Method for measuring contents of hydrofluoric acid and nitric acid in mixed acid by potentiometric titration

Abstract

The invention discloses a method for measuring the content of hydrofluoric acid and nitric acid in mixed acid by potentiometric titration, which comprises the following steps: step 1, selecting a titration mixed acid system of a potassium hydroxide, methanol and isopropanol solution, titrating a mixed acid sample to be detected, and obtaining two equivalent points by adopting a step-by-step titration mode; calculating the percentage content of hydrofluoric acid in the mixed acid according to the numerical values of the two equivalent points, and calculating the total amount of nitric acid and hydrochloric acid in the mixed acid; step 2, titrating the percentage content of nitric acid in the mixed acid by selecting an ammonium ferrous sulfate solution; and step 3, calculating the percentage content of hydrochloric acid in the mixed acid through a subtraction method. The invention belongs to the technical field of chemical content determination, and solves the problems of numerous chemicals, complex operation and long time consumption in the existing mixed acid system determination process.

Description

Technical Field

[0001] The present invention belongs to the technical field of determination of chemical substance content, and relates to a method for potentiometric titration to determine the contents of hydrofluoric acid and nitric acid in mixed acids. Background Art

[0002] Pickling solutions for superalloys usually consist of hydrochloric acid, nitric acid, and hydrofluoric acid. The use of mixed acids in pickling solutions has a wide range of applications in many industrial fields such as metal surface treatment, semiconductor manufacturing, and glass etching. The concentration of the pickling solution has a great influence on the pickling effect. If the concentration of the pickling solution is too high, it will corrode the equipment (hydrogen depolarization), and if it is too low, it will affect the pickling effect. Whether under-pickling or over-pickling, it will have an adverse impact on the material surface. Therefore, correctly controlling the concentration of the pickling solution is the key to successful pickling. Accurately determining the content of each component in the mixed acid is of great significance for controlling the production process, ensuring product quality, and environmental protection.

[0003] Currently, the manual titration method is used to determine the content of chemical components in the ash removal tank solution. The end point of the titration is determined by visually observing the color change during the titration of the solution to be tested. Since each person has different sensitivities to color changes, especially for some titration tests where the color change is not particularly obvious (such as titrating nitric acid in the tank solution with a potassium dichromate standard solution, the color of the solution changes from yellow to brown, which is the end point of the titration and is relatively difficult to determine), it will lead to large differences in test results. Using potentiometric titration for titration analysis can effectively avoid this problem, no longer using color reagents, and reducing the impact on personnel and the environment. In addition, traditional determination methods such as acid-base neutralization titration and redox titration are more effective for the determination of single acids. However, for mixed acid systems, due to the mutual influence of the acidity of various acids, there are many chemical reagents, complex operations, and long time consumption during the determination process. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for potentiometric titration to determine the contents of hydrofluoric acid and nitric acid in mixed acids, which solves the problems of numerous chemical reagents, complex operation process, and long time consumption involved in the determination process of the existing mixed acid system.

[0005] The technical solution adopted by the present invention is a method for potentiometric titration to determine the contents of hydrofluoric acid and nitric acid in mixed acids, which is implemented according to the following steps: Step 1: Select a potassium hydroxide methanol isopropanol solution to titrate the mixed acid system, titrate the mixed acid sample to be detected, and obtain two equivalence points by using a stepwise titration method; then, according to the values of the two equivalence points, calculate the percentage content of hydrofluoric acid in the mixed acid, and at the same time, calculate the total amount of nitric acid and hydrochloric acid in the mixed acid; Step 2: Select an ammonium ferrous sulfate solution to titrate the percentage content of nitric acid in the mixed acid; Step 3: Obtain the percentage content of hydrochloric acid in the mixed acid by the subtraction method.

[0006] The beneficial effects of the present invention are as follows: When using the potentiometric titration method to determine the content of the bath solution, it is very different from the existing manual titration method. There is no need to use a color reagent anymore, and there will be no human error caused by manual titration. There is less waste liquid, which is more environmentally friendly, has lower energy consumption, and high detection efficiency; it shortens the detection cycle and improves the detection technology and progress. Description of the Drawings

[0007] Figure 1 is the potentiometric titration method (sodium hydroxide solution) of the present invention for titrating the mixed acid system; Figure 2 is the potentiometric titration method (potassium hydroxide methanol isopropanol solution) of the present invention for titrating the mixed acid system; Figure 3 is the titration curve of the potentiometric titration method (for titrating nitric acid in the mixed acid system) of the present invention. Detailed Embodiments

[0008] The present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0009] The method of the present invention uses a stepwise titration method for quantification respectively and is implemented according to the following steps: Step 1: Select the potassium hydroxide methanol isopropanol solution to titrate the mixed acid system, titrate the mixed acid sample to be detected, and obtain two equivalence points by the stepwise titration method; then, according to the values of the two equivalence points, calculate the percentage content of hydrofluoric acid in the mixed acid, and at the same time, calculate the total amount of nitric acid and hydrochloric acid in the mixed acid; Step 2: Select the ammonium ferrous sulfate solution to titrate the percentage content of nitric acid in the mixed acid; The ammonium ferrous sulfate solution with a concentration of 0.500 mol / L is prepared and calibrated according to GB / T 601-2016 "Chemical Reagents - Preparation of Standard Titration Solutions"; In addition, accurately weigh 6 g (accurate to 0.001 g) of potassium dichromate that has been dried to constant weight at 120 °C to obtain a potassium dichromate solution with a concentration of 0.5 mol / L. After dissolution, transfer it to a 250 mL volumetric flask for constant volume, which is used for the calibration of the ammonium ferrous sulfate solution; Step 3: Obtain the percentage content of hydrochloric acid in the mixed acid by the subtraction method.

[0010] Use the method of the present invention to determine the acid content of the prepared simulated mixed acid sample 2#, and compare it with the determination results of the single acid content in the same sample by the traditional titration method. Each level is determined in parallel 6 times, and the average value is taken; and F-test and t-test are performed on the 6 test results. The comparison experimental results are shown in Table 1.

[0011] Table 1. Results of Comparative Experiments (n = 6)

[0012] As can be seen from the results in Table 1, at α = 0.05 (general significance level), F < F0.05(4,4), that is, the data are homoscedastic; at α = 0.05 (two-tailed), |t| < t0.025(8), that is, there is no significant difference in the means between the data.

[0013] Therefore, it can be seen from the F-test and t-test that there is no significant difference in the determination results of each component of the mixed acid system between the method of the present invention and the other traditional titration methods, and the consistency is good.

[0014] Interference Experiment: An inductively coupled plasma emission spectrometer from Thermo Fisher Scientific of the United States was used to determine the elemental components in the pickling solution.

[0015] Model: iCAPPRO, Serial Number: iCAPPRO60455.

[0016] Operating Parameters: Spectral Range: 167 nm - 852 nm, Plasma Power: 1150 W, Working Gas: High-purity Argon (≥99.995%), Working Pressure: 0.55 MPa - 0.6 MPa, Circulating Cooling Water Temperature: 20 °C, Minimum Flow Rate: 2 L / min, Exhaust Velocity: 5.5 m / s, Sample Processing Time: 60 s, Exposure Time: 30 s.

[0017] Since the contents of elements such as phosphorus, copper, vanadium, titanium, lead, and magnesium in the detection object are relatively low, it can be basically considered that they have no influence on the determination of the total acidity and are not considered here. While the contents of elements such as aluminum, manganese, calcium, and zinc are slightly higher, and the specific data are shown in Table 2.

[0018] Table 2. Contents of Interfering Elements

[0019] For these four components with higher contents, in order to verify their interference situations, 10 times the determined contents of these four elements were added to the prepared simulated mixed acid sample No. 2, that is, the aluminum element standard solution was 100 μg / mL, the manganese element standard solution was 110 μg / mL, the calcium element standard solution was 110 μg / mL, and the zinc element standard solution was 100 μg / mL. The results of the determination of each acid content before and after adding the above four element standard solutions are shown in Table 3.

[0020] Table 3. Results of Interference Experiment

[0021] As can be seen from the results in Table 3, the relative standard deviations of the acid contents before and after adding the standard solution are both less than 1%. It can be seen that the determination of the acid contents by potentiometric titration is not affected by other elements in the mixed acid.

[0022] Precision experiment: Select two simulated mixed acid samples 1# and 3# with different acidities, and measure each 11 times according to the experimental method of this experiment. Calculate their respective averages and relative standard deviations. The results are shown in Table 4 and Table 5.

[0023] Table 4. Precision experiment results of sample 1# (n = 11)

[0024] Table 5. Precision experiment results of sample 3# (n = 11)

[0025] As can be seen from Table 4 and Table 5, for the two simulated mixed acid samples 1# and 3# with different acidities, the relative standard deviations of the 11 measurement values obtained according to the experimental method of this experiment are both less than 1%. The experimental results show that the results of detecting each component in the mixed acid system by the method of the present invention have a small degree of dispersion and high precision, and the method of the present invention has obvious effects.

[0026] Spiked recovery experiment: Add a certain amount of nitric acid, hydrofluoric acid and hydrochloric acid to the simulated mixed acid sample 2# respectively, and conduct parallel tests six times. Calculate their respective spiked recoveries, as shown in Table 6.

[0027] Table 6. Spiked recovery test results

[0028] As can be seen from Table 6, the spiked recoveries of each component are all between 99.34% and 101.20%, indicating that the spiked recovery effect of the method of the present invention is good. During the pickling process, as the three acid components in the pickling mixed acid are consumed, the bath solution needs to accurately supplement raw materials to control the ratio of the three acids. The potentiometric titration method has a good spiked recovery effect and can accurately analyze the content of the added sample, providing a solid theoretical support for product quality control.

[0029] Verification experiment: 1. Experimental method.

[0030] 1.1) Preparation of mixed acid samples.

[0031] The grades of the reagents used are all analytical pure (including analytical pure) and above. The water for analytical experiments should be distilled water or deionized water or pure water of equivalent purity, meeting the specifications of grade 3 water in GB / T 6682-2008 "Specifications and Test Methods for Laboratory Water".

[21] In the specifications of grade 3 water.

[0032] To verify that mixed acid samples with different ratios can all be determined by potentiometric titration, three solutions with different ratios were prepared according to the ratios of factory production. The total amount of each solution was 50 g, and the mass ratio of hydrofluoric acid to hydrochloric acid was determined to be 5:2, with the nitric acid content ranging from 1% to 10%, as shown in Table 7.

[0033] Table 7. Mixed acid samples with different ratios

[0034] 1.2) Titrate the total acidity in the mixed acid system by potentiometric titration.

[0035] The mixed acid system contains two strong acids (nitric acid and hydrochloric acid) and one weak acid (hydrofluoric acid). The basic standard titration solution will react with all three of nitric acid, hydrochloric acid, and hydrofluoric acid in the mixed acid system. Therefore, quantitative analysis cannot be carried out by simple acid-base titration. However, the principle of acid-base titration can be used to quantify the total acidity, and then the content of some components that are difficult to directly measure can be obtained by subtraction.

[0036] The automatic potentiometric titrator uses the 916Ti-Touch model from Metrohm AG, Switzerland. Magnetic stirrer: 801Stirrer; drive head: 800 Dosino; liquid addition unit (20 mL): 20 mL Dosing Unit; electrodes: redox electrode, pH aqueous electrode, pH non-aqueous electrode, silver electrode, fluoride ion selective electrode, reference electrode.

[0037] 1.2.1) Control group, select sodium hydroxide solution as the titrant with a concentration of 0.500 mol / L, and prepare and standardize it according to GB / T 601-2016 "Chemical Reagents - Preparation of Standard Titration Solutions". Weigh the prepared mixed acid sample into a polytetrafluoroethylene titration cup, dilute it with secondary water, select the pH aqueous electrode, and titrate it with the standardized sodium hydroxide solution, as Figure 1 shown in Potentiometric Titration of the Mixed Acid System (Titration with Sodium Hydroxide Solution). Figure 1 Among them, except for the first equivalence point (EP1) that is clearly shown, the titration endpoints of the other equivalence points EP2-EP7 are close to each other and interfere greatly with each other. Therefore, the equivalence points EP2-EP7 cannot be used to calculate the component content of the mixed acid system.

[0038] 1.2.2) Select potassium hydroxide methanol isopropanol solution as the titrant with a concentration of 0.100 mol / L, and prepare and standardize it according to GB / T 601-2016 "Chemical Reagents - Preparation of Standard Titration Solutions". Weigh the prepared mixed acid sample into a polytetrafluoroethylene titration cup, using ethanol as the solvent, with a density of 0.789 - 0.791 g / mL; select a non-aqueous pH electrode and titrate with the calibrated potassium hydroxide methanol isopropanol solution, as shown in Figure 2 Potentiometric titration of the mixed acid system (titration with potassium hydroxide methanol isopropanol solution). Figure 2 In this method, both the first equivalence point (EP1) and the second equivalence point (EP2) are clearly shown, with a large jump, which can be used to calculate the component content of the mixed acid system.

[0039] It can be seen that compared with using sodium hydroxide solution as the titrant, the method of the present invention preferably uses potassium hydroxide methanol isopropanol solution to titrate the mixed acid system, and the principle is as follows: First, the reason for choosing potassium hydroxide methanol isopropanol solution is analyzed: Hydrofluoric acid (HF) is a weak acid, which means that it is not completely ionized in water. Therefore, when reacting in an aqueous solution, the ionization degree of hydrofluoric acid is relatively low, resulting in a slow reaction rate. In a non-aqueous ethanol solvent, the ionization degree of hydrofluoric acid may be higher, thus accelerating the reaction rate. Therefore, in the experiment of the present invention, potassium hydroxide methanol isopropanol solution should be selected to titrate the mixed acid system, which can not only ensure an obvious potential jump but also keep the titration volume within a reasonable range, improving the accuracy of the determination; Secondly, the reason for the appearance of two obvious equivalence points is analyzed: The binding ability between hydrogen atoms and fluorine atoms is relatively strong, making hydrofluoric acid not completely ionized in water. At 25 °C, the Ka value of hydrofluoric acid is 7.2×10 -4 , so theoretically, low-concentration hydrofluoric acid is a weak acid; while hydrochloric acid and nitric acid are both strong acids. First, they are titrated by potassium hydroxide methanol isopropanol solution to show the first equivalence point EP1; then the titration of hydrofluoric acid starts, showing the second equivalence point EP2. According to the two equivalence points, the total amount of HF and HNO 3 +HCl can be calculated, and the reaction equations are as follows: HCl + H 2 O → H 3 O + + Cl － Strong acid dissociation, H 3 O + + OH- → 2H 2 O neutralization, HNO 3 + H 2 O → H 3 O + + NO 3 － Strong acid dissociation, H 3 O + + OH － → 2H 2 O neutralization; HF + H2 O→H 3 O + +F － Weak acid dissociation, H 3 O + +OH － →2H 2 O neutralization.

[0040] Among them, the first four chemical formulas are collectively called EP1, and the last two chemical formulas are collectively called EP2.

[0041] 1.3) Potentiometric titration of hydrofluoric acid in a mixed acid system, Accurately weigh m 1 g of the prepared mixed acid sample into a polytetrafluoroethylene titration cup, where m 1 g is accurate to four decimal places. Add 50 mL of ethanol as the solvent, select a pH non-aqueous phase electrode, and titrate with a calibrated potassium hydroxide methanol isopropanol solution. The volume consumed at the first equivalence point EP1 is V 1 mL, and the volume consumed at the second equivalence point EP2 is V 2 mL. Then the percentage content of hydrofluoric acid in the mixed acid is: (1) Among them, ω HF ——Percentage content of hydrofluoric acid in the mixed acid, %; C(KOH)——Molar concentration of potassium hydroxide, mol / L; V 1 ——Volume consumed at the first equivalence point EP1, mL; V 2 ——Volume consumed at the second equivalence point EP2, mL; m 1 ——Mass of the prepared mixed acid sample weighed, g; M HF ——Molar mass of HF, the value is 20.01 g / mol.

[0042] 1.4) Potentiometric titration of nitric acid in a mixed acid system, The reaction equation of nitric acid and ammonium ferrous sulfate under acidic conditions is as follows: 4Fe 2+ +2NO 3 - +6H + →4Fe 3 + +N 2 O 3 +3H 2 O, Accurately weigh m 2Weigh m g of the prepared mixed acid sample into a PTFE titration cup, where m g is accurate to four decimal places. Add 40 mL of sulfuric acid (4+1). Select the redox electrode and titrate with the ammonium ferrous sulfate solution. The consumed volume is V mL. Then the percentage content of nitric acid in the mixed acid is: 2 g accurate to four decimal places, add 40 mL of sulfuric acid (4+1), select the redox electrode, and titrate with the ammonium ferrous sulfate solution. The consumed volume is V 3 mL. Then the percentage content of nitric acid in the mixed acid is: (2) Where ω HNO3 ——The percentage content of nitric acid in the mixed acid, %; C(NH 4 ) 2 Fe(SO 4 ) 2 ——The molar concentration of ammonium ferrous sulfate, mol / L; V 3 ——The volume of ammonium ferrous sulfate solution consumed for titrating nitric acid, mL; m 2 ——The mass of the prepared mixed acid sample weighed, g; MHNO 3 ——HNO 3 The molar mass of which is 63.02 g / mol.

[0043] For potentiometric titration of nitric acid in the mixed acid system, its titration curve (potentiometric titration of nitric acid in the mixed acid system) is as Figure 3 shown.

[0044] 1.5) The hydrochloric acid in the mixed acid system is obtained by the subtraction method. Although the determination of hydrochloric acid content is not required in this experiment, since they belong to the same mixed acid system and there are mutual influences. Therefore, it is hoped that in addition to accurately determining nitric acid and hydrofluoric acid in the mixed acid system, hydrochloric acid in the mixed acid system can also be accurately determined simultaneously.

[0045] From step 1.2) to step 1.4), it can be obtained that hydrochloric acid and nitric acid are first titrated by the potassium hydroxide methanol isopropanol solution to appear the first equivalence point EP1, and then hydrofluoric acid begins to be titrated to appear the second equivalence point EP2. Then, based on the two equivalence points, the total amounts of HF and HNO 3 +HCl can be calculated.

[0046] Accurately weigh m 1 g of the prepared mixed acid sample into a PTFE titration cup, where m 1 g is accurate to four decimal places. Add 50 mL of ethanol as the solvent. Select the pH non-aqueous phase electrode and titrate with the calibrated potassium hydroxide methanol isopropanol solution. The first equivalence point EP1 consumes a volume of V 1 mL, and the second equivalence point EP2 consumes a volume of V 2If it is mL, the following calculation formula is available: (3) Among them, ω HCl ——Percentage content of hydrochloric acid in the mixed acid, %; C(KOH)——Molar concentration of potassium hydroxide, mol / L; V 1 ——Volume consumed at the first equivalence point EP1, mL; m 1 ——Mass of the prepared mixed acid sample weighed, g; M HCl —— HCl Molar mass of, the value is 36.46 g / mol; n HNO3 ——Amount of substance of nitric acid in the mixed acid obtained through the aforementioned 1.4), mol.

[0047] The determination results of the single acid content in the same sample by the method of the present invention and the existing titration method are compared. It can be known by F-test and t-test that there is no significant difference in the determination results of each component of the mixed acid system between the method of the present invention and the existing titration method, and the consistency is good. Through the interference experiment, it can be seen that the relative standard deviations of the acid contents before and after adding the standard solution are both less than 1%. It can be seen that the potentiometric titration method is not affected by other elements in the mixed acid when determining the acid contents. The relative standard deviations of the 11 determination values of each component of the mixed acid system detected by the method of the present invention are all less than 1%, that is, the degree of result dispersion is small and the precision is high. The spiked recovery rates of each component are all between 99.34% and 101.20%, indicating that the spiked recovery effect of the method of the present invention is good. During the pickling process, as the three acid components in the pickling mixed acid are consumed, the tank solution needs to accurately supplement raw materials to control the ratio of the three acids. The potentiometric titration method has a good spiked recovery effect, can accurately analyze the content of the added sample, and provides a theoretical support for product quality control. The method of the present invention overcomes the traditional titration method with a large number of chemical reagents, complex operations, and long time consumption in the prior art detection process. The whole process does not require sample treatment, and it only takes about 15 minutes from the start of the experiment to obtain the analysis data results, improving the analysis speed, greatly reducing the workload of the operators, and reducing the influence of errors in the manual titration process. At the same time, it also reduces the cost, can meet the needs of production analysis and detection, and provides a new idea for the analysis method of determining the content of the mixed acid in the tank solution by potentiometric titration method.

[0048] Example 1 The determination object is the simulated mixed acid sample 2#.

[0049] This Example 1 is implemented according to the data in Table 2 according to the following steps: First, select a potassium hydroxide methanol isopropanol solution to titrate the mixed acid system, and obtain two equivalence points by using a stepwise titration method; calculate the percentage content of hydrofluoric acid in the mixed acid and the total amount of nitric acid and hydrochloric acid in the mixed acid according to the two equivalence points; After that, select an ammonium ferrous sulfate solution to titrate the percentage content of nitric acid in the mixed acid; The ammonium ferrous sulfate solution, 0.500 mol / L, is prepared and calibrated in accordance with GB / T 601-2016 "Chemical Reagents - Preparation of Standard Titration Solutions"; In addition, accurately weigh 6 g (accurate to 0.001 g) of primary potassium dichromate that has been dried to constant weight at 120 °C, accurate to four decimal places, to obtain a 0.5 mol / L potassium dichromate solution. After dissolution, transfer it to a 250 mL volumetric flask for volume fixation, and use it for the calibration of the ammonium ferrous sulfate solution; Finally, obtain the percentage content of hydrochloric acid in the mixed acid by the method of subtraction.

[0050] Using the method of the present invention described above, the percentage detection results are 1.99 for nitric acid, 70.05 for hydrofluoric acid, and 27.97 for hydrochloric acid respectively. After the comparative analysis following Table 2, the results are relatively accurate and meet the detection accuracy requirements.

[0051] Example 2 The object to be measured is the simulated mixed acid sample 2#.

[0052] In this Example 2, according to the data in Table 2, it is implemented according to the steps described above.

[0053] Using the method of the present invention described above, the percentage detection results are 1.98 for nitric acid, 70.03 for hydrofluoric acid, and 27.95 for hydrochloric acid respectively. After the comparative analysis following Table 2, the results are relatively accurate and meet the detection accuracy requirements.

[0054] Example 3 The object to be measured is the simulated mixed acid sample 2#.

[0055] In this Example 3, according to the data in Table 2, it is implemented according to the steps described above.

[0056] Using the method of the present invention described above, the percentage detection results are 1.98 for nitric acid, 70.03 for hydrofluoric acid, and 27.98 for hydrochloric acid respectively. After the comparative analysis following Table 2, the results are relatively accurate and meet the detection accuracy requirements.

[0057] Example 4 The object to be measured is the simulated mixed acid sample 2#.

[0058] In this Example 4, according to the data in Table 2, it is implemented according to the steps described above.

[0059] Using the method of the present invention described above, the percentage of the detection results are 2.02 for nitric acid, 70.06 for hydrofluoric acid, and 27.99 for hydrochloric acid respectively. After the comparative analysis with reference to Table 2, the results are relatively accurate and meet the requirements of detection accuracy.

[0060] Example 5 The object to be measured is the simulated mixed acid sample 2#.

[0061] This Example 5 is implemented according to the data in Table 2 and the steps described above.

[0062] Using the method of the present invention described above, the percentage of the detection results are 2.01 for nitric acid, 70.01 for hydrofluoric acid, and 27.94 for hydrochloric acid respectively. After the comparative analysis with reference to Table 2, the results are relatively accurate and meet the requirements of detection accuracy.

[0063] Example 6 The object to be measured is the simulated mixed acid sample 2#.

[0064] This Example 6 is implemented according to the data in Table 2 and the steps described above.

[0065] Using the method of the present invention described above, the percentage of the detection results are 2.00 for nitric acid, 70.04 for hydrofluoric acid, and 27.97 for hydrochloric acid respectively. After the comparative analysis with reference to Table 2, the results are relatively accurate and meet the requirements of detection accuracy.

[0066] It can be seen that the potentiometric titration method of the present invention is an analytical method based on the change of electrode potential to determine the titration end point, which has the advantages of high accuracy, high sensitivity, and automation. It is applicable to the determination of each component in the mixed acid system, can improve the accuracy of the determination results, avoid the harm of toxic chemicals to the human body, improve the analysis efficiency, and has certain guiding significance for the determination of the components of the acid pickling solution of superalloy by a potentiometric titrator.

Claims

1. A method for determining the content of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration, characterized in that: Follow these steps to implement: Step 1, selecting a titration mixed acid system of potassium hydroxide methanol isopropanol solution, titrating the mixed acid sample to be tested, and obtaining two equivalence points by step titration; then calculating the percentage of hydrofluoric acid in the mixed acid according to the values ​​of the two equivalence points, and at the same time, calculating the total amount of nitric acid and hydrochloric acid in the mixed acid; Step 2, selecting ammonium ferrous sulfate solution to titrate the percentage of nitric acid in the mixed acid; Step 3, calculating the percentage of hydrochloric acid in the mixed acid by subtraction method.

2. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 1, characterized in that: In step 1, two equivalence points are obtained by step-by-step titration. The specific process is: Weigh the mixed acid sample into a polytetrafluoroethylene titration cup, select a pH non-aqueous phase electrode, and titrate with a calibrated potassium hydroxide methanol-isopropanol solution; Hydrochloric acid and nitric acid are both strong acids. They are first titrated with potassium hydroxide methanol isopropanol solution to produce the first equivalence point EP1; then hydrofluoric acid is titrated to produce the second equivalence point EP2.

3. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 1, characterized in that: In step 1, the total amount of HF and HNO3+HCl is calculated based on the two equivalence points. The reaction equation is as follows: HCl+H2O→H3O + +Cl － Strong acid dissociation, H3O + +OH-→2H2O neutralizes, HNO3+H2O→H3O + +NO3 － Strong acid dissociation, H3O + +OH － →2H2O neutralizes; HF+H2O→H3O + +F － Weak acid dissociation, H3O + +OH － →2H2O neutralizes; The first four chemical formulas are collectively referred to as EP1, and the last two chemical formulas are collectively referred to as EP2.

4. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 1, characterized in that: In step 1, the hydrofluoric acid in the mixed acid system is titrated by potentiometric titration. The specific process is: Weigh m1g of the prepared mixed acid sample into a polytetrafluoroethylene titration cup, where m1g is accurate to four decimal places, add 50mL of ethanol as solvent, select a pH non-aqueous phase electrode, and titrate with a calibrated potassium hydroxide methanol isopropanol solution; The first equivalence point EP1 consumes a volume of V1mL, and the second equivalence point EP2 consumes a volume of V2mL. The percentage of hydrofluoric acid in the mixed acid is: (1) in, ω HF ——Percentage of hydrofluoric acid in mixed acid, %; C(KOH)——molar concentration of potassium hydroxide, mol / L; V1——consumption volume of the first equivalence point EP1, mL; V2——the volume consumed at the second equivalence point EP2, mL; m1——Weigh the mass of the prepared mixed acid sample, g; M HF ——The molar mass of HF is 20.01 g / mol.

5. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 1, characterized in that: In step 2, the nitric acid in the mixed acid system is titrated by potentiometric titration. The specific process is: The reaction equation of nitric acid and ammonium ferrous sulfate under acidic conditions is as follows: <h2 style=";text-align:left;direction:ltr">4Fe<h2 style=";text-align:left;direction:ltr"> 2+ <h2 style=";text-align:left;direction:ltr"> +2NO3<h2 style=";text-align:left;direction:ltr"> - <h2 style=";text-align:left;direction:ltr"> +6H<h2 style=";text-align:left;direction:ltr"> + <h2 style=";text-align:left;direction:ltr"> →4Fe3<h2 style=";text-align:left;direction:ltr"> + <h2 style=";text-align:left;direction:ltr"> +N2O3+3H2O, Weigh m2g of the prepared mixed acid sample into a polytetrafluoroethylene titration cup, where m2g is accurate to four decimal places, add 40mL of sulfuric acid (4+1), select a redox electrode, and titrate with ammonium ferrous sulfate solution. The volume consumed is V3mL, and the percentage of nitric acid in the mixed acid is: (2) in, ω HNO3 ——The percentage of nitric acid in the mixed acid, %; C(NH4)2Fe(SO4)2——molar concentration of ammonium ferrous sulfate, mol / L; V3——volume of ammonium ferrous sulfate solution consumed in titration of nitric acid, mL; m2——Weigh the mass of the prepared mixed acid sample, g; MHNO3 – The molar mass of HNO3 is 63.02 g / mol.

6. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 5, characterized in that: In step 2, the ammonium ferrous sulfate solution has a concentration of 0.500 mol / L and is prepared and calibrated in accordance with GB / T601-2016 "Preparation of Standard Titration Solutions for Chemical Reagents".

7. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 5, characterized in that: In step 2, accurately weigh 6 g of reference potassium dichromate that has been dried to constant weight at 120° C. to obtain a potassium dichromate solution with a concentration of 0.5 mol / L. After dissolving, transfer to a 250 mL volumetric flask and make up to volume for the calibration of the ammonium ferrous sulfate solution.

8. The method for determining the contents of hydrofluoric acid and nitric acid in a mixed acid by potentiometric titration according to claim 1, characterized in that: In step 3, the hydrochloric acid in the mixed acid system is obtained by subtraction, and the specific process is: Weigh m1g of the prepared mixed acid sample into a polytetrafluoroethylene titration cup, where m1g is accurate to four decimal places, add 50mL of ethanol as solvent, select a pH non-aqueous phase electrode, and titrate with a calibrated potassium hydroxide methanol isopropanol solution. The first equivalence point EP1 consumes a volume of V1mL, and the second equivalence point EP2 consumes a volume of V2mL. The following calculation formula is used: (3) in, ω HCl ——The percentage of hydrochloric acid in the mixed acid, %; C(KOH)——molar concentration of potassium hydroxide, mol / L; V1——consumption volume of the first equivalence point EP1, mL; m1——Weigh the mass of the prepared mixed acid sample, g; M HCl —— HCl The molar mass of is 36.46 g / mol; n HNO3 ——The amount of nitric acid in the mixed acid obtained by the above step 1.4), mol.

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