Method for measuring acidity of iron-chromium electrolyte based on complexing mass difference method

Abstract

The invention is applicable to the technical field of electrochemical analysis, and provides a method for measuring the acidity of an iron-chromium electrolyte based on a complexing mass difference method.The method comprises the following steps that a pretreated iron-chromium electrolyte is prepared, a complexing reagent playing a complexing role on iron and chromium ions is added into the pretreated iron-chromium electrolyte, and the weight is recorded as M1; the pretreated iron-chromium electrolyte added with the complexing reagent is stirred, then a standard alkali solution is dropwise added, after titration is stopped, the weight is weighed and recorded as M2, a quantitative determination method of mass difference is used for calculation, and the acidity calculation formula is {(M2-M1)-K} * C. In conclusion, a quantitative determination method for the quality difference is adopted, and the complexing reagent containing carboxyl is added, so that the side reaction of iron and chromium ions is reduced and avoided, the selectivity and the accuracy of determining the acidity of the iron-chromium electrolyte are improved, and the rapid and accurate determination of the free acid is realized.

Description

Technical Field

[0001] This invention relates to the field of electrochemical analysis technology, and in particular to a method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method. Background Technology

[0002] In the field of electrochemical energy storage (such as iron-chromium flow batteries), the acidity of the electrolyte (free H+) is crucial. + Concentration is a core control parameter: too high a concentration will exacerbate electrode corrosion, while too low a concentration will cause the electrochemical reaction rate to decrease by more than 30%, and may even lead to metal ion precipitation and blockage of the flow channel. Iron-chromium electrolytes contain Fe... 2+ / Fe 3+ Cr 3+ / Cr 2+ For multivalent metal ions, acidity determination requires avoiding side reactions between the metal ions and acids / bases, and current technologies struggle to meet the demands for precise control. Therefore, developing a simple, rapid, and accurate acidity determination method that effectively overcomes metal ion interference is crucial for advancing technologies such as iron-chromium redox flow batteries. Currently, various techniques exist for determining the concentration of free acid in electrolytes, each with its own advantages and disadvantages.

[0003] Some existing methods for testing the acidity of iron-chromium electrolytes may have the following drawbacks: 1. Acid-base titration method: For iron-chromium electrolyte with an acidity of 2.5, the acid-base titration method according to GB / T 6379.2-2004 was used for determination. The error of three parallel experiments ranged from -0.1 to +0.16 (see Table 1 Comparative Example 1), and the relative standard deviation (RSD) reached 6.8%, which could not meet the requirements of industrial control.

[0004] Poor compatibility with iron-chromium electrolyte: The iron-chromium electrolyte is dark green (Fe... 3+ / Cr 3+ (Color development), the color change of traditional indicators (such as phenolphthalein and methyl orange) is masked, and the error in judging the pH endpoint is > ±0.2; Metal ion side reaction: Fe 3+ Hydrolysis at pH > 2 produces Fe(OH)3 precipitate (reaction formula: Fe 3+ +3OH - =Fe(OH)3↓), Cr 3+ It is oxidized to CrO4 under alkaline conditions. 2- (4Cr) 3+ +8OH - +O2=4CrO4 2- (+4H2O) consumes standard alkaline solution, resulting in a 10-15% higher reading for the free acid.

[0005] 2. pH meter measurement method: Severe ion interference: High concentration of Fe in iron-chromium electrolyte 3+ Cr 3+ It will complex and adsorb with the pH electrode sensitive membrane, resulting in a prolonged electrode response time of 5-10 min (normal <1 min), and a pH value deviation of ±0.1 or more. Poor stability: Metal hydroxide precipitates easily form on the electrode surface. After three uses, the pH measurement error increases from ±0.02 to ±0.08, requiring frequent acid washing and regeneration, each time taking about 30 minutes.

[0006] 3. Ion chromatography: Pretreatment cannot eliminate metal ion interference: Fe in iron-chromium electrolyte needs to be removed by solid-phase extraction. 3+ Cr 3+ (To avoid column clogging), but the loss rate of free acid during extraction is 8-12%, resulting in lower measured values; cost and efficiency are contradictory: a single analysis takes more than 1 hour (including pretreatment), and the equipment purchase cost exceeds 500,000 yuan, which cannot meet the needs of industrial online detection.

[0007] 4. Potentiometric titration: The cost of the instruments and equipment is relatively high; the titration curve has no obvious jump: the free acid in the iron-chromium electrolyte coexists with H⁺ generated by the hydrolysis of metal ions, resulting in a slope of <5mV / mL for potentiometric titration (>20mV / mL for normal strong acid titration), and an endpoint judgment error of ±0.2mL (corresponding to an acidity error of ±0.1).

[0008] In summary, existing methods for determining the acidity of iron-chromium electrolytes share a common bottleneck: the presence of metal ions (Fe... 3+ Cr 3+ Ni 2+ Side reactions between iron-chromium electrolytes and acids / bases cannot be effectively eliminated, leading to large measurement errors (±0.1 to ±0.5%) and poor stability (RSD > 5%). Furthermore, methods such as ion chromatography are costly and complex to operate, failing to meet the demands for accurate and rapid determination of free acids in industrial applications such as iron-chromium flow batteries. Therefore, developing a method for determining the acidity of iron-chromium electrolytes that avoids interference from metal ions is of great significance. Summary of the Invention

[0009] In view of this, the present invention proposes a method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method. A complexing reagent containing a carboxyl group is added to preferentially complex metal ions, blocking their side reaction with alkali. Combined with the quantitative determination method of mass difference, the amount of alkali consumed is accurately measured, thereby achieving accurate determination of free acid.

[0010] The technical solution of this invention is implemented as follows: This invention provides a method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method, comprising the following steps: Step 1: Take the iron-chromium electrolyte, disperse it evenly, and then dilute it to obtain the pretreated iron-chromium electrolyte; Step 2: Add a complexing reagent that can complex iron and chromium ions to the pretreated iron-chromium electrolyte, mix and weigh, and record as M1. Step 3: Stir the pretreated iron-chromium electrolyte with added complexing reagent, then add standard alkali solution dropwise to obtain a mixed solution. After stopping the titration, weigh and record the solution as M2. Step four involves calculating the acidity using a quantitative method based on mass difference. The acidity calculation formula is {(M2-M1)-K}×C. Here, K is taken as 0.03, which is the error correction factor; C is the molar concentration of the standard alkaline solution.

[0011] In this invention, the iron-chromium electrolyte contains Fe 2+ Or Fe 3+ Or Cr 2+ Or Cr 3+ At least one of the following iron-chromium electrolytes.

[0012] Preferably, the total molar concentration of metal ions in the pretreated iron-chromium electrolyte is 0.05-0.1 mol / L. The pretreatment method for the iron-chromium electrolyte is as follows: the electrolyte is dispersed using an ultrasonic disperser with an oscillation frequency of 30-40 Hz at 20-25°C for 14-20 min; then deionized water is added and diluted with stirring at a speed of 500-600 r / min to obtain the pretreated iron-chromium electrolyte. In this invention, mechanical dispersion can also be used instead of oscillation dispersion.

[0013] Preferably, the amount of complexing reagent added is 0.8-1.2 mL, and the molar concentration of the complexing reagent is 3-3.5 mol / L.

[0014] Preferably, the complexing agent contains at least two carboxyl groups. The complexing agent is any one of citric acid, oxalic acid, succinic acid, or tartaric acid.

[0015] Preferably, in step three, the stirring method for the pretreated iron-chromium electrolyte with added complexing reagent is as follows: the pH value of the solution is monitored using a pH meter, and stirring is carried out at 20-25℃ and 500-600r / min.

[0016] Preferably, in step three, the standard alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution; the molar concentration C of the standard alkaline solution is 0.5-1 mol / L; and the pH of the mixed solution is 2-4 when the titration is stopped.

[0017] The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method of the present invention has the following advantages over the prior art: 1. The quantitative determination method for determining the acidity of iron-chromium electrolyte using a low-quality method improves accuracy by 5-25 times compared to the traditional acid-base titration method; reagent consumption is reduced by 40% and the single determination time is <30min, which is suitable for iron-chromium electrolyte systems.

[0018] 2. Although using a low-quality quantitative determination method can avoid the shortcomings of other methods to some extent, the iron and chromium ions in the iron-chromium electrolyte are prone to redox reactions, leading to an increase in the amount of alkaline solution titrated and resulting in large fluctuations in the acidity measurement results. After adding alkaline solution to the iron-chromium electrolyte, the iron ions react with the alkaline solution to undergo redox reactions, causing significant deviations in the acidity measurement results. Therefore, this invention adds a complexing reagent to the iron-chromium electrolyte. The complexing reagent complexes the iron and chromium ions, preventing them from undergoing redox reactions themselves and avoiding large fluctuations in the acidity measurement results. Because the complexing reagent complexes the iron and chromium ions, it also prevents the iron ions from undergoing redox reactions with the alkaline solution, thus avoiding small deviations in the acidity measurement results.

[0019] 3. During the preparation of iron-chromium electrolyte, nickel ions are inevitably present in the raw materials. Nickel ions react with sodium hydroxide or potassium hydroxide, resulting in a higher acidity measurement value. By adding a complexing reagent, a complexing reaction can be formed with the nickel ions, avoiding the side reaction of nickel ions in the iron-chromium electrolyte and preventing the acidity measurement value from being too high.

[0020] 4. Since it needs to react with iron, chromium and nickel ions, the complexing reagent of the present invention is selected as an organic acid containing multiple carboxyl groups.

[0021] 5. To ensure that the iron, chromium, and nickel ions in the iron-chromium electrolyte react completely with the complexing reagent without side reactions, the amount of complexing reagent added should be 0.8-1.2 mL, with a molar concentration of 3-3.5 mol / L. If the amount of complexing reagent added is too low, side reactions cannot be completely prevented, and the acidity measurement results will still be inaccurate. If the amount of complexing reagent added is too high, the accuracy of the results will not be improved, but the cost of reagents and consumables will be increased.

[0022] 6. Add standard alkali solution dropwise to the pretreated iron-chromium electrolyte with added complexing reagent. The titration endpoint pH is 2-4. If the endpoint pH is less than 2, the acidity measurement value will be too high. The acidity value of the vanadium electrolyte can be accurately obtained within the range of 2-4, and the amount of standard alkali solution used is the least, saving the cost of reagents and consumables. When the pH is greater than 4, the acidity value of the vanadium electrolyte can also be accurately obtained, but the amount of standard alkali solution used is large, and the cost of reagents and consumables is high.

[0023] 7. The present invention uses a quantitative determination method based on mass difference. In the process of calculating the acidity of iron-chromium electrolyte, the error correction coefficient K is 0.03 obtained through the standard curve. Therefore, the calculation formula for the acidity of iron-chromium electrolyte is {(M2-M1)-K}×C. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 1. Take 45 mL of iron-chromium electrolyte and disperse it using an ultrasonic disperser with an oscillation frequency of 30 Hz at 20 ℃ for 14 min to make the sample uniform. Then take 0.8 mL of the uniform iron-chromium electrolyte into a 100 mL glass beaker, add 26 mL of deionized water for pretreatment dilution, and place a magnetic stir bar to stir and mix at a stirring speed of 500 r / min to obtain a pretreated iron-chromium electrolyte with a molar concentration of 0.05 mol / L.

[0026] 2. Add 0.8 mL of citric acid to the pretreated iron-chromium electrolyte, then weigh it and record it as M1; the molar concentration of the complexing reagent is 3 mol / L.

[0027] The preparation method of citric acid is as follows: Weigh 38-39g of citric acid solid into a volumetric flask, add deionized water to 60ml, stir until the solution is completely dissolved, and then add water to make up to 100ml.

[0028] 3. Monitor the pH value of the solution using a Mettler pH meter. Stir the solution at 20℃ and 500r / min. During stirring, keep the solution in the beaker from splashing and maintain a small vortex on the surface. Then add standard sodium hydroxide solution dropwise to obtain a mixed solution. Stop the titration when the pH of the mixed solution reaches 2, and record the weight as M2. The molar concentration C of the standard sodium hydroxide solution is 0.5mol / L.

[0029] 4. The acidity is calculated using the quantitative determination method based on the mass difference: Acidity = {(M2-M1)-K}×C; where K is 0.03, which is the error correction coefficient; and C is the molar concentration of the standard alkaline solution.

[0030] Example 2 1. Take 50 mL of iron-chromium electrolyte and disperse it using an ultrasonic disperser with an oscillation frequency of 33 Hz at 22℃ for 17 min to make the sample uniform. Then take 1 mL of the uniform iron-chromium electrolyte into a 100 mL glass beaker, add 28 mL of deionized water for pretreatment dilution, and place a magnetic stir bar to stir and mix at a stirring speed of 560 r / min to obtain a pretreated iron-chromium electrolyte with a molar concentration of 0.08 mol / L.

[0031] 2. Add 1 mL of oxalic acid to the pretreated iron-chromium electrolyte, weigh it, and record it as M1; the molar concentration of the complexing reagent is 3.2 mol / L.

[0032] The preparation method of oxalic acid is as follows: Weigh 38-39g of solid oxalic acid into a volumetric flask, add deionized water to 65ml, stir until the solution is completely dissolved, and then add water to make up to 100ml.

[0033] 3. Monitor the pH value of the solution using a Mettler pH meter. Stir the solution at 23℃ and 560r / min. During stirring, keep the solution in the beaker from splashing and maintain a small vortex on the surface. Then add standard sodium hydroxide solution dropwise to obtain a mixed solution. Stop the titration when the pH of the mixed solution reaches 3, and record the weight as M2. The molar concentration C of the standard sodium hydroxide solution is 0.8mol / L.

[0034] 4. The acidity is calculated using the quantitative determination method based on the mass difference: Acidity = {(M2-M1)-K}×C; where K is 0.03, which is the error correction coefficient; and C is the molar concentration of the standard alkaline solution.

[0035] Example 3 1. Take 55 mL of iron-chromium electrolyte and disperse it using an ultrasonic disperser with an oscillation frequency of 40 Hz at 25 ℃ for 20 min to make the sample uniform. Then take 1.2 mL of the uniform iron-chromium electrolyte into a 100 mL glass beaker, add 30 mL of deionized water for pretreatment dilution, and place a magnetic stir bar to stir and mix at a stirring speed of 600 r / min to obtain a pretreated iron-chromium electrolyte with a molar concentration of 0.1 mol / L.

[0036] 2. Add 1.2 mL of succinic acid to the pretreated iron-chromium electrolyte, weigh it, and record it as M1; the molar concentration of the complexing reagent is 3.5 mol / L.

[0037] The preparation method of succinic acid is as follows: Weigh 39g of succinic acid solid into a volumetric flask, add deionized water to 70ml, stir until the solution is completely dissolved, and then add water to make up to 100ml.

[0038] 3. Monitor the pH value of the solution using a Mettler pH meter. Stir the solution at 25℃ and 600r / min. During stirring, keep the solution in the beaker from splashing and maintain a small vortex on the surface. Then add standard potassium hydroxide solution dropwise to obtain a mixed solution. Stop the titration when the pH of the mixed solution reaches 4. Record the weight as M2. The molar concentration C of the standard potassium hydroxide solution is 1 mol / L.

[0039] 4. The acidity is calculated using the quantitative determination method based on the mass difference: Acidity = {(M2-M1)-K}×C; where K is 0.03, which is the error correction coefficient; and C is the molar concentration of the standard alkaline solution.

[0040] Comparative Example 1 The acidity of the iron-chromium electrolyte was determined by acid-base titration according to the national standard GB / T 6379.2-2004.

[0041] Comparative Example 2 The acidity of the iron-chromium electrolyte was determined according to the pH meter measurement method in the national standard GB / T 6920-1986 Determination of pH value of water - Glass electrode method.

[0042] Comparative Example 3 The acidity of the iron-chromium electrolyte was determined by ion chromatography according to the national ecological and environmental standard "HJ 84-2016 Water Quality - Determination of Inorganic Anions".

[0043] Comparative Example 4 The acidity of the iron-chromium electrolyte was determined by potentiometric titration according to the national standard GB / T 1668-2008 Determination of Acid Value and Acidity of Plasticizers.

[0044] Comparative Example 5 Based on Example 2, in step 2, no complexing reagent is added, and the pretreated iron-chromium electrolyte is weighed directly, while other conditions remain the same.

[0045] Comparative Example 6 Based on Example 2, the complexing reagent in step 2 is sodium acetate, a non-acid reagent containing a carboxyl group, while other conditions remain the same.

[0046] Comparative Example 7 Based on Example 1, the molar concentration of the complexing reagent was 2 mol / L, and all other conditions were the same.

[0047] Comparative Example 8 Based on Example 1, the molar concentration of the complexing reagent was 4 mol / L, and all other conditions were the same.

[0048] Comparative Example 9 Based on Example 1, a standard alkaline solution was added dropwise, and the titration endpoint pH was 1, with all other conditions remaining the same.

[0049] Comparative Example 10 Based on Example 1, a standard alkaline solution was added dropwise, and the titration endpoint pH was 5, with all other conditions remaining the same.

[0050] Comparative Example 11 Based on Comparative Example 3, the amount of complexing reagent added was 0.7 mL, and all other conditions were the same.

[0051] Comparative Example 12 Based on Comparative Example 3, the amount of complexing reagent added was 1.5 mL, and all other conditions were the same.

[0052] A ferrochromium electrolyte with an acidity of 2.5 was prepared. The actual acidity of the ferrochromium electrolyte was determined using the methods described in the three embodiments above, as well as the comparative method, and the error was calculated. Error = Actual acidity - Standard acidity. The measurement results for each embodiment and comparative example are shown in Table 1.

[0053]

[0054]

[0055] The data above shows that: In the various embodiments of this invention, the measured acidity error is small, ≤ ±0.02, and the results of multiple measurements are relatively stable, with RSD ≤ 0.5%. Compared with the traditional acid-base titration method, the accuracy is improved by 5-25 times; reagent consumption is reduced by 40%, and the single measurement time is < 30 min, making it suitable for iron-chromium electrolyte systems.

[0056] Both acid-base titration and potentiometric titration methods introduce significant errors in acidity detection due to interference from iron-chromium ions. pH meter measurement, which directly measures acidic solutions, results in highly unstable data as the measured values ​​fall below the pH meter's lower limit, making calculations impossible. Ion chromatography, a method for determining inorganic anions in water, suffers from errors due to the presence of organic anions in the iron-chromium electrolyte, which combine with hydrogen ions, leading to an underestimation of hydrogen ion levels.

[0057] The results obtained without adding a complexing reagent are very unstable. The presence of iron-chromium ions and other impurity metal ions greatly interferes with the detection of free hydrogen ions and is irregular.

[0058] Sodium acetate is a non-acid reagent containing a carboxyl group. Although it contains a carboxyl group, its aqueous solution is weakly alkaline. When added to a diluted iron-chromium electrolyte, its acetate ions will combine with hydrogen ions, promoting the positive shift of the hydrolysis equilibrium of ferric ions. This may eventually produce a reddish-brown precipitate of ferric hydroxide. At the same time, acetic acid will be present in the solution, leading to a significantly overestimated result.

[0059] If the amount of complexing reagent added is too low, the side reaction cannot be completely prevented, and the acidity measurement result will still be inaccurate. If the amount of complexing reagent added is too high, hydrogen ions will be released when the carboxylic acid complexes with the metal cation, resulting in a higher total hydrogen ion concentration, a higher measured actual acidity value, and an increase in the cost of reagents and consumables.

[0060] If the pH at the titration endpoint is too low, the acidity measurement result will be inaccurate. If the pH at the titration endpoint is too high, the accuracy of the result will not be improved, but the cost of reagents and consumables will be increased.

[0061] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method, characterized in that, Includes the following steps: Step 1: Take the iron-chromium electrolyte, disperse it evenly, and then dilute it to obtain the pretreated iron-chromium electrolyte; Step 2: Add a complexing reagent that can complex iron and chromium ions to the pretreated iron-chromium electrolyte, mix and weigh, and record as M1. Step 3: Stir the pretreated iron-chromium electrolyte with added complexing reagent, then add standard alkali solution dropwise to obtain a mixed solution. After stopping the titration, weigh and record the solution as M2. Step four involves using a quantitative method based on mass difference to calculate the acidity. The acidity calculation formula is {(M2-M1)-K}×C, where K is 0.03, which is the error correction coefficient, and C is the molar concentration of the standard alkaline solution.

2. The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method as described in claim 1, characterized in that, The molar concentration of the pretreated iron-chromium electrolyte is 0.05-0.1 mol / L. The pretreatment method of the pretreated iron-chromium electrolyte is as follows: use an ultrasonic disperser with an oscillation frequency of 30-40 Hz to disperse the electrolyte at 20-25℃ for 14-20 min; then add deionized water and stir to dilute the electrolyte at a stirring speed of 500-600 r / min to obtain the pretreated iron-chromium electrolyte.

3. The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method as described in claim 1, characterized in that, The amount of the complexing reagent added is 0.8-1.2 mL, and the molar concentration is 3-3.5 mol / L.

4. The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method as described in claim 1, characterized in that, The complexing agent contains at least two carboxyl groups; the complexing agent is any one of citric acid, oxalic acid, succinic acid or tartaric acid.

5. The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method as described in claim 1, characterized in that, In step three, the stirring method for the pretreated iron-chromium electrolyte with added complexing reagent is as follows: the pH value of the solution is monitored using a pH meter, and stirring is carried out at 20-25℃ and 500-600r / min.

6. The method for determining the acidity of iron-chromium electrolyte based on the complexation mass difference method as described in claim 1, characterized in that, In step three, the molar concentration C of the standard alkaline solution is 0.5-1 mol / L; when the titration is stopped, the pH of the mixed solution is 2-4.

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