the electrolyte to be tested is:

[0061] If X = 3C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +2;

[0062] If 2C 总 <X<3C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +2 to +3, and the concentration of +2 vanadium ions C V(II) For X-2C 总 , +3-valent vanadium ion concentration C V(III) For 3C 总 -X;

[0063] If X = 2C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +3;

[0064] If C 总 <X<2C 总 , then the average valence of vanadium ions in the electrolyte to be tested is +3 to +4, and the concentration of +3 vanadium ions C V(III) For XC 总 , +4 vanadium ion concentration C V(IV) 2C 总 -X;

[0065] If X=C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +4;

[0066] If 0<X <C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +4 to +5, and the concentration of +4 vanadium ions is C V(IV) =X, the concentration of +5-valent vanadium ions is C V(V) =C 总 -X;

[0067] If X=0, the valence state of the vanadium ions in the electrolyte to be tested is all +5. In the present invention, the oxidant solution can be a potassium permanganate solution or a potassium dichromate solution. In the present invention, when the oxidant solution is a potassium permanganate solution, C ox Equal to 5 times the concentration of potassium permanganate solution; when the oxidant solution is potassium dichromate solution, C oxEqual to 6 times the concentration of potassium dichromate solution. In the present invention, the concentration of the oxidant solution can be 0.005-0.030 mol / L, such as 0.005 mol / L, 0.010 mol / L, 0.015 mol / L, 0.020 mol / L, 0.025 mol / L, 0.030 mol / L. In the present invention, the volume of the oxidant solution can be 5-50 mL, such as 5 mL, 10 mL, 15 mL, 20 mL, 25 mL, 30 mL, 35 mL, 40 mL, 45 mL, 50 mL.

[0068] In the present invention, the buffer solution can be a sulfuric acid-phosphoric acid mixed solution or a phosphoric acid solution. The volume of the buffer solution can be 10-100mL, for example 10mL, 20mL, 30mL, 40mL, 50mL, 60mL, 70mL, 80mL, 90mL, 100mL. In the sulfuric acid-phosphoric acid mixed solution of the present invention, the volume ratio of 98wt% sulfuric acid, 85wt% phosphoric acid and water can be (0.5-2):(0.5-2):(0.8-4).

[0069] In the present invention, the volume V of the electrolyte to be tested can be 0.1-5.0 mL, for example, 0.1 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, 4.5 mL, 5 mL.

[0070] In the present invention, the concentration of the ammonium ferrous sulfate solution can be 0.02-0.20 mol / L, for example, 0.02 mol / L, 0.05 mol / L, 0.10 mol / L, 0.15 mol / L, 0.20 mol / L.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] (1) The present invention provides a universal method for determining the vanadium content and vanadium valence state in a vanadium electrolyte of any valence state. The concentration of each valence state and the total vanadium concentration can be simultaneously determined through a single titration process. There is no need to pay attention to the valence state before the test, and the method is applicable to vanadium electrolytes in any environment.

[0073] (2) The electrolyte to be tested in the present invention reacts immediately with a quantitative strong oxidizing standard solution when sampling, and is directly oxidized into stable pentavalent vanadium ions, thereby avoiding the problem of uncontrollable oxidation of low-valent electrolyte;

[0074] (3) In the conventional titration method, potassium permanganate solution is added to the electrolyte to be tested. The electrolyte to be tested is easily oxidized when in contact with air for a long time. In the present invention, the electrolyte to be tested is added to the potassium permanganate solution to avoid reaction with oxygen, thereby solving the problem of oxidation of the vanadium ion solution. Furthermore, potassium permanganate is always in excess, and two transitions will occur during titration.

[0075] The present invention will be described below in the form of specific examples. It should be understood that these examples are merely illustrative and are not intended to limit the scope of the present invention. The methods, reagents and materials used in the examples are, unless otherwise stated, conventional methods, reagents and materials in the art. The raw material compounds in the examples can all be purchased through commercial routes.

[0076] Example 1

[0077] This example measures the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the vanadium electrolyte before charge and discharge, and the determination method is as follows:

[0078] S1: Accurately pipette 10 mL of 0.015 mol / L potassium permanganate standard solution into a titration cup, and add 40 mL of a sulfuric acid-phosphoric acid mixed solution, wherein the volume ratio of 98 wt % sulfuric acid, 85 wt % phosphoric acid and water is 1:1:4, to obtain a standby solution;

[0079] S2: Accurately pipette a volume of V=0.2mL of the electrolyte to be tested at the sampling site, add it to the standby solution, stir it thoroughly, and the solution turns slightly red. After standing for 5 minutes, the red color does not fade. Because the potassium permanganate standard solution is in excess, this process causes all the trivalent vanadium ions and tetravalent vanadium ions in the electrolyte to be tested to be oxidized to pentavalent vanadium ions, obtaining a mixed solution containing potassium permanganate and pentavalent vanadium ions, i.e., the intermediate solution;

[0080] S3: Start the potentiometric titrator, and dynamically add a standard solution of ammonium ferrous sulfate with a concentration of 0.0401 mol / L to the intermediate solution until two jump points appear; wherein, the first jump point is the excess potassium permanganate reduction potential jump, at which the corresponding horizontal coordinate, i.e., the volume of the titrant consumed by the excess potassium permanganate reduction, is V1, V1=5.99 mL, and the red color of the solution disappears completely, and the solution is slightly yellow; the second jump point is the pentavalent vanadium reduction potential jump, at which the corresponding horizontal coordinate, i.e., the total volume of the titrant consumed, is V2, V2=14.47 mL, and the solution is light blue, and the specific corresponding volume of the titrant consumed to reduce the pentavalent vanadium ions to tetravalent vanadium ions is V2-V1;

[0081] According to C 总 =C Fe (V2–V1) / V, calculate the total concentration C of vanadium ions in the electrolyte to be tested 总 is 1.70 mol / L; according to X=(C ox V ox –C Fe V1) / V, the unit valence state concentration X of vanadium ions in the electrolyte to be tested is calculated to be 2.55 mol / L; according to S=5-X / C 总 , C 总 <X<2C 总, the average valence state S of vanadium ions in the electrolyte to be tested is calculated to be +3.5; the average valence state of vanadium ions in the electrolyte to be tested is +3 to +4, according to the concentration C of +3 vanadium ions V(III) For XC 总 , C V(III) =0.85mol / L, based on the concentration of +4 vanadium ions C V(IV) 2C 总 -X,C V(IV) =0.85mol / L.

[0082] The above steps were repeated twice. The results of the three tests are shown in Table 1. According to the test results, the average total concentration of vanadium ions was calculated to be 1.70 mol / L, with a relative standard deviation of 0.9%, the average concentration of trivalent vanadium ions was 0.85 mol / L, with a relative standard deviation of 1.2%, and the average concentration of trivalent vanadium ions was 0.85 mol / L, with a relative standard deviation of 0.7%.

[0083] Table 1: Three determination results of the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the vanadium electrolyte before charging and discharging

[0084]

[0085]

[0086] Example 2

[0087] This example determines the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the negative electrode vanadium electrolyte during charge and discharge, and the determination method is as follows:

[0088] S1: Accurately pipette 20 mL of 0.025 mol / L potassium dichromate standard solution into a titration cup, add 30 mL of a sulfuric acid-phosphoric acid mixed solution, wherein the volume ratio of 98 wt% sulfuric acid, 85 wt% phosphoric acid and water is 0.5:2:4, to obtain a standby solution;

[0089] S2: Accurately pipette a volume of V=0.5 mL of the electrolyte to be tested at the sampling site, add it to the standby solution, stir it thoroughly and let it stand for 5 minutes. Since the potassium dichromate standard solution is in excess, this process causes all the divalent vanadium ions and trivalent vanadium ions in the electrolyte to be tested to be oxidized to pentavalent vanadium ions, obtaining a mixed solution containing potassium dichromate and pentavalent vanadium ions, i.e., the intermediate solution;

[0090] S3: Start the potentiometric titrator, and dynamically add a standard solution of ammonium ferrous sulfate with a concentration of 0.1498 mol / L to the intermediate solution until two jump points appear; the first jump point is the excess potassium dichromate reduction potential jump, and the corresponding horizontal coordinate, i.e., the volume of titrant consumed by the excess potassium dichromate reduction, is V1, V1=7.01 mL; the second jump point is the pentavalent vanadium reduction potential jump, and the corresponding horizontal coordinate, i.e., the total volume of titrant consumed, is V2, V2=12.13 mL. At this time, the solution is light blue, and the specific volume of titrant consumed to reduce the pentavalent vanadium ions to tetravalent vanadium ions is V2-V1;

[0091] According to C 总 =C Fe (V2–V1) / V, calculate the total concentration C of vanadium ions in the electrolyte to be tested 总 is 1.53 mol / L; according to X=(C ox V ox –C Fe V1) / V, the unit valence concentration X of vanadium ions in the electrolyte to be tested is calculated to be 3.90 mol / L; according to S=5-X / C 总 , 2C 总 <X<3C 总 , the average valence state S of vanadium ions in the electrolyte to be tested is calculated to be +2.45, the average valence state of vanadium ions in the electrolyte to be tested is +2 to +3, and the concentration of +2 vanadium ions C V(II) For X-2C 总 , C V(II) =0.84mol / L, +3-valent vanadium ion concentration C V(III) For 3C 总 -X,C V(III) =0.69mol / L.

[0092] The above steps were repeated twice. The results of the three tests are shown in Table 2. According to the test results, the average total concentration of vanadium ions was calculated to be 1.52 mol / L, with a relative standard deviation of 1.0%, the average concentration of divalent vanadium ions was 0.83 mol / L, with a relative standard deviation of 1.2%, and the average concentration of trivalent vanadium ions was 0.69 mol / L, with a relative standard deviation of 0.8%.

[0093] Table 2: Three determination results of the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the negative electrode vanadium electrolyte during charge and discharge

[0094] Number of tests <![CDATA[C 总 (mol / L)]]> <![CDATA[C V(II) (mol / L)]]> <![CDATA[C V(III) (mol / L)]]> 1st time 1.53 0.84 0.69 2nd time 1.52 0.83 0.69 3rd 1.50 0.82 0.68

[0095] Example 3

[0096] This example measures the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the positive electrode vanadium electrolyte during charge and discharge, and the determination method is as follows:

[0097] S1: Accurately pipette 5 mL of 0.011 mol / L potassium permanganate standard solution into a titration cup, and add 50 mL of a sulfuric acid-phosphoric acid mixed solution, wherein the volume ratio of 98 wt% sulfuric acid, 85 wt% phosphoric acid and water is 0.5:1:4, to obtain a standby solution;

[0098] S2: Accurately pipette a volume of V=0.1 mL of the electrolyte to be tested at the sampling site, add it to the standby solution, stir thoroughly, and the solution turns slightly red. After standing for 5 minutes, the red color does not fade. Because the potassium permanganate standard solution is in excess, this process causes all the tetravalent vanadium ions in the electrolyte to be tested to be oxidized to pentavalent vanadium ions, obtaining a mixed solution containing potassium permanganate and pentavalent vanadium ions, i.e., the intermediate solution;

[0099] S3: Start the potentiometric titrator, and dynamically add a 0.020 mol / L ammonium ferrous sulfate standard solution to the intermediate solution until two jump points appear; wherein, the first jump point is the excess potassium permanganate reduction potential jump, at which the corresponding horizontal coordinate, i.e., the volume of the titrant consumed by the excess potassium permanganate reduction, is V1, V1=9.75 mL, and the red color of the solution disappears completely, and the solution is slightly yellow; the second jump point is the pentavalent vanadium reduction potential jump, at which the corresponding horizontal coordinate, i.e., the total volume of the titrant consumed, is V2, V2=18.65 mL, and the solution is light blue, and the specific corresponding volume of the titrant consumed to reduce the pentavalent vanadium ions to tetravalent vanadium ions is V2-V1;

[0100] According to C 总 =C Fe (V2–V1) / V, calculate the total concentration C of vanadium ions in the electrolyte to be tested 总 is 1.78 mol / L; according to X=(C ox V ox –C Fe V1) / V, the unit valence concentration X of vanadium ions in the electrolyte to be tested is calculated to be 0.80 mol / L; according to S=5-X / C 总 , 0<X <C 总 , the average valence state S of vanadium ions in the electrolyte to be tested is calculated to be +4.55, the average valence state of vanadium ions in the electrolyte to be tested is +4 to +5, and the concentration of +4 vanadium ions is C V(IV) =X,C V(IV) =0.80mol / L, the concentration of +5-valent vanadium ions is C V(V) =C 总 -X,C V(V) =0.98mol / L.

[0101] The above steps were repeated twice. The results of the three tests are shown in Table 3. According to the test results, the average total concentration of vanadium ions was calculated to be 1.78 mol / L, with a relative standard deviation of 0.9%, the average concentration of tetravalent vanadium ions was 0.79 mol / L, with a relative standard deviation of 1.3%, and the average concentration of pentavalent vanadium ions was 0.99 mol / L, with a relative standard deviation of 1.2%.

[0102] Table 3: Three determination results of the total concentration of vanadium ions and the concentration of vanadium ions of various valence states in the positive electrode vanadium electrolyte during charge and discharge

[0103] Number of tests <![CDATA[C 总 (mol / L)]]> <![CDATA[C V(IV) (mol / L)]]> <![CDATA[C V(V) (mol / L)]]> 1st time 1.78 0.80 0.98 2nd time 1.80 0.79 1.00 3rd 1.77 0.78 0.98

[0104] Comparative Example 1

[0105] This comparative example determines the total concentration of vanadium ions and the concentration of vanadium ions of each valence state in the positive electrode vanadium electrolyte during charge and discharge, and the determination method is as follows:

[0106] S1: Accurately pipette 0.1 mL of the electrolyte to be tested (the electrolyte to be tested is the same as that in Example 3) into a titration cup, and add 50 mL of a sulfuric acid-phosphoric acid mixed solution thereto, wherein the volume ratio of 98 wt % sulfuric acid, 85 wt % phosphoric acid and water is 0.5:1:4, to obtain a standby solution;

[0107] S2: Start the potentiometric titrator and dynamically add 0.020 mol / L ammonium ferrous sulfate standard solution to the test solution until the potential is -100 mV. At this time, there is only one jump point, which is the pentavalent vanadium reduction potential jump. The corresponding abscissa, i.e. the total volume of the titrant consumed, is V0, V0 = 4.90 mL. At this time, the solution is light blue;

[0108] According to formula C V(V) =C Fe V0 / V can only measure the concentration of pentavalent vanadium C V(V)＝ 0.98mol / L, if you need to get the tetravalent vanadium concentration C V(IV) , other methods are needed to detect C 总 Then calculate.

Claims

1. A method for testing vanadium content and vanadium valence, characterized in that: The method comprises the following steps: S1: adding a buffer solution to an excess oxidant solution to obtain a standby solution; S2: adding a volume V of the electrolyte to be tested containing vanadium element to the reserve solution, and letting it stand to obtain an intermediate solution containing excess oxidant and pentavalent vanadium ions; S3: Use a potentiometric titrator to add a titrant to the intermediate solution until two jump points appear; wherein the first jump point is a potential jump of excess oxidant reduction, and the volume of titrant consumed for reducing the excess oxidant is V1; the second jump point is a potential jump of pentavalent vanadium reduction, and the total volume of titrant consumed at this time is V2, and the volume of titrant consumed for reducing pentavalent vanadium ions to tetravalent vanadium ions is V2-V1; wherein the titrant is ammonium ferrous sulfate solution; The total concentration of vanadium ions in the electrolyte to be tested is C 总 The calculation formula is: C 总 =C Fe (V2–V1) / V; The calculation formula of the total concentration X of the valence change of vanadium ions in the electrolyte to be tested during the oxidation process is: X = (C ox V ox –C Fe V1) / V; The calculation formula for the average valence state S of vanadium ions in the electrolyte to be tested is: S = 5-X / C 总 ; Among them, C 总 The unit is mol / L; C Fe is the concentration of ammonium ferrous sulfate solution, in mol / L; The unit of V, V1 and V2 is L; The unit of X is mol / L; C ox is the total concentration of the valence change of the oxidant solution during the oxidation process, in mol / L; V ox is the volume of the oxidant solution, in L; The calculation method of the valence state of vanadium ions and the concentration of the corresponding valence state in the electrolyte to be tested is: If X = 3C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +2; If 2C 总 <X<3C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +2 to +3, and the concentration of +2 vanadium ions C V(II) For X-2C 总 , +3-valent vanadium ion concentration C V(III) For 3C 总 -X; If X = 2C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +3; If C 总 <X<2C 总 , then the average valence of vanadium ions in the electrolyte to be tested is +3 to +4, and the concentration of +3 vanadium ions C V(III) For XC 总 , +4 vanadium ion concentration C V(IV) 2C 总 -X; If X=C 总 , then the valence state of vanadium ions in the electrolyte to be tested is all +4; If 0<X <C 总 , then the average valence of vanadium ions in the electrolyte to be tested is from +4 to +5, and the concentration of +4 vanadium ions is C V(IV) =X, the concentration of +5-valent vanadium ions is C V(V) =C 总 -X; If X=0, the valence states of all vanadium ions in the electrolyte to be tested are +5.

2. The method according to claim 1, characterized in that The oxidant solution is a potassium permanganate solution or a potassium dichromate solution.

3. The method according to claim 2, characterized in that When the oxidant solution is potassium permanganate solution, C ox Equal to 5 times the concentration of potassium permanganate solution; when the oxidant solution is potassium dichromate solution, C ox Equivalent to 6 times the concentration of potassium dichromate solution.

4. The method according to claim 1, characterized in that The concentration of the oxidant solution is 0.005-0.030 mol / L.

5. The method according to claim 1, characterized in that The volume of the oxidant solution is 5-50 mL.

6. The method according to claim 1, characterized in that The buffer solution is a sulfuric acid and phosphoric acid mixed solution or a phosphoric acid solution.

7. The method according to claim 6, characterized in that The sulfuric acid-phosphoric acid mixed solution is prepared by mixing 98 wt % sulfuric acid, 85 wt % phosphoric acid and water in a volume ratio of (0.5-2):(0.5-2):(0.8-4).

8. The method according to claim 1, characterized in that The volume of the buffer solution is 10-100 mL.

9. The method according to claim 1, characterized in that The volume V of the electrolyte to be tested is 0.1-5.0 mL.

10. The method according to claim 1, characterized in that The concentration of the ammonium ferrous sulfate solution is 0.02-0.20 mol / L.

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