A method for characterizing surface charge of inorganic salts based on electrochemical means

Abstract

The application belongs to the technical field of inorganic salt surface charge characterization, and relates to a method for characterizing the surface charge of inorganic salt based on an electrochemical method. In the method, the inorganic salt is processed into an electrode sheet, a saturated solution of the inorganic salt is used as an electrolyte, and the zero charge potential of the inorganic salt in the saturated solution is measured. The method does not need to grind the inorganic salt, guarantees the integrity of the crystal surface of the inorganic salt, and can accurately characterize the charge condition of the surface of the inorganic salt in the saturated solution.

Description

Technical Field

[0001] This invention belongs to the field of inorganic salt surface charge characterization technology, and relates to a method for characterizing the surface charge of inorganic salts based on electrochemical means. Background Technology

[0002] Characterizing the surface charge of inorganic salts has significant theoretical and practical implications. It not only helps in understanding the fundamental physicochemical properties of materials but also plays a crucial role in numerous application fields. Understanding the surface charge state of inorganic salt crystals is essential for studying their surface chemical reactivity, adsorption behavior, and interfacial phenomena. Surface charge influences processes such as ion exchange, solubility, and precipitation; the distribution of surface charge can reveal the kinetics and thermodynamics of crystal growth, helping to explain changes in crystal morphology under different conditions. In mineral processing, flotation utilizes the selective adhesion between mineral particles and bubbles, and surface charge is one of the key factors determining flotation efficiency. Understanding the surface charge of fertilizer particles helps in developing more efficient slow-release fertilizers, reducing nutrient loss, and improving crop absorption rates. Characterizing the surface charge of inorganic salts provides a bridge to understanding the microstructure and macroscopic behavior of materials, promoting widespread progress from basic research to practical applications. In-depth exploration of the mechanisms of surface charge can not only optimize existing materials and technologies but also inspire the development of novel functional materials, driving innovation in multiple fields.

[0003] Currently, commonly used methods for characterizing the surface charge of inorganic salts include Zeta potential measurement, Kelvin probe force microscopy, rheology, and dynamic light scattering. However, these methods have very high requirements for the particle size of inorganic salts, requiring particles to be at the micrometer or even nanometer scale, and also demanding excellent dispersibility. In practical applications, inorganic salt particles are relatively large and poorly dispersed. To determine their surface charge, physical grinding is required, but grinding destroys the inorganic salt crystal structure, leading to distorted surface charge data that cannot accurately reflect the actual surface charge of the crystal.

[0004] The Zeta potential method for characterizing the surface potential of inorganic salts requires salt particles to be at the micrometer scale, stable in a dispersant, and well-dispersible. This limits the method's ability to characterize inorganic salts. Water and ethanol are commonly used dispersants in the Zeta potential method. However, water is unsuitable as a dispersant for inorganic salts because they are readily soluble in water. For some hydrated inorganic salts, such as carnallite (KCl·MgCl2·6H2O) and potassium magnesium alum (KCl·MgSO4·3H2O), they decompose in ethanol, thus requiring dispersion in their corresponding saturated solutions. Furthermore, the Zeta potential method has limitations on the conductivity of the dispersant; excessively high conductivity in saturated inorganic salt solutions often exceeds the range of the Zeta potential meter. In conclusion, even with methods like grinding to achieve the required particle size, the Zeta potential method struggles to meet the characterization requirements for the surface potential of most inorganic salts due to the selectivity of the dispersant.

[0005] Kelvin probe force microscopy is a technique based on atomic force microscopy (AFM) that can measure surface potential at the nanoscale, thereby reflecting the surface charge distribution. Characterizing the surface charge of inorganic salts using this method requires controlling the particle size to the nanoscale through physical grinding or other methods. Inorganic salts exist in crystalline form; grinding disrupts the crystal structure and alters the exposed crystal faces, thus affecting the surface charge of the inorganic salt. Therefore, the surface charge data measured by this method is distorted and cannot accurately reflect the surface charge of inorganic salts.

[0006] Therefore, a method is needed to accurately characterize the surface charge of inorganic salts. Summary of the Invention

[0007] The potential of zero charge (PZC) is the electric potential at which the surface charge of an electrode is zero. In electrochemistry, PZC is an important parameter describing the properties of the electrode-electrolyte interface. By measuring PZC, the charging state and charge properties of the electrode surface at different potentials can be understood. When PZC < 0, it indicates that decreasing the electrode surface potential results in zero surface charge, indicating the presence of a positive charge on the electrode surface. When PZC > 0, it indicates that increasing the electrode surface potential results in zero surface charge, indicating the presence of a negative charge on the electrode surface. Using this principle, inorganic salts can be used as electrodes, and the surface charge of the inorganic salts can be determined by characterizing PZC.

[0008] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for characterizing the surface charge of inorganic salts based on electrochemical means. This method involves processing the inorganic salt into electrode sheets and using its saturated solution as the electrolyte to measure the zero-charge potential of the inorganic salt in its saturated solution. This method eliminates the need for grinding the inorganic salt, ensuring the integrity of the inorganic salt crystal planes and enabling accurate characterization of the surface charge of the inorganic salt in its saturated solution.

[0009] The technical solution adopted by this invention to solve the technical problem is:

[0010] A method for characterizing the surface charge of inorganic salts based on electrochemical means, comprising the following steps:

[0011] (1) Preparation of salt electrode sheet:

[0012] The inorganic salt to be tested is pressed into a salt electrode sheet, with the pressing pressure controlled at 12-14 MPa and the sheet thickness controlled at 0.2-1.5 mm.

[0013] (2) Electrochemical characterization of the salt electrode sheet, including:

[0014] 1) Constructing the device:

[0015] Using an electrochemical workstation, the prepared salt electrode sheet was mounted on the working electrode, and the counter electrode and reference electrode were selected. The electrolyte was a saturated solution of the inorganic salt to be tested at the corresponding temperature.

[0016] 2) Set the parameters of the electrochemical workstation:

[0017] Select the Staircase Potentialoelectrochemical Impedance Spectroscopy (Mott-schottky)-SPEIS function on the electrochemical workstation. The scan voltage range can be adjusted appropriately according to different types of salt electrodes, set to -1.0 to 1.0 V. A higher number of scan steps results in higher accuracy, but also longer testing time; set the step size to 0.02 to 0.05 V. The frequency should not exceed 200 mHz; set it to 10 to 50 mHz. The electrochemical workstation will measure the capacitance between the salt electrode surface and the reference electrode at different potentials, ultimately obtaining a curve showing the capacitance of the salt electrode as a function of potential.

[0018] 3) Data processing:

[0019] In the curve showing the capacitance of the salt electrode as a function of potential, the potential corresponding to the minimum capacitance value is the zero-charge potential (PZC). To more clearly determine the PZC, the data is normalized. By comparing the capacitance values ​​at different potentials with the minimum capacitance value and plotting the resulting ratios, a normalized capacitance curve can be obtained. The potential value corresponding to the lowest point of capacitance is the zero-charge potential (PZC).

[0020] The tableting pressure should be controlled between 12 and 14 MPa. If the tableting pressure is too low, the mechanical strength of the salt electrode sheet will not meet the standard. It will easily disperse in the electrolyte solution during the measurement process, or even fail to form. If the tableting pressure is too high, it will easily damage the mold and equipment, and the salt tablet will be difficult to remove from the mold, making it difficult to obtain a flat and intact salt electrode sheet.

[0021] The thickness of the pressed sheet should be controlled between 0.2 and 1.5 mm. If the thickness of the pressed sheet is too small, the mechanical strength of the salt electrode sheet will not meet the standard, and it will be easy to bend or break in the electrolyte solution during the measurement process. If the thickness of the pressed sheet is too large, it will not be easy to be fixed by the clamp of the working electrode, and at the same time, it will increase the resistance of the salt electrode sheet, which is not conducive to electrochemical measurement.

[0022] Furthermore, the inorganic salt is anhydrous salt or hydrated salt. For anhydrous salt that does not contain water of crystallization, it is directly pressed into a salt electrode sheet using a tablet press. For inorganic salt (hydrated salt) that contains a certain number of water molecules of crystallization, a metal mesh is placed at the bottom of the tablet press mold, and the pressed salt sheet is loaded onto the metal mesh. This can effectively prevent the salt electrode sheet from breaking and dispersing during the measurement process, thus obtaining the salt electrode sheet.

[0023] Furthermore, the metal mesh is a copper mesh, a platinum mesh, or a gold mesh.

[0024] Furthermore, in step (1), for the salt electrode sheet prepared by hydrated salt, when assembling the working electrode, the side without metal mesh faces the counter electrode, and the side with metal mesh is supported by an inert electrode sheet and mounted together on the working electrode.

[0025] Furthermore, the counter electrode is an inert electrode, which is a carbon fiber plate or a platinum sheet.

[0026] Furthermore, the reference electrode is any one of a saturated calomel electrode, a silver / silver chloride electrode, or a dual salt bridge reference electrode.

[0027] The advantages and positive effects of this invention are:

[0028] This invention uses a tablet pressing method to prepare inorganic salt electrodes. The zero-charge potential of the salt electrode sheet is measured by the capacitance potential method to characterize the surface charge of the inorganic salt. This method does not require grinding the inorganic salt, thus ensuring the integrity of the crystal plane of the inorganic salt. It can accurately characterize the surface charge of the inorganic salt in its saturated solution. The adsorption effect on anions or cations can be determined based on the sign of the zero-charge potential value. Attached Figure Description

[0029] Figure 1 Photograph of a sodium chloride electrode sheet;

[0030] Figure 2 The normalized capacitance curve of sodium chloride in a saturated solution at 25℃ is shown.

[0031] Figure 3 Normalized capacitance curve of potassium magnesium alum in saturated solution at 25℃;

[0032] Figure 4 The normalized capacitance curve of carnallite in a saturated solution at 25℃ is shown. Detailed Implementation

[0033] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0034] Example 1

[0035] A method for characterizing the surface charge of inorganic salts based on electrochemical means, comprising the following steps:

[0036] Sodium chloride was directly compressed into salt electrode sheets using a tablet press. The compression pressure was 14 MPa, and the sheet thickness was 1.2 mm. The sodium chloride electrode sheets were prepared as follows: Figure 1 As shown.

[0037] The surface charge of sodium chloride in its saturated solution at 25°C was characterized using an electrochemical workstation. The prepared sodium chloride electrode was mounted on the working electrode, with a saturated calomel electrode as the reference electrode, a carbon fiber plate as the counter electrode, and the saturated sodium chloride solution at 25°C as the electrolyte.

[0038] Select the stepped potential electrochemical impedance spectroscopy function on the electrochemical workstation, set the voltage range to 0–0.5 V, the voltage step size to 0.05 V, and the frequency to 50 mHz. The normalized capacitance curve of the sodium chloride electrode in a saturated solution at 25 °C is shown below. Figure 2 As shown.

[0039] The zero-charge potential of sodium chloride in a saturated solution at 25℃ is 0.2V, and PZC>0, indicating that the surface potential of sodium chloride in a saturated solution at 25℃ is negative. Therefore, it can be concluded that the surface of sodium chloride crystals in a saturated solution at 25℃ has an electrostatic adsorption effect on sodium ions (cations).

[0040] Example 2

[0041] A method for characterizing the surface charge of inorganic salts based on electrochemical means, comprising the following steps:

[0042] Potassium magnesium alum (KCl·MgSO4·3H2O) was compressed into tablets using a tablet press at a pressure of 14 MPa and a thickness of 1.0 mm. A copper mesh was placed at the bottom of the tablet press mold, and the compressed salt tablets were loaded onto the copper mesh to prepare potassium magnesium alum electrode sheets.

[0043] The surface charge of potassium magnesium alum (KCl·MgSO4·3H2O) in its saturated solution at 25°C was characterized using an electrochemical workstation. The potassium magnesium alum electrode was mounted on the working electrode with the unsupported copper mesh facing the counter electrode, and the copper mesh-supported side supported by a carbon fiber plate. A saturated calomel electrode was selected as the reference electrode, the carbon fiber plate as the counter electrode, and the 25°C saturated potassium magnesium alum solution as the electrolyte.

[0044] Select the stepped potential electrochemical impedance spectroscopy function on the electrochemical workstation, set the voltage range to -0.8 to 0.8 V, the voltage step size to 0.05 V, and the frequency to 50 mHz. The normalized capacitance curve of the potassium-magnesium alum electrode in a saturated solution at 25 °C is shown below. Figure 3 As shown.

[0045] The zero-charge potential of potassium magnesium alum in a saturated solution at 25℃ is -0.65V, and PZC < 0, indicating that the surface potential of potassium magnesium alum in a saturated solution at 25℃ is positive. Therefore, it can be concluded that the surface of potassium magnesium alum crystals in a saturated solution at 25℃ has an electrostatic adsorption effect on anions.

[0046] Example 3

[0047] A method for characterizing the surface charge of inorganic salts based on electrochemical means, comprising the following steps:

[0048] Carnallite (KCl·MgCl2·6H2O) was compressed into tablets using a tablet press at a pressure of 12 MPa and a thickness of 1.0 mm. A platinum mesh was placed at the bottom of the tablet press mold, and the compressed salt tablets were loaded onto the platinum mesh to prepare carnallite electrode sheets.

[0049] The surface charge of carnallite (KCl·MgCl2·6H2O) in its saturated solution at 25 °C was characterized using an electrochemical workstation. The carnallite electrode was mounted on the working electrode with the unsupported platinum mesh facing the counter electrode, and the platinum mesh-supported side supported by a carbon fiber plate. A saturated calomel electrode was selected as the reference electrode, the carbon fiber plate as the counter electrode, and the 25 °C saturated carnallite solution as the electrolyte.

[0050] Select the stepped potential electrochemical impedance spectroscopy function on the electrochemical workstation, set the voltage range to -0.8 to 0.8 V, the voltage step size to 0.05 V, and the frequency to 30 mHz. The normalized capacitance curve of the carnallite electrode in a saturated solution at 25 °C is shown below. Figure 4 As shown.

[0051] The zero-charge potential of carnallite in a saturated solution at 25℃ is -0.55V, and PZC < 0, indicating that the surface potential of carnallite in a saturated solution at 25℃ is positive. Therefore, it can be concluded that the surface of carnallite crystals has an electrostatic adsorption effect on anions in a saturated solution of carnallite at 25℃.

[0052] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A method for characterizing the surface charge of an inorganic salt based on electrochemical means, characterized in that, The steps are as follows: (1) preparing a salt electrode sheet: The inorganic salt to be tested is pressed into a salt electrode sheet, the pressing pressure is controlled at 12-14 MPa, and the thickness of the pressed sheet is controlled at 0.2-1.5 mm; (2) electrochemically characterizing the salt electrode sheet, including: 1) building a device: Using an electrochemical workstation, the prepared salt electrode sheet is mounted on the working electrode, the counter electrode and the reference electrode are selected, and the electrolyte is a saturated solution of the inorganic salt to be tested at the corresponding temperature; 2) setting the parameters of the electrochemical workstation: Select the step potential electrochemical impedance spectroscopy function of the electrochemical workstation, the scan voltage range is-1.0-1.0 V, the step size is set to 0.02-0.05 V, the frequency is set to 10-50 mHz, and the electrochemical workstation tests the capacitance value between the surface of the salt electrode sheet and the reference electrode at different potentials, and finally obtains the curve of the capacitance value of the salt electrode sheet changing with the potential; 3) data processing: In the curve of the capacitance value of the salt electrode sheet changing with the potential, the potential corresponding to the minimum capacitance value is the zero charge potential, the capacitance value is normalized, the capacitance value at different potentials is compared with the minimum capacitance value, the obtained ratio is plotted, and the normalized capacitance curve is obtained, and the potential value corresponding to the minimum point of the capacitance is the zero charge potential.

2. The method of claim 1, wherein, The inorganic salt is anhydrous salt or hydrated salt, for anhydrous salt, the salt electrode sheet is directly prepared by a tablet press.

3. The method of claim 2, wherein, For hydrated salt, a layer of metal mesh is placed at the bottom of the tablet press, and the prepared salt sheet is loaded on the metal mesh to prepare the salt electrode sheet.

4. The method of claim 3, wherein, The metal mesh is a copper mesh, a platinum mesh or a gold mesh.

5. The method of claim 3, wherein, In step (1), for the salt electrode sheet prepared from hydrated salt, when assembling the working electrode, the side without the metal mesh is faced to the counter electrode, and the side with the metal mesh is supported by an inert electrode sheet and mounted on the working electrode together.

6. The method of claim 1, wherein, The counter electrode is an inert electrode.

7. The method of claim 6, wherein, The inert electrode is a carbon fiber plate or a platinum sheet.

8. The method of claim 1, wherein, The reference electrode is any one of a saturated calomel electrode, a silver / silver chloride electrode and a double salt bridge reference electrode.

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