A method for determining the chromium content of iron-chromium mixtures

Abstract

The application discloses a method for determining the content of chromium in a mixture of iron and chromium, and comprises the following steps: S1, oxidizing the divalent iron ions in the mixture of iron and chromium into trivalent iron ions under an acidic condition; S2, adding sodium hydroxide solution and hydrogen peroxide under stirring, so that the trivalent iron ions form iron hydroxide precipitate, and the trivalent chromium ions are oxidized into hexavalent chromium ions; S3, adding an acidic solution, so that the iron hydroxide precipitate is dissolved, and the wrapped trivalent chromium ions are released; adding sodium hydroxide solution and hydrogen peroxide under stirring, so that the trivalent iron ions form iron hydroxide precipitate, and the released trivalent chromium ions are oxidized into hexavalent chromium ions; S4, repeating the step S3 X times; X is an integer greater than 0; and S5, determining the content of chromium in the mixture. The method for determining the content of chromium in the mixture of iron and chromium has the advantages of accurate measurement, high precision, simple operation and improved efficiency of the determination of the content of chromium.

Description

Technical Field

[0001] This invention relates to the field of determination of iron-chromium mixtures, and more particularly to a method for determining the chromium content in iron-chromium mixtures. Background Technology

[0002] The testing of chromium in the electrolyte of iron-chromium flow batteries and its waste electrolyte has long been a challenge for industry professionals. The method of using the color of trivalent chromium ions and spectrophotometry requires a 24-hour waiting period before testing, resulting in a long testing cycle and unstable test results. This method cannot be well applied to the production and acceptance of electrolytes.

[0003] HG / T4311-2012 Industrial Chromium Chloride is a test method for pure chromium trichloride. When used for mixed ferrochromium chlorides, the test results are often too low or no color change occurs during the test, leading to test failure. GB / T4699.2-2008 Determination of Chromium Content in Ferrochromium and Silicon-Chromium Alloys by Ammonium Persulfate Oxidation Titration and Potentiometric Titration requires the addition of silver nitrate as an oxidation catalyst. If used for testing chloride-containing compounds, this will result in the precipitation of excessive silver chloride, significantly increasing the amount of silver nitrate required and leading to high testing costs. Furthermore, the oxidation-reduction process is complex, making it unsuitable for the rapid testing of chromium in mixed ferrochromium chlorides.

[0004] Therefore, there is an urgent need for a method that can quickly and accurately measure chromium in iron-chromium mixed chlorides to guide the production and acceptance of electrolytes. Summary of the Invention

[0005] This invention aims to at least partially solve one of the problems in related technologies. Therefore, the object of this invention is to provide a method for determining the chromium content in an iron-chromium mixture, which is accurate, highly precise, simple to operate, and improves the efficiency of chromium content determination.

[0006] To achieve the above objectives, this application adopts the following technical solution: a method for determining the chromium content in an iron-chromium mixture, comprising the following steps:

[0007] S1: Oxidizes ferrous ions in an iron-chromium mixture to ferric ions under acidic conditions;

[0008] S2: Add sodium hydroxide solution and hydrogen peroxide under stirring conditions, so that ferric ions form ferric hydroxide precipitate, and ferric ions are oxidized to hexavalent chromium ions.

[0009] S3: Add an acidic solution to dissolve the ferric hydroxide precipitate and release the encapsulated trivalent chromium ions; add sodium hydroxide solution and hydrogen peroxide under stirring to form ferric hydroxide precipitate from the trivalent ferric ions, while the released trivalent chromium ions are oxidized to hexavalent chromium ions.

[0010] S4: Repeat step S3 X times; X is an integer greater than 0;

[0011] S5: Determine the chromium content in the mixture.

[0012] Furthermore, in step S1, an acidic solution is used to adjust the iron-chromium mixture to acidic conditions, wherein the acidic solution is one or more of hydrochloric acid, sulfuric acid, and phosphoric acid.

[0013] Furthermore, in step S1, an oxidant is used to oxidize the ferrous ions in the iron-chromium mixture to ferric ions. The oxidant is one or more of sodium chlorate, hydrogen peroxide, potassium chlorate, sodium hypochlorite, potassium hypochlorite, and ozone.

[0014] Furthermore, in step S1, the oxidant causes all ferrous ions in the iron-chromium mixture to be oxidized to ferric ions, and the acidic solution reacts completely with the remaining oxidant.

[0015] Furthermore, in step S2, distilled water is first added to reduce the concentration in the iron-chromium mixture, and then sodium hydroxide solution and hydrogen peroxide are added under stirring conditions to form ferric hydroxide precipitate of trivalent iron ions. When the pH of the solution is greater than 10, hydrogen peroxide oxidizes trivalent chromium ions to hexavalent chromium ions.

[0016] Furthermore, in step S2, the concentration of the sodium hydroxide solution is 50 g / L.

[0017] Furthermore, in step S3, an excess of acidic solution is added to completely dissolve the ferric hydroxide precipitate.

[0018] Furthermore, in step S5, the test solution is mixed with sulfuric acid and phosphoric acid, and then titrated with ferrous ammonium sulfate standard titration solution until it turns yellow-green. N-benzoic acid indicator is added, and titration continues until the solution changes from purple-red to green as the endpoint. The mass content of chromium in the iron-chromium mixture is calculated based on the titration results.

[0019] Compared with the prior art, the technical solution provided in this application has the following advantages: This application first completely oxidizes the iron in the iron-chromium mixture, so that Fe... 2+ It does not affect subsequent Cr 3+ The test, and this application also proposes tests for ferric hydroxide and Cr 3+ The technical solution of multiple acid dissolution and re-oxidation of the mixture allows the Cr encapsulated in the ferric hydroxide precipitate to be dissolved. 3+ The present application's technical solution is simple and easy to implement, and the test results are highly precise and accurate with short testing time, which significantly improves the efficiency of determining the chromium content in iron-chromium mixtures. Detailed Implementation

[0020] The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the invention. Modifications or substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and substance of the invention are all within the scope of protection of the present invention. Unless otherwise specified, the experimental materials, reagents, instruments, etc., used in the embodiments of the present invention are all commercially available; unless specifically specified, all technical means in the embodiments of the present invention are conventional means well known to those skilled in the art.

[0021] This application provides a method for determining the chromium content in an iron-chromium mixture, comprising the following steps:

[0022] S1: Oxidizes ferrous ions in an iron-chromium mixture to ferric ions under acidic conditions.

[0023] Specifically, the iron-chromium mixture is adjusted to acidity using an acidic solution, wherein the acidic solution is one or more of hydrochloric acid, sulfuric acid, and phosphoric acid. Distilled water can be added before adding the acidic solution to reduce the concentration of the mixture.

[0024] The ferrous ions in the iron-chromium mixture are oxidized to ferric ions using an oxidizing agent. The oxidizing agent can be one or more of sodium chlorate, hydrogen peroxide, potassium chlorate, sodium hypochlorite, potassium hypochlorite, and ozone. In this step, the oxidizing agent needs to be able to oxidize Fe under acidic conditions. 2+ Furthermore, it can react with acids and decompose under appropriate conditions. The purpose of this step is to reduce the Fe content in the solution. 2+ Oxidation must be complete; otherwise, in step S5, adjusting the solution to acidity will result in Fe in the solution. 2+ and Cr 6+ A reaction occurs, causing the chromium test result to be lower than expected or directly reaching the reaction endpoint, resulting in test failure. This step requires, in addition to removing Fe... 2+ Complete oxidation requires the removal of excess oxidant; otherwise, the excess oxidant will oxidize the ferrous ammonium sulfate standard solution during titration in S5, leading to higher test results. Therefore, the oxidant should be selected that can react with excess acid or decompose into gas upon heating.

[0025] In this step, the amount of oxidant added is relative to the Fe content in the iron-chromium mixture. 2+ The amount is excessive, the purpose of which is to ensure Fe 2+ Oxidation is complete. Simultaneously, the amount of acidic solution relative to the oxidant is also in excess, ensuring the removal of excess oxidant.

[0026] To accelerate Fe 2+ The oxidation rate can be adjusted by slightly heating the mixture, allowing Fe to... 2+ Oxidation is complete.

[0027] S2: Add sodium hydroxide solution and hydrogen peroxide under stirring conditions, so that ferric ions form ferric hydroxide precipitate, and ferric ions are oxidized to hexavalent chromium ions.

[0028] Specifically, in this step, distilled water is first added to reduce the concentration of the iron-chromium mixture. When the concentration of iron-chromium in the mixture is high, Fe(OH)3 encapsulates Cr. 3+ The increased probability of Fe(OH)3 coating on Cr makes the test value too low. Therefore, the concentration can be reduced to decrease the amount of Fe(OH)3 coating on Cr. 3+ The probability of this happening is low. Adding too much distilled water results in a longer heating time for the test; therefore, the concentration can be appropriately reduced to a specific range.

[0029] Adding sodium hydroxide solution under stirring causes ferric ions to form ferric hydroxide precipitate. The purpose of stirring is to reduce the particle size of Fe(OH)3 and prevent the precipitate from encapsulating Cr. 3+ , making Cr 3+ Incomplete oxidation led to lower test results. Additionally, the sodium hydroxide concentration used in this step was relatively low; using a lower concentration makes it easier to reduce the particle size of the Fe(OH)3 precipitate than a higher concentration. Specifically, a 50 g / L sodium hydroxide solution can be used.

[0030] This step involves adding both sodium hydroxide solution and hydrogen peroxide, and requires appropriate heating of the mixture. The solution pH should be greater than 10, and heating is necessary to ensure the Cr content is maintained. 3+ It can be oxidized by hydrogen peroxide.

[0031] This step can also reduce the rate of alkali addition, thereby decreasing the amount of Fe(OH)3 coating Cr. 3+ The probability of Cr; this step can promote the reaction by increasing the amount of hydrogen peroxide, lowering the heating temperature, and extending the reaction time. 3+ Oxidation is complete.

[0032] To accelerate the reaction of hydrogen peroxide and Cr 3+ The reaction can be accelerated by heating the mixture.

[0033] S3: Add an acidic solution to dissolve the ferric hydroxide precipitate and release the encapsulated trivalent chromium ions; add sodium hydroxide solution and hydrogen peroxide under stirring conditions to form ferric hydroxide precipitate from the trivalent ferric ions, while the released trivalent chromium ions are oxidized to hexavalent chromium ions.

[0034] This step is specifically divided into:

[0035] S31: Add acidic solution to dissolve the ferric hydroxide precipitate, releasing the encapsulated trivalent chromium ions. This step requires adding an excess of acidic solution to completely dissolve the ferric hydroxide precipitate. The purpose of adding acidic solution is to adjust the solution to acidity, dissolving Fe(OH)3 to release the encapsulated chromium ions.3+ The purpose of adding an excess of acidic solution is to reduce human error in judgment and to prevent the complete dissolution of trace amounts of non-visible flocculent Fe(OH)3. Hydrochloric acid can be used as the specific acidic solution.

[0036] S32: Same as step S2, causing ferric ions to form ferric hydroxide precipitate, while the released ferric ions are oxidized to hexavalent chromium ions.

[0037] S4: Repeat step S3 X times; X is an integer greater than 0. If the iron concentration in the iron-chromium mixture is less than 5%, step S3 can be repeated only 1 times to obtain relatively accurate results, with a spiked recovery rate greater than 90%. If the iron concentration in the iron-chromium mixture is high, step S3 can be repeated two or more times to ensure accurate determination results.

[0038] S5: Determination of chromium content. Mix the test solution with sulfuric acid and phosphoric acid, then titrate with ferrous ammonium sulfate standard titration solution until a yellow-green color is reached. Add N-benzoic acid indicator and continue titrating until the solution changes from purple-red to green, which is the endpoint. Calculate the mass content of chromium in the iron-chromium mixture based on the titration results.

[0039] Example 1

[0040] A method for determining the chromium content in an iron-chromium mixture includes:

[0041] S1: Weigh 0.6-0.7g of iron-chromium flow battery electrolyte, add 10mL of distilled water, 2mL of 1+1 hydrochloric acid solution, and 3 drops of approximately 40% sodium chlorate solution. Heat to the desired temperature. 2+ Once oxidation is complete, allow it to cool slightly; heating in this step helps accelerate the oxidation reaction.

[0042] S2: Stir with a rotor, add 50-100mL of distilled water, 10-15mL of 50g / L sodium hydroxide solution, and 0.5-3mL of hydrogen peroxide, and heat until the hydrogen peroxide reacts completely; heating in this step helps to accelerate the oxidation reaction.

[0043] S31: Add hydrochloric acid dropwise to completely dissolve the precipitate in the solution, and add one drop in excess;

[0044] S32: Add 10-15 mL of 50 g / L sodium hydroxide solution and 0.5-3 mL of hydrogen peroxide, and heat until the hydrogen peroxide reacts completely; this will oxidize the released trivalent chromium ions to hexavalent chromium.

[0045] S5: Cool to room temperature, add 150 mL of water, 4 mL of sulfuric acid, and 5 mL of phosphoric acid. Titrate with 0.1 N ferrous ammonium sulfate standard solution until yellow-green. Add N-phenyl-o-aminobenzoic acid indicator and continue titrating until the solution changes from purple-red to green as the endpoint.

[0046] S6: Result Calculation

[0047] Chromium is expressed as the mass fraction ω of elemental chromium (Cr), with the value expressed as a percentage, and is calculated according to formula (1):

[0048]

[0049] In the formula:

[0050] V – The numerical value of the volume of ferrous ammonium sulfate standard titration solution consumed in the titration test solution, in milliliters (mL);

[0051] C – The accurate value of the concentration of the ferrous ammonium sulfate standard titration solution, in moles per liter (mol / L);

[0052] m — the numerical value of the mass of the sample, in grams (g);

[0053] M – The numerical value of the molar mass of elemental chromium (1 / 3Cr), in grams per mole (g / mol) (M = 17.33)

[0054] The arithmetic mean of the parallel measurement results is taken as the measurement result, and the absolute value of the two parallel measurement results is not greater than 0.06%.

[0055] Experimental Example 1: Detection Limit of the Method in this Application

[0056] The detection limit of titration analysis methods is generally calculated based on the titration volume of one drop of standard solution (i.e., 0.05 mL). The concentration of ferrous ammonium sulfate standard solution is approximately 0.1 mol / L. Substituting into formula (1), the detection limit of chromium in this invention method is: 0.1 * 0.05 * 17.33 / 10 / 0.5 = 0.0173, in %. The detection limit of chromium in this application method is very low, approximately 0.02%, which can meet the requirements for routine testing of iron-chromium mixtures.

[0057] Experimental Example 2: Precision of the Method of this Application

[0058] According to the test method in this patent application, the same person shall test different samples at least 6 times according to the method in Example 1 to determine the indoor standard deviation and indoor repeatability.

[0059] The method of the present invention was evaluated through precision tests, and the results obtained for the unknown sample are shown in Table 1.

[0060] Table 1. Precision of the test method (n=6)

[0061]

[0062]

[0063] As can be seen from the data in Table 1, the chromium test values ​​of the three iron-chromium mixtures with different concentrations are very close, and the relative standard deviations are all very low, less than 0.5%, indicating that the test method has high precision, which is a basic condition for evaluating the accuracy of the test method.

[0064] Experimental Example 3: Method Accuracy

[0065] Accuracy tests were conducted using an unknown iron-chromium mixture. Triple parallel tests were performed on iron-chromium mixtures #4 and #5 according to the method in Example 1, and the average value was taken to obtain the "chromium test value". Analytical grade chromium chloride hexahydrate was added to the test sample according to the calculated amount, and the "total chromium test value" was obtained from the sample test. The spiked recovery rate was calculated, and the test method was evaluated. The test results are shown in Table 2.

[0066] Table 2 Chromium test spiked recovery rate

[0067]

[0068] As can be seen from the data in Table 2, the chromium test values ​​range from 2.2% to 18.56%, almost covering the content range of iron-chromium mixtures. The chromium spike recovery rate is between 89.0% and 99.4%, and the recovery rate is stable and very good, which proves that the test method of the present invention has high accuracy and is suitable for the production, acceptance and regeneration of electrolytes and waste electrolytes.

[0069] Experiment Example 4: Sample size, amount of distilled water added, and whether step S3 was used to verify the experiment.

[0070] Different amounts of iron-chromium mixture samples (3#, 6#, 7#, and 8#) were weighed, and different amounts of distilled water were added in step S2 according to the method in Example 1, as well as whether step S3 was omitted. The test results are summarized in Table 3.

[0071] Table 3 Summary of test results for iron-chromium mixtures under different conditions

[0072]

[0073]

[0074] The test results in the table above show that the higher the iron concentration in the iron-chromium mixture, the smaller the amount weighed, the more water added, and the higher the Cr content. 3+ The number of oxidation cycles helps obtain more accurate test results. The test results are consistent with theory; reducing the sample size and increasing the water volume are both ways to dilute the concentration of the sample, thereby reducing the possibility of flocculent ferric hydroxide encapsulating chromium ions under alkaline conditions, and thus improving the accuracy of the test results.

[0075] In this invention, the iron in the iron-chromium mixture is first completely oxidized, so that the Fe...2+ It does not affect subsequent Cr 3+ In practical applications, tests have shown that when the iron content is high, ferric hydroxide precipitates encapsulate Cr. 3+ Seriously, leading to Cr 3+ Since it cannot be completely oxidized by hydrogen peroxide, this application innovatively proposes a process of multiple acid dissolution and re-oxidation of ferric hydroxide precipitate to ensure that Cr... 3+ Complete oxidation is achieved. The technical solution of this application is simple and easy to implement, and the test results have high precision and accuracy. The test time is short (about 15 minutes), which has high promotional value.

[0076] This invention provides a rapid method for determining the content of iron and chromium mixtures. It does not limit the relative content of iron and chromium, the total iron and chromium content, or the presence of impurity ions and anions in the mixture, indicating that these conditions do not affect the test results. The method has a wide range of applications. The above embodiments illustrate the detailed method of this invention, but this invention is not limited to the detailed method described above; that is, it does not mean that this invention must rely on the detailed method described above to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials in the method, the addition of auxiliary components, and the selection of specific methods, without departing from the principles of this invention, all fall within the protection and disclosure scope of this invention.

Claims

1. A method for determining the chromium content in an iron-chromium mixture, characterized in that, Includes the following steps: S1: Oxidizes ferrous ions in an iron-chromium mixture to ferric ions under acidic conditions; S2: First, add distilled water to reduce the concentration in the iron-chromium mixture. Then, add sodium hydroxide solution and hydrogen peroxide under stirring conditions to form ferric hydroxide precipitate of ferric ions. When the pH of the solution is greater than 10, hydrogen peroxide oxidizes ferric ions to hexavalent chromium ions. S3: Add an acidic solution to dissolve the ferric hydroxide precipitate and release the encapsulated trivalent chromium ions; add sodium hydroxide solution and hydrogen peroxide under stirring to form ferric hydroxide precipitate from the trivalent ferric ions, while the released trivalent chromium ions are oxidized to hexavalent chromium ions. S4: Repeat step S3 X times; X is an integer greater than 0; S5: Determine the chromium content in the mixture.

2. The method for determining the chromium content in an iron-chromium mixture according to claim 1, characterized in that, In step S1, an acidic solution is used to adjust the iron-chromium mixture to acidity, wherein the acidic solution is one or more of hydrochloric acid, sulfuric acid, and phosphoric acid.

3. The method for determining the chromium content in an iron-chromium mixture according to claim 2, characterized in that, In step S1, an oxidant is used to oxidize the ferrous ions in the iron-chromium mixture to ferric ions. The oxidant is one or more of sodium chlorate, hydrogen peroxide, potassium chlorate, sodium hypochlorite, potassium hypochlorite, and ozone.

4. The method for determining the chromium content in an iron-chromium mixture according to claim 3, characterized in that, The oxidant in step S1 causes all ferrous ions in the iron-chromium mixture to be oxidized to ferric ions, and the acidic solution reacts completely with the remaining oxidant.

5. The method for determining the chromium content in an iron-chromium mixture according to claim 1, characterized in that, In step S2, the concentration of the sodium hydroxide solution is 50 g / L.

6. The method for determining the chromium content in an iron-chromium mixture according to claim 1, characterized in that, In step S3, an excess of acidic solution is added to completely dissolve the ferric hydroxide precipitate.

7. The method for determining the chromium content in an iron-chromium mixture according to claim 1, characterized in that, In step S5, the test solution is mixed with sulfuric acid and phosphoric acid, and then titrated with ferrous ammonium sulfate standard titration solution until it turns yellow-green. N-benzoic acid indicator is added, and titration continues until the solution changes from purple-red to green as the endpoint. The mass content of chromium in the iron-chromium mixture is calculated based on the titration results.

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