A method for determining the number of hydrated water molecules carried during the process of salt ions passing through a polyamide separation membrane

Abstract

The present invention provides a method for determining the hydration number carried by salt ions during the process of passing through a polyamide separation membrane, belonging to the field of membrane technology. The method of the present invention comprises the following operating steps: S1. Stripping the support non-woven fabric and the intermediate layer of the polyamide separation membrane to obtain the active layer of the polyamide separation membrane; S2. Coating the active layer of the polyamide separation membrane obtained in step S1 on the counter electrode platinum sheet and the gold chip; S3. Placing the counter electrode platinum sheet and the gold chip obtained in step S2 into the EQCM-D module and connecting them to an electrochemical workstation through a three-electrode system, respectively introducing ultrapure water and a salt solution with a known concentration until equilibrium is reached, applying a constant voltage after equilibrium, recording the frequency and current changes during the whole process, and quantitatively determining the hydration number carried by salt ions during the process of passing through the polyamide separation membrane. This method is simple to operate, has high sensitivity and a wide range of applications, and for the first time realizes the quantitative determination of the hydration number of salt ions in the process of passing through a polyamide separation membrane by an electrochemical method.

Description

Technical Field

[0001] The present invention relates to the field of membrane technology, and particularly to a method for determining the number of water molecules carried by salt ions during the process of passing through a polyamide separation membrane. Background Art

[0002] Dissolved ions capture water molecules through electrostatic interactions to form a hydration layer called hydrated ions. Within the hydration layer, water molecules are interconnected by hydrogen bonds, and different ions exhibit different hydration structures (hydrate size and configuration) due to their different ionic properties (such as ionic size and charge density). The transport of hydrated ions through (sub)nanopores is ubiquitous in nature and in many industrial and biomedical applications. Since the interaction between salt ions and the pore wall is amplified under (sub)nanoscale conditions, the phenomenon behavior and properties of ion-limiting are different from those in their bulk solution state. The membrane pore size is usually smaller than the hydrated ions, and ions will inevitably dehydrate during transmembrane transport, which further affects the selective transport of different ions. The degree of dehydration specifically depends on the hydration energy of the ions, which is an overall reflection of ionic characteristics. Therefore, for ions with the same charge and similar hydrated diameters, dehydration becomes the key factor determining ion transmembrane selectivity. By optimizing the solution chemistry conditions and focusing on dehydration-dominated transport in membrane design, the differences in ion hydration structures can be amplified, which is expected to improve ion selectivity. This research has an important impact on enhancing the ion selectivity of membrane separation and is of great significance for selective resource recovery and efficient desalination technologies based on membrane technology, such as seawater desalination, lithium extraction, valuable metal recovery, and separation of harmful metal ions.

[0003] However, the influence of ion hydration structure on membrane separation has long been ignored. The traditional polyamide separation theory fails to include hydration characteristics to explain the microscopic ion transport mechanism, and little is known about the complex influence of aqueous solution chemistry on ion hydration structure and the corresponding transmembrane transport behavior. It is necessary to clarify the mutual relationship among aqueous solution chemistry, hydration structure, and hydration energy to gain a deeper understanding of selective transmembrane transport. Most of the dehydration mechanisms in previous studies were discussed through theoretical simulations, lacking in-situ characterization techniques. Therefore, it is necessary to systematically study the number of water molecules during the ion selective transport process to comprehensively analyze the transmembrane mechanism, which can promote the design of solute-solute selective membranes required for precise separation, thereby improving energy utilization efficiency and process performance efficiency.

[0004] There is an urgent need to develop a method for in-situ determining the number of water molecules carried by salt ions during the process of passing through a polyamide separation membrane to analyze the dehydration behavior of salt ions passing through a polyamide separation membrane and overcome the limitations of the existing technology. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a method for determining the hydration number carried by salt ions during the process of passing through a polyamide separation membrane. The method for determining the hydration number carried by salt ions during the process of passing through a polyamide separation membrane according to the present invention has high sensitivity and a wide range of applications, and can quantitatively characterize the hydration number carried by salt ions during the process of passing through a polyamide separation membrane under different aqueous solution chemical conditions, providing a new strategy and method for clarifying the potential mechanism of salt ion transmembrane mass transfer.

[0006] The object of the present invention is to provide a method for determining the hydration number carried by salt ions during the process of passing through a polyamide separation membrane, including the following steps:

[0007] S1. Peel off the support non-woven fabric and the intermediate layer of the polyamide separation membrane to obtain the active layer of the polyamide separation membrane;

[0008] S2. Coat the active layer of the polyamide separation membrane obtained in step S1 on the counter electrode platinum sheet and the QCM-D gold chip;

[0009] S3. Place the counter electrode platinum sheet and the gold chip obtained in step S2 in the EQCM-D module and connect them to an electrochemical workstation through a three-electrode system. Respectively introduce water and salt solution until equilibrium. After equilibrium, apply a constant voltage, and record the frequency and current changes during the whole process to quantitatively determine the hydration number carried by salt ions during the process of passing through the polyamide separation membrane.

[0010] Specifically, the method of the present invention is as follows: Peel off the support layer and the intermediate layer of the polyamide separation membrane; coat the active layer of the polyamide separation membrane from which the support layer and the intermediate layer have been peeled off on the quartz crystal microbalance gold chip (QCM-D Sensor 301) and the counter electrode platinum sheet of the electrochemical quartz crystal microbalance module (EQCM-D); connect the assembled EQCM-D module to an electrochemical workstation through a three-electrode system and place it on the quartz crystal microbalance device (QCM-D). Respectively introduce ultrapure water and salt ion solution until equilibrium, and record the frequency change; use the electrochemical workstation to give a constant voltage, and record the current and frequency changes to quantitatively determine the hydration number carried by salt ions during the process of passing through the polyamide separation membrane.

[0011] In some embodiments of the present invention, in step S1, the polyamide separation membrane is a nanofiltration membrane, a reverse osmosis membrane or a self-made membrane.

[0012] In some embodiments of the present invention, in step S1, impurity removal of the polyamide separation membrane is further included: soak the polyamide separation membrane in an isopropanol solution and then wash it with ultrapure water to remove impurities and the anti-corrosion layer on the surface of the polyamide separation membrane. After drying, peel off the support non-woven fabric.

[0013] Furthermore, the concentration of the isopropanol solution is 10-50 %v / v;

[0014] The soaking time is 30-60 min;

[0015] The number of cleaning times is at least three times.

[0016] In some embodiments of the present invention, in step S2, the method for coating the active layer of the polyamide separation membrane on the counter electrode platinum sheet and the gold chip: Immerse the active layer of the polyamide separation membrane in an organic solvent, place the counter electrode platinum sheet and the gold chip at the bottom of the active layer of the polyamide separation membrane, and then fish out the counter electrode platinum sheet and the gold chip coated with the active layer of the polyamide separation membrane.

[0017] In some embodiments of the present invention, the organic solvent is selected from one or more of N,N-dimethylformamide, chloroform, and dimethyl sulfoxide.

[0018] In some embodiments of the present invention, in step S2, it further includes cleaning the counter electrode platinum sheet and the gold chip: Use an alcohol cotton sheet to wipe off the excess active layer of the polyamide separation membrane on the back of the counter electrode platinum sheet and the gold chip, and pierce the active layer of the polyamide separation membrane at the flow channel of the counter electrode platinum sheet.

[0019] In some embodiments of the present invention, in step S3, the calculation formula for the number of hydration water molecules carried during the process of salt ions passing through the polyamide separation membrane is as follows:

[0020] ,

[0021] ,

[0022] ,

[0023] ,

[0024] Wherein, the mass change Δm of the hydrated salt ions passing through the polyamide separation membrane i is defined as the product of the crystal constant C and the frequency change Δf, and the relative atomic mass MW' of the hydrated salt ions i is defined as the ratio of the product of Δm i and the Faraday constant F to the charge quantity Q i , and the number of hydration water molecules N i is defined as the ratio of the difference between MW' i and the relative atomic mass MW of the bare salt ions to the relative molecular mass MW i of water; H2O ;

[0025] n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequencies before and after applying a constant voltage, F = 96485 C / mol, Q i is the charge quantity required for the i ions to enter the polyamide separation membrane, that is, the total charge quantity Q transferred during the process of applying a constant voltaget Subtract the electric charge It, Q caused by the electric double layer t Obtained by integrating the I-t curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability and t is the time corresponding to the system stability, MW i and MW' i are the relative atomic masses of the non-hydrated and hydrated i ions, N i is the number of water molecules carried by the i ions during the process of entering the polyamide separation membrane.

[0026] In some embodiments of the present invention, in step S3, the method for the complete adsorption equilibrium of salt ions on the polyamide separation membrane is, for example: when the salt ion concentration is 100 mM, the peristaltic pump speed is 30 μL / min, and the equilibrium time is 0.5 - 2 h.

[0027] In some embodiments of the present invention, in step S3, the pH value of the salt solution is 3 - 11, the salt solution is a single salt solution, and the ion concentration is 100 mmol / L - 1000 mmol / L.

[0028] In some embodiments of the present invention, in step S3, the method for adjusting the pH is the acid-base titration method, and a Leici PHSJ-3F type laboratory pH meter is used to characterize the pH value.

[0029] In some embodiments of the present invention, in step S3, both water and the salt solution run for 0.5 - 2 h to reach a stable equilibrium.

[0030] Specifically, in step S4, the water is ultrapure water produced by a Milli-Q IQ 7000 ultrapure water machine, and the measured conductivity < 0.2 μS / cm

[0031] In some embodiments of the present invention, in step S3, the number of water molecules of anions or cations is measured by controlling the positive and negative of the applied constant voltage. A negative voltage is applied to measure the number of water molecules of cations, and a positive voltage is applied to measure the number of water molecules of anions.

[0032] In some embodiments of the present invention, in step S3, the cation of the salt solution is one of lithium, sodium, potassium, calcium, or magnesium, and the anion is chloride ion or sulfate ion.

[0033] The above technical solution of the present invention has the following advantages compared with the prior art:

[0034] The present invention adopts the technology based on quartz crystal microbalance and electrochemical workstation to quantitatively characterize the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane. By measuring the mass difference and the transferred charge amount before and after applying a constant voltage balance between the polyamide separation membrane and the salt solution, the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane is accurately quantified. This method is simple to operate, real-time and intuitive, and can be widely applied to thin-film composite reverse osmosis membranes, nanofiltration membranes and self-made membranes, providing an important analysis tool for measuring the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to make the content of the present invention easier to be clearly understood, the following further describes the present invention in detail according to specific embodiments of the present invention in conjunction with the drawings, wherein,

[0036] Figure 1 is a schematic flow chart of the method for measuring the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane in the embodiment;

[0037] Figure 2 is a schematic diagram of the frequency change under the condition of applying a constant voltage of -0.3 V in Example 1 for 100 mM Na + ;

[0038] Figure 3 is a schematic diagram of the current change under the condition of applying a constant voltage of -0.3 V in Example 1 for 100 mM Na + ;

[0039] Figure 4 is a schematic diagram of the number of hydrated water molecules carried by Li + and Na + during the process of passing through the NF90 polyamide separation membrane measured in the embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The following further describes the present invention in conjunction with the drawings and specific embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the embodiments cited do not limit the present invention.

[0041] The purpose of the present invention is to provide a method for measuring the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane, including the following steps:

[0042] S1. Peel off the support non-woven fabric and the intermediate layer of the polyamide separation membrane, and only retain the active layer;

[0043] S2. Flatly coat the active layer of the polyamide separation membrane from which the support layer and the intermediate layer are peeled off in step S1 on the counter electrode platinum sheet and the QCM-D gold chip;

[0044] S3. Place the counter electrode platinum sheet and the gold chip in step S2 in a ventilated place until completely dry;

[0045] S4. Place the dried counter electrode platinum sheet and gold chip in the EQCM-D module and connect them to the electrochemical workstation through a three-electrode system. Then, introduce ultrapure water and a salt solution with a known concentration until equilibrium is reached. After equilibrium, apply a known constant voltage and record the frequency and current changes during the whole process to quantitatively determine the number of hydrated water molecules carried by salt ions during the process of passing through the polyamide separation membrane.

[0046] Example 1:

[0047] This example provides a method for measuring the number of hydrated water molecules carried by Na + ions during the process of passing through the NF90 polyamide separation membrane. The specific steps are as follows:

[0048] (1) Immerse the commercial NF90 polyamide separation membrane in 25% v / v isopropanol for 60 min, then wash it three times with ultrapure water. After stripping the support layer of the washed NF90 polyamide separation membrane, place it in an N,N-dimethylformamide solution and strip the intermediate layer to obtain the active layer of the NF90 polyamide separation membrane.

[0049] (2) Place the counter electrode platinum sheet and gold chip at the bottom of the active layer of the NF90 polyamide separation membrane immersed in the N,N-dimethylformamide solution. Take out the counter electrode platinum sheet and gold chip coated with the active layer of the NF90 polyamide separation membrane and place them in a fume hood until completely dry. After drying, use an alcohol cotton sheet to wipe off the excess active layer of the NF90 polyamide separation membrane on the back of the counter electrode platinum sheet and gold chip, and use a needle to pierce the active layer of the NF90 polyamide separation membrane at the flow channel of the counter electrode platinum sheet.

[0050] (3) Prepare a NaCl solution with a concentration of 100 mM and a pH of 5.70

[0051] (4) Place the counter electrode platinum sheet and gold chip obtained in step (2) in the EQCM-D module and connect them to the electrochemical workstation. Set the peristaltic pump speed to 30 μL / min and introduce ultrapure water for 1 h until the frequency is stable. Then, introduce the 100 mM NaCl solution obtained in step (3) for 30 min until the frequency is stable. After stability, apply a constant voltage of -0.3 V for 10 min and record the frequency and current changes during the whole process to quantitatively determine the number of hydrated water molecules carried by Na + ions during the process of passing through the NF90 polyamide separation membrane;

[0052] (5) The mass change of Na + ions passing through the polyamide separation membrane △m Na is defined as the product of the crystal constant C and the frequency change Δf during the process of passing through the NaCl salt solution. The relative atomic mass of the + hydrated ion MW 'Na + is defined as △m Na + the ratio of the product of the Faraday constant F and the charge quantity Q Na + to the hydration number N Na + is defined as MW ' Na + the difference between + the relative atomic mass of the Na MW Na + ion and the relative molecular mass of water MW H2O The calculation formula is as follows:

[0053] ,

[0054] ,

[0055] ,

[0056] ,

[0057] where n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequencies before and after applying a constant voltage, F = 96485 C / mol, Q Na + is the charge quantity required for the Na + + ion to enter the NF90 polyamide separation membrane process, that is, the total charge quantity Q transferred during the application of the constant voltage t minus the charge quantity It caused by the electric double layer, Q t is obtained by integrating the I-t curve recorded by the electrochemical workstation, I is the current corresponding to the system stability, t is the time corresponding to the system stability, MW Na + and MW ' Na + are the relative atomic masses of the non-hydrated and hydrated Na + + ions, N Na+ is the hydration number carried by the Na + + ion during the process of entering the NF90 polyamide separation membrane;

[0058] (6) Calculate that when the constant voltage of -0.3 V is applied and t is 400 s, the concentration of Na + + with a concentration of 100 mM and a pH of 5.70 Na + has a △m MW ' Na + of 68.4 g / mol, Q t is 3.75×10 -3 C, I is 8.86×10 -6 A, Q Na+ is 2.06×10 - 4 C, N Na + is 2.52;

[0059] Figure 2 is 100 mM Na in this example + Schematic diagram of frequency change under the condition of applying a constant voltage of -0.3 V. As can be seen from the figure, before applying a constant voltage of -0.3 V (setting the time of applying the constant voltage as T = 0), 100 mM Na is introduced + for about 30 min until the frequency stabilizes ( Figure 2 as shown is the frequency corresponding to introducing 100 mM Na + and the corresponding frequency after stabilization and applying a constant voltage of -0.3 V). After the frequency stabilizes, a constant voltage of -0.3 V is applied, and it is observed that the frequency continuously decreases with time, and the corresponding frequency change Δf corresponds to the mass change.

[0060] Figure 3 is 100 mM Na in this example + Schematic diagram of current change under the condition of applying a constant voltage of -0.3 V. As can be seen from the figure, as time extends, the current continuously decreases and finally reaches a stable value. The total charge transferred during the process of applying a constant current is Q t , and the charge corresponding to this stable value is the charge It caused by the electric double layer. The difference between the two is the charge Q + required for Na ions to enter the polyamide separation membrane Na +.

[0061] Example 2:

[0062] This example provides a method for measuring the number of hydrated water molecules carried by Li + ions during the process of passing through the NF90 polyamide separation membrane. The specific steps are as follows:

[0063] (1) Immerse the commercial NF90 polyamide separation membrane in 25% v / v isopropanol for 60 min, then wash it three times with ultrapure water. After peeling off the support layer of the washed NF90 polyamide separation membrane, place it in an N,N-dimethylformamide solution and peel off the intermediate layer to obtain the active layer of the NF90 polyamide separation membrane;

[0064] (2) Place the counter electrode platinum sheet and the gold chip at the bottom of the active layer of the NF90 polyamide separation membrane immersed in the N,N-dimethylformamide solution. Take out the counter electrode platinum sheet and the gold chip coated with the active layer of the NF90 polyamide separation membrane, and place the counter electrode platinum sheet and the gold chip coated with the active layer of the NF90 polyamide separation membrane in the fume hood until completely dry. After drying, use an alcohol cotton sheet to wipe off the excess active layer of the NF90 polyamide separation membrane on the back of the counter electrode platinum sheet and the gold chip, and use a needle to pierce the active layer of the NF90 polyamide separation membrane at the flow channel of the counter electrode platinum sheet;

[0065] (3) Prepare a LiCl solution with a concentration of 100 mM and a pH of 5.70;

[0066] (4) Place the counter electrode platinum sheet and the gold chip obtained in step (2) into the EQCM-D module and connect it to the electrochemical workstation. Set the peristaltic pump speed to 30 μL / min and introduce ultrapure water for 1 h until the frequency is stable. Then introduce the 100 mM LiCl solution obtained in step (3) for 30 min until the frequency is stable. After stability, apply a constant voltage of -0.3 V for 10 min, and record the frequency and current changes during the whole process to quantitatively determine the number of water molecules carried by Li + ions passing through the NF90 polyamide separation membrane;

[0067] (5) The mass change of Li + ions passing through the polyamide separation membrane △m Li + is defined as the product of the crystal constant C and the frequency change Δf during the process of passing the LiCl salt solution. The relative atomic mass of Li + hydrated ions MW ' Li + is defined as △m Li + the ratio of the product of and the Faraday constant F to the charge quantity Q Li +. The number of water molecules N Li + is defined as MW ' Li + the difference between and the relative atomic mass of Li + ions and the relative molecular mass of water MW Li +. The calculation formula is as follows: MW H2O The ratio, and the calculation formula is as follows:

[0068] ,

[0069] ,

[0070] ,

[0071] ,

[0072] Among them, n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequencies before and after applying a constant voltage, F = 96485 C / mol, Q Li + is Li + The amount of electric charge required for the Li t + to enter the NF90 polyamide separation membrane process, which is the total amount of electric charge Q transferred during the application of the constant voltage, minus the amount of electric charge It caused by the electric double layer. Q t is obtained by integrating the I-t curve recorded by the electrochemical workstation. I is the current corresponding to the system stability, and t is the time corresponding to the system stability. MW Li + and MW ' Li + are the relative atomic masses of non-hydrated and hydrated Li + ions, and N Li + is the number of water molecules carried by Li + ions during the process of entering the NF90 polyamide separation membrane;

[0073] (6) It is calculated that when a constant voltage of -0.3 V is applied and t is 100 s, the △m + of Li Li + with a concentration of 100 mM and a pH of 5.70 is 44.4 ng, MW ' Li + is 97.4 g / mol, Q t is 5.74×10 -4 C, I is 5.30×10 -6 A, Q Li + is 4.40×10 - 4 C, and N Li + is 5.03.

[0074] Figure 4 is the schematic diagram of the number of water molecules carried by Li + and Na + ions during the process of passing through the NF90 polyamide separation membrane in the examples. It can be seen from the figure that the average value of the number of water molecules carried by Li + ions during the process of passing through the polyamide separation membrane is 5.03, and the average value of the number of water molecules carried by Na + ions during the process of passing through the NF90 polyamide separation membrane is 2.52.

[0075] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for determining the hydration number of salt ions carried through a polyamide separation membrane, characterized in that: The following steps are involved: S1, peeling off the supporting layer non-woven fabric and the middle layer of the polyamide separation membrane to obtain the active layer of the polyamide separation membrane; S2, coating the polyamide separation membrane active layer obtained in step S1 on the counter electrode platinum sheet and gold chip; S3, placing the counter electrode platinum sheet and gold chip obtained in step S2 in the EQCM-D module and connecting the electrochemical workstation through a three-electrode system, respectively passing water and salt solution until equilibrium, applying constant voltage after equilibrium, recording the frequency and current changes of the whole process, and quantitatively determining the hydration number carried by the salt ions through the polyamide separation membrane; the calculation formula for the hydration number carried by the salt ions through the polyamide separation membrane is as follows: Q i =Q t -It, Among them, the mass change of hydrated salt ions passing through the polyamide separation membrane is Δm i Defined as the product of the crystal constant C and the frequency change Δf, the relative atomic mass of the hydrated salt ion MW' i Defined as Δm i The product of the Faraday constant F and the charge Q i The ratio of hydration number N i Defined as MW' i Relative atomic mass MW to bare salt ion i The difference between the molecular weight of water and ratio; n=5,C=17.7ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequency before and after applying constant voltage, F = 96485C / mol, Q i is the charge required for the i ion to enter the polyamide separation membrane, that is, the total charge Q transferred during the constant voltage application process t Subtract the charge It, Q caused by the double layer t It is obtained by integrating the It curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability, and t is the time corresponding to the system stability.

2. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S1, the polyamide separation membrane is a nanofiltration membrane, a reverse osmosis membrane or a homemade membrane.

3. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: Step S1 also includes removing impurities from the polyamide separation membrane: soaking the polyamide separation membrane in an isopropanol solution and then washing it with ultrapure water to remove impurities and the anti-corrosion layer on the surface of the polyamide separation membrane, and peeling off the supporting layer non-woven fabric after drying.

4. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: Step S2 includes the following steps: immersing the polyamide separation membrane active layer in an organic solvent, placing a counter electrode platinum sheet and a gold chip at the bottom of the polyamide separation membrane active layer, and then fishing out the counter electrode platinum sheet and the gold chip coated with the polyamide separation membrane active layer.

5. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 4, characterized in that: The organic solvent is selected from one or more of N,N-dimethylformamide, chloroform and dimethyl sulfoxide.

6. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: Step S2 also includes cleaning the counter electrode platinum sheet and gold chip: using an alcohol cotton pad to wipe off the excess polyamide separation membrane active layer on the back of the counter electrode platinum sheet and gold chip, and piercing the polyamide separation membrane active layer at the flow channel of the counter electrode platinum sheet.

7. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, both the water and the salt solution run for 0.5 to 2 hours to reach a stable equilibrium.

8. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, the hydration number of anions or cations is measured by controlling the positive and negative of the applied constant voltage. The hydration number of cations is measured by applying a negative voltage, and the hydration number of anions is measured by applying a positive voltage.

9. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, the cation of the salt solution is one of lithium, sodium, potassium, calcium or magnesium, and the anion is chloride ion or sulfate ion.

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